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#### Question 1:

What colour do the following indicators turn when added to a base or alkali (such as sodium hydroxide)?
(a) methyl orange
(b) litmus
(c) red cabbage extract

(a) Methyl orange: Methyl orange changes its colour to yellow when added to a base or an alkali.
(b) Litmus paper: Red litmus paper turns blue, whereas blue litmus paper remains blue in a base or an alkali.
(c) Red cabbage extract: Red cabbage extract turns greenish-yellow in basic solutions.

#### Question 2:

What colours do the following indicators turn when added to an acid (such as hydrochloric acid)?
(a) litmus
(b) methyl orange

(a) Litmus paper: Blue litmus paper turns red to an acid, whereas red litmus paper remains red.
(b) Methyl orange: Methyl orange changes to red when added to an acid.

#### Question 3:

Name an indicator which is red in acid solution but turns blue in basic solution.

Red litmus paper is red in an acid solution and turns blue in a basic solution.

#### Question 4:

Name an indicator which is pink in alkaline solution but turns colourless in acidic solution.

Phenolphthalein is pink in an alkaline solution but turns colourless in an acidic solution.

#### Question 5:

When a solution is added to a cloth strip treated with onion extract, then the smell of onion cannot be detected. State whether the given solution contains an acid or a base.

Basic solutions subdue the pungent smell of onion, however acidic solutions have no such effect. Hence we can say that, the cloth strip must have been treated with a basic solution.

#### Question 6:

When a solution is added to vanilla extract, then the characteristic smell of vanilla cannot be detected. State whether the given solution is an acid or a base.

When a basic solution is added to vanilla extract, it destroys the characteristic smell of vanilla.  Conversely, it is retained in an acidic solution. Hence, the given solution is a base.

#### Question 7:

How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?

Hydrogen gas is liberated when an active metal reacts with hydrochloric acid.
For testing the presence of hydrogen gas, bring a burning candle near the mouth of the test tube in which the reaction is taking place. If the gas burns with a popping sound, then it can be concluded that it is hydrogen.

#### Question 8:

Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How is it recognised?

Carbon dioxide gas is liberated when dilute hydrochloric acid reacts with sodium hydrogen carbonate.
When the gas liberated in the reaction is passed through lime water, lime water turns milky or a white precipitate of calcium carbonate is formed. This confirms that the gas is carbon dioxide.

#### Question 9:

Give the names and formulae of two strong acids and two weak acids.

The names and formulae of two strong acids and two weak acids are as follows.

(a) Strong acids:
1. Hydrochloric acid: HCl
2. Sulphuric acid: H2SO4

(b) Weak acids:
1. Formic acid: HCOOH
2. Acetic acid: CH3COOH

#### Question 10:

Name one natural source of each of the following acids:
(a) Citric acid
(b) Oxalic acid
(c) Lactic acid
(d) Tartaric acid

(a) Citric acid: Lemons and oranges

(b) Oxalic acid: Tomatoes

(c) Lactic acid: Sour milk

(d) Tartaric acid: Unripe grapes and tamarind

#### Question 11:

Name one animal and one plant whose stings contain formic acid (or methanoic acid).

Animal sting containing formic acid: Ant sting
Plant sting containing formic acid: Nettle leaf sting

#### Question 12:

How is the concentration of hydronium ions (H3O+) affected when the solution of an acid is diluted?

When the solution of an acid is diluted, the concentration of hydronium ions decreases because the total volume of the solution increases on addition of water.

#### Question 13:

Write word equations and then balanced equations for the reactions taking place when:
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.

(a)  Sulphuric acid     +  Zinc    →  Zinc Sulphate solution  + Hydrogen gas
dil. H2SO4(aq)     +   Zn(s)  →         ZnSO4(aq)              +  H2(g)

(b) Hydrochloric acid + Magnesium  → Magnesium chloride solution  +  Hydrogen gas
dil. 2HCl(aq)       +      Mg(s)       →            MgCl2(aq)                    +  H2(g)

(c) Sulphuric acid  + Aluminium →  Aluminium sulphate solution  +  Hydrogen gas
dil. 3H2SO4(aq) +   2Al(s)       →            Al2(SO4)3(aq)                +  3H2(g)

(d) Hydrochloric acid +  Iron    →   Iron(III) chloride solution + Hydrogen gas
dil. 6HCl(aq)      +  2Fe(s)  →         2FeCl3(aq)                 +   3H2(g)

#### Question 14:

Complete and balance the following chemical equations:
(a)
(b)
(c)
(d)
(e)

(a) Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

(b) Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

(c) NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)

(d) NaOH(aq) + HCl(aq)→ NaCl(aq) + H2O(l)

(e) CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)

#### Question 15:

Fill in the blanks in the following sentences:
(a) Acids have a ..........................taste and they turn..........................litmus to..........................
(b) Substances do not show their acidic properties without.......................... .
(c) Acids produce.......................... ions on dissolving in water.
(d) Those substances whose smell (or odour) changes in acidic or basic solution are called .......................... indicators.
(e) Onion and vanilla extract are..........................indicators.

(a) Acids have a sour taste and they turn blue litmus to red.
(b) Substances do not show their acidic properties without water.
(c) Acids produce hydronium ions on dissolving in water.
(d) Those substances whose smell (or odour) changes in acidic or basic solution are called olfactory indicators.
(e) Onion and vanilla extract are olfactory indicators.

#### Question 16:

(a) What is an indicator? Name three common indicators.
(b) Name the acid-base indicator extracted from lichen.
(c) What colour does the turmeric paper turn when put in an alkaline solution?

(a) Indicators are substances that show a distinct change (colour or odour) simultaneous to a change in concentration of ions such as H+ and OH- ions.
Examples: Methyl orange, phenolphthalein and litmus solution.

(b) A purple dye is extracted from a plant called lichen. This dye is called a litmus solution and is commonly used as an indicator.

(c) When turmeric paper is put into an alkaline solution, it turns red.

#### Question 17:

What is an olfactory indicator? Name two olfactory indicators. What is the effect of adding sodium hydroxide solution to these olfactory indicators?

Olfactory indicators are those indicators that change their odour when added to an acidic or a basic solution. Onion and vanilla are examples of olfactory indicators.

The characteristic smell of an olfactory indicator is lost when it is added to a sodium hydroxide solution.

#### Question 18:

(a) What happens when an acid reacts with a metal? Give chemical equation of the reaction involved.
(b) Which gas is usually liberated when an acid reacts with a metal? How will you test for the presence of this gas?

(a) When a metal is treated with an acid, it liberates hydrogen gas from the acid and combines with the remaining part of the acid to form a compound called a salt.

Chemical equation:
Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

(b) Hydrogen gas is usually liberated when an acid reacts with an active metal.
For testing the presence of hydrogen gas, bring a burning candle near the test tube in which reaction has taken place. If the gas burns with a popping sound, the gas is hydrogen.

#### Question 19:

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

While diluting an acid, the acid should always be added slowly to water with constant stirring and in small amounts. The dilution of an acid is an exothermic reaction. So, when we add acid to water, the evolved heat gets absorbed by the water that is in bulk. But when we add water to acid, a large amount of heat is evolved, the water turns into vapour and the acid splashes onto our clothes or face, causing severe burns.

#### Question 20:

What happens when an acid reacts with a metal hydrogencarbonate? Write equation of the reaction which takes place.

When an acid is added to a metal hydrogen carbonate, carbon dioxide gas is liberated and metal salt and water are formed.

This reaction can be represented as follows.
Acid + Metal hydrogen carbonate → Salt + Water + Carbon dioxide gas

Example: When sodium hydrogen carbonate is treated with hydrochloric acid, carbon dioxide gas is liberated and sodium chloride is formed.

The reaction that takes place can be written as follows:
NaHCO3(s)  +  HCl(aq) →  NaCl(aq)  +  H2O(l) + CO2(g)

#### Question 21:

(a) What happens when dilute hydrochloric acid is added to sodium carbonate? Write a balanced chemical equation of the reaction involved.
(b) Which gas is liberated when dilute hydrochloric acid reacts with sodium carbonate? How will you test for the presence of this gas?

(a) When sodium carbonate is treated with dilute hydrochloric acid, sodium chloride is formed and carbon dioxide gas is liberated.

Na2CO3(s) + 2HCl(aq) →  2NaCl(aq) + CO2(g) + H2O(l)

(b) Carbon dioxide gas is liberated when dilute hydrochloric acid reacts with sodium carbonate.
When the gas, liberated during the reaction, is passed through lime water, the lime water turns milky or a white precipitate of calcium carbonate is formed. This confirms that the gas is none other than carbon dioxide.

#### Question 22:

What happens when an acid reacts with a base? Explain by taking the example of hydrochloric acid and sodium hydroxide. Give equation of the chemical reaction which takes place. What is the special name of such a reaction?

When an acid reacts with a base, it nullifies the effect of the base. Such a reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.

The reaction can be represented as given below.
Acid + Base → Salt + Water

For example, when sodium hydroxide is treated with hydrochloric acid, the following reaction takes place,
NaOH(aq) + HCl(aq) → H2O(aq) + NaCl(aq)

This kind of reaction is called a 'neutralisation reaction'.

#### Question 23:

What happens when an acid reacts with a metal oxide? Explain with the help of an example. Write a balanced equation for the reaction involved.

When an acid is treated with a metal oxide, a salt and water are obtained.
The general reaction can be written as follows:

Acid + Metal oxide → Salt + Water

For example, when a small amount of copper oxide is added to sulphuric acid,  colour of the solution becomes blue and the copper oxide dissolves. The blue colour of the solution indicates the formation of copper (II) sulphate.

The reaction taking place can be written as follows:
CuO(s) +  H2SO4(aq) → CuSO4(aq) + H2O(l)

#### Question 24:

(a) What are organic acids and mineral acids?
(b) Give two examples each of organic acids and mineral acids.
(c) State some of the uses of mineral acids in industry.

(a) Organic acids: An organic acid is an acid that is found naturally in plants and animals. Generally, organic acids are weak acids and do not dissociate completely in water.

Mineral acids: Mineral acids are man-made and are derived from one or more inorganic compounds. They are also known as inorganic acids. They range from acids of great strength to those that are very weak.

(b) Here are a few common examples.

Organic acids

•  Lactic acid
•  Citric acid
Mineral acids
• Hydrochloric acid (HCl)
• Nitric acid (HNO3)

(c) Uses of mineral acids in industry:
• Mineral acids are used in many sectors of the chemical industry for the synthesis of various chemicals.
• Dilute solutions of hydrochloric acid are used for removing the deposits from inside boilers, with precautions to prevent the corrosion of the boilers by the acid.
• They are also used in the processing of leather, purification of common salt, construction of buildings, etc.

#### Question 25:

What is meant by strong acids and weak acids? Classify the following into strong acids and weak acids:

Strong acids are those that completely dissociate into their ions when mixed with water, whereas weak acids are those that dissociate only partially into their ions when added to water.

Strong acids – HCl, H2SO4, HNO3
Weak acids – CH3COOH, H2CO3, H2SO3

#### Question 26:

Why do HCl, H2SO4, HNO3, etc., show acid character in aqueous solutions while solutions of compounds like C6H12O6 (glucose) and C2H5OH (alcohol) do not show acidic character?

HCl, H2SO4, HNO3, etc., dissociate in aqueous solutions to give H+(aq) ions, which are a vital part of an acid and determine the acidic property of acids.
C6H12O6 (glucose) and C2H5OH (alcohol) do not dissociate in aqueous solutions even though they contain hydrogen atoms.
Hence, HCl, H2SO4, HNO3, etc., show acidic character in aqueous solutions, whereas solutions of compounds such as C6H12O6 (glucose) and C2H5OH (alcohol) do not show acidic character.

#### Question 27:

What is a neutralisation reaction? Explain with an example. Give the chemical equation of the reaction which takes place.

When an acid reacts with a base, the acid nullifies the effect of the base, and conversely, the effect of an acid is nullified by a base. Such a type of reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.

The reaction can be represented as follows:
Acid + Base → Salt + Water

For example, when sodium hydroxide is treated with hydrochloric acid, sodium hydroxide and hydrochloric acid neutralise each other to form common salt (NaCl) and water. The reaction can be given as below:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)

#### Question 28:

Why should curd and other sour foodstuffs (like lemon juice, etc.) not be kept in metal containers (such as copper and brass vessels)?

Curd and other foodstuffs (such as lemon juice) are acidic in nature. Acids react with metals to give hydrogen gas. Hence, when these acidic foodstuffs are kept in metal containers, hydrogen gas is liberated, and it spoils the food and makes it inedible.

#### Question 29:

(a) What is produced if an acid is added to a base?
(b) Why does dry HCl gas not change the colour of dry litmus paper?
(c) What colour does phenolphthalein indicator turn when added to an alkali (such as sodium hydroxide)?

(a) When an acid is added to a base, the acid nullifies the effect of the base, and conversely, the effect of an acid is nullified by a base to give a salt and water. Such a type of reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
The reaction can be represented as follows.

Acid + Base → Salt + Water

For example, when sodium hydroxide is treated with hydrochloric acid, the reaction can be written as follows.

NaOH(aq) + HCl(aq) →  NaCl(aq) +  H2O(l)

(b) Dry HCl gas does not undergo dissociation to form ions, because of the absence of an aqueous medium, whereas the colour of litmus paper changes only in the presence of ions. Hence, dry HCl gas does not change the colour of dry litmus paper.

(c) A phenolphthalein indicator turns pink when added to an alkali (such as sodium hydroxide).

#### Question 30:

(a) Why do acids not show acidic behaviour in the absence of water?
(b) Why does an aqueous solution of an acid conduct electricity?
(c) Why does distilled water not conduct electricity whereas rain water does?

(a) The acidic behaviour of a substance is due to the presence of H+ ions. As acids dissociate into their ions only in the presence of water, the acidic character of acids cannot be seen in the absence of water.

(b) Acids dissociate to release H+ ions and an anion, when dissolved in water. When electric current is passed through an aqueous solution, the ions start moving toward oppositely charged terminals of the battery, henceforth, conducting electricity.

(c) Distilled water is the purest form of water. The process of distillation makes the water free from all the ionic species, whereas rainwater consists of plenty of  impurities. Thus, it contains numerous ions such as H+, CO3-, SO42- etc. Therefore, rainwater conducts electricity.

#### Question 31:

(a) What happens when an acid reacts with a metal carbonate? Explain with the help of an example. Write chemical equation of the reaction involved.
(b) What happens when carbon dioxide gas is passed through lime water:
(i) for a short time?
(ii) for a considerable time?
Write equations of the reactions involved.

(a) Metal carbonates react with acids to give a corresponding salt, carbon dioxide and water.
The reaction can be represented as follows.
Metal carbonate + Acid → Salt + Carbon dioxide + Water.
For example, when sodium carbonate reacts with dilute hydrochloric acid,  sodium chloride, water and carbon dioxide gas are produced. The reaction occurs as follows.

Na2CO3(s) + 2HCl(aq)  →  2NaCl(aq) + H2O(l) + CO2(g)

(b) When carbon dioxide gas is passed through lime water, the lime water turns milky. This confirms that the gas liberated in the reaction is none other than carbon dioxide.

(i) The reaction that occurs when carbon dioxide is passed through lime water for a short time can be represented as follows:

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

When carbon dioxide gas is passed through lime water, it turns the lime water milky. This happens because of the formation of white precipitate of calcium carbonate.

(ii) When carbon dioxide is passed through lime water for a considerable time (in excess), the following reaction occurs with the formation of calcium bicarbonate:

CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)

When carbon dioxide gas is passed through lime water for a considerable time, a white precipitate of calcium carbonate is dissolved. This happens because of the formation of calcium hydrogen carbonate, which is soluble in water.

#### Question 32:

With the help of labelled diagrams, describe an activity to show that acids produce ions only in aqueous solutions.

Procedure:

• Take about 1 g of sodium chloride (NaCl) in a clean and dry test tube.
• Add some concentrated sulphuric acid (H2SO4).
• A gas is produced and comes out through the delivery tube.
• Test the gas evolved with dry and wet strips of litmus paper.

Observation:
(i) When the gas evolved is passed through dry litmus paper, there is no change in the colour of litmus paper.
(ii) However, when the gas is passed through wet blue litmus paper, the paper turns red, which indicates that the gas evolved is an acid, HCl.

Conclusion:
This activity concludes that hydrogen ions in HCl are produced in the presence of water because only wet blue litmus turns red.

#### Question 33:

(a) Which element is common to all acids?
(b) Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

(a) All acids have hydrogen as a common element in them.
Examples:  Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3).

(b) Activity to show that alcohol and glucose also contain hydrogen but are not acids:
Procedure:
i) Take samples of alcohol, glucose and hydrochloric acid.
ii) Take a 100 ml beaker. Take a cork and fix two nails on it.
iii) Place the cork inside the beaker as shown in the figure.
iv) Connect the nails to two terminals of a 6-volt battery through a bulb and a switch.
v) Now pour some dilute hydrochloric acid in the beaker so that the nails and the cork are immersed in it.
vi) Switch on the current flow.
vii) Repeat the same with alcohol and glucose.

Observation :

The bulb starts glowing in the case of acid, whereas it does not glow in the case of glucose and alcohol solutions. Glowing of the bulb signifies that there is a flow of electric current through the solution. The electric current is carried through the solution by ions.

Conclusion:
Acids dissociate in aqueous solutions to give H+(aq) ions, which determine their acidic property. Glucose and alcohol do not dissociate and do not furnish H+ ions in aqueous solutions even though they contain hydrogen atoms. Hence, HCl shows acidic character in aqueous solutions, whereas solutions of compounds such as C6H12O6 (glucose) and C2H5OH (alcohol) do not show acidic character.

#### Question 34:

10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be:
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL

(d) 16 mL

It is given that 10 mL of a solution of NaOH is completely neutralised by 8 mL of a solution of HCl. Hence, 20 mL of  the solution of NaOH, which is double the amount taken earlier, will require double the solution of HCl taken earlier, i.e., 16 mL, to be completely neutralised.

#### Question 35:

Which of the following types of medicine is used for treating indigestion caused by over-eating?
(a) antibiotic
(b) analgesic
(c) antacid
(d) antiseptic

(c) antacid

Indigestion is caused by the formation of excess acid in the stomach. An antacid is a medicine that is generally a base or a basic salt that neutralises acidity in the stomach.

#### Question 36:

A solution reacts with marble chips to produce a gas which turns lime water milky. The solution contains:
(a) Na2SO4
(b) CaSO4
(c) H2SO4
(d) K2SO4

(c) H2SO4

Marble chips contain calcium carbonate (CaCO3), which reacts with sulphuric acid (H2SO4) to form carbon dioxide gas (CO2). Carbon dioxide gas, when passed through lime water [Ca(OH)2], turns it milky.

#### Question 37:

One of the following is not an organic acid. This is:
(a) ethanoic acid
(b) formic acid
(c) citric acid
(d) carbonic acid

(d) carbonic acid

Carbonic acid H2CO3 is formed when  CO2 is dissolved in water. Hence, carbonic acid is an inorganic or mineral acid. Ethanoic acid,  formic acid and citric acid are found naturally.

#### Question 38:

The property which is not shown by acids is:
(a) they have sour taste
(b) they feel soapy
(c) they turn litmus red
(d) their pH is less than seven

(b) they feel soapy

Acids have a sour taste, they turn blue litmus paper red and their pH is less than seven. But they do not feel soapy.

#### Question 39:

The indicators which turn red in acid solution are:
(a) turmeric and litmus
(b) phenolphthalein and methyl orange
(c) litmus and methyl orange
(d) phenolphthalein and litmus

(c) litmus and methyl orange

Litmus and methyl orange turn red in an acid solution.

#### Question 40:

The discomfort caused by indigestion due to overeating can be cured by taking:
(a) vinegar
(b) lemon juice
(c) baking soda
(d) caustic soda

(c) baking soda

Baking soda, being a base, neutralises the excess acid formed in the stomach and cures the discomfort caused by indigestion.

#### Question 41:

The property which is common between vinegar and curd is that they:
(a) have sweet taste
(b) have bitter taste
(c) are tasteless
(d) have sour taste

(d) have sour taste

Vinegar and curd are acidic. They both have a sour taste.

#### Question 42:

The indicator which produces a pink colour in an alkaline solution is:
(a) methyl orange
(b) turmeric paper
(c) phenolphthalein
(d) litmus paper

(c) phenolphthalein

Phenolphthalein produces a pink colour in an alkali solution.

#### Question 43:

A solution reacts with zinc granules to give a gas which burns with a 'pop' sound. The solution contains:
(a) Mg(OH)2
(b) Na2CO3
(c) NaCl
(d) HCl

(d) HCl

Zinc granules react with HCl to give hydrogen gas, which burns with a ‘pop’ sound.

#### Question 44:

When a piece of limestone reacts with dilute HCl, a gas X is produced. When gas X is passed through lime water then a white precipitate Y is formed. On passing excess of gas X, the white precipitate dissolves forming a soluble compound Z.
(a) What are X, Y and Z?
(b) Write equations for the reactions which take place:
(i) when limestone reacts with dilute HCl
(ii) when gas X reacts with lime water to form white precipitate Y
(iii) when excess of gas X dissolves white precipitate Y to form a soluble compound Z

(a) X is carbon dioxide gas (CO2).
Y is calcium carbonate (CaCO3).
Z is calcium hydrogen carbonate (calcium bicarbonate) [Ca(HCO3)2].

(b)
(i) When a piece of limestone or calcium carbonate is treated with dilute hydrochloric acid, carbon dioxide gas (CO2 , here given as X) is produced.
CaCO3(s) + 2HCl(aq) → 2NaCl(aq) +  H2O(l) + CO2(g)

(ii) When carbon dioxide (X), liberated in the reaction is passed through lime water, the lime water turns milky or a white precipitate of calcium carbonate (CaCO3 , Here given as Y) is formed.

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

(iii) When excess of carbon dioxide is passed through lime water, a solution of calcium hydrogen carbonate [Z, Ca(HCO3)2] is formed.

CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)

#### Question 45:

If someone is suffering from the problem of acidity after overeating, which of the following would you suggest as remedy?
Lemon juice, Vinegar, Banking soda solution

Baking soda can be suggested as a remedy to the person suffering from acidity. Acidity is caused by the production hydrochloric acid in the stomach, and excess of acid causes pain and burning. Baking soda (NaHCO3) is basic in nature and it neutralizes the HCl produced in the stomach, bringing relief.

NaHCO3 (s)+ HCl (aq) → NaCl (aq) + CO2 (g)+ H2O(l)

#### Question 46:

On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green.
(a) Predict the new compound formed which imparts a blue-green colour to solution.
(b) Write a balanced chemical equation of the reaction which takes place.
(c) On the basis of the above reaction, what can you say about the nature of copper oxide?

(a) When cupric oxide is treated with dilute hydrochloric acid, copper(II)chloride (blue-green salt solution) and water are obtained.

(b) CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)

(c) The reaction between a metal oxide and hydrochloric acid is similar to the neutralisation reaction in which, after the reaction, a salt and water are formed. So, we can conclude that copper oxide is basic in nature.

#### Question 47:

A white shirt has a yellow stain of curry. When soap is rubbed on this shirt during washing, the yellow stain turns reddish-brown. On rinsing the shirt with plenty of water, the reddish-brown stain turns yellow again.
(a) Name the natural indicator present in curry stain.
(b) Explain the changes in colour of this indicator which take place during washing and rinsing the shirt.
(c) What is the nature of soap (acidic/basic) as shown by the indicator present in curry stain?

(a) The curry contains turmeric which works as a natural indicator.

(b) When the shirt with a yellow stain of curry is washed with soap, the stain turns reddish brown because soap is basic and turmeric gives a reddish-brown colour when reacts with a base. When rinsed with plenty of water, the reddish-brown stain turns yellow again due to removal soap.

(c) Soap is basic in nature because it turns the yellow turmeric stain reddish brown.

#### Question 48:

You have been provided with three test-tubes. One  of these test-tubes contains distilled water and the other two contain an acidic and a basic solution respectively. If you are given only blue litmus paper, how will you identify the contents of each test-tube?

We have been provided with three test tubes, containing distilled water, an acidic solution and a basic solution. The test tube containing distilled water will not bring about any change in the colour of the blue litmus paper. Now, when you dip the same litmus paper in the test tube containing the acidic solution, it will turn the blue colour of the litmus paper to red. The basic solution in the third test tube will have no effect on blue litmus paper but will revert back the colour of litmus paper to blue which was earlier turned red by the acidic solution.

#### Question 49:

A substance X which is used as an antacid reacts with dilute hydrochloric acid to produce a gas Y which is used in one type of fire-extinguisher. Name the substance X and gas Y. Write a balanced equation for the chemical reaction which takes place.

Sodium hydrogen carbonate (baking soda) is used as an antacid because it neutralises excess acid present in the stomach. It reacts with dilute hydrochloric acid to produce carbon dioxide gas, which is used in fire extinguishers.

Hence, substance X is sodium hydrogen carbonate (baking soda) and Y is carbon dioxide gas.

The reaction between sodium hydrogen carbonate and hydrochloric acid gives sodium chloride, carbon dioxide and water.

HCl(aq) + NaHCO3(s) → NaCl(s) + H2O(l) + CO2(g)

#### Question 50:

How is the neutralisation of a carbonate with an acid different from the neutralisation of an oxide or a hydroxide?

Metal carbonates react with acids to give a corresponding salt, carbon dioxide and water.

The reaction can be represented as follows:
Metal carbonate + Acid → Salt + Carbon dioxide + Water

Example:
Na2CO3(s) +  2HCl(aq) → 2NaCl(aq) + H2O(l)  + CO2(g)

However, the neutralisation of an oxide or a hydroxide with an acid gives only a salt and water.

Metal oxide + Acid → Salt + Water

Example:
Na2O + 2HCl  → 2NaCl + H2O

Metal hydroxide + Acid → Salt + Water
Example:
NaOH + HCl →  NaCl + H2O

#### Question 51:

What happens to (a) the H+ ions, and (b) temperature of the solution, when an acid is neutralised?

(a) When a base is added to an acid, it nullifies the effect of the acid and removes the H+ ions from it and turns them into water. During a neutralisation reaction, the H+ ions of an acid are converted into water.

The reaction can be represented as follows.
Acid + Base → Salt + Water
For example, when sodium hydroxide is treated with hydrochloric acid, sodium chloride and water are produced.

NaOH(aq) + HCl(aq) →  NaCl(aq) + H2O(l)

(b) The temperature of the solution increases, because a neutralisation reaction is an exothermic reaction in which heat is produced.

#### Question 1:

Name the gas evolved when zinc granules are treated/heated with:
(a) hydrochloric acid solution
(b) sodium hydroxide solution

(a) When zinc granules are treated with hydrochloric acid, zinc chloride is formed and hydrogen gas is liberated.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

(b) Zinc reacts with sodium hydroxide, giving sodium zincate and hydrogen gas.
Zn(s) + 2NaOH(aq)→ Na2ZnO2(aq) + H2(g)

#### Question 2:

What is the common name of water soluble bases?

Water-soluble bases are commonly known as alkalies. An alkali is a unique base that dissolves in water without any chemical reaction. All alkalies are bases, whereas only some bases are alkalies.

#### Question 3:

What is common in all the water soluble bases (or alkalis)?

An alkali is a unique base that is soluble in water and forms hydroxide ions (OH-) when dissolved in water.

#### Question 4:

Why does tooth decay start when the pH of mouth is lower than 5.5?

Tooth enamel is made up of calcium phosphate and is the hardest part of the body. When the pH in the mouth goes down and becomes 5.5, it is strong enough to corrode the enamel of the tooth. The mouth turns acidic because the bacteria present in it produce acids by breaking down sugar and food particles left behind in it after eating.

#### Question 5:

What is the pH of a neutral solution?

A neutral solution has a pH value of 7. It is free of ions such as H+ and OH-.

#### Question 6:

Which is more acidic : a solution of pH = 2 or a solution of pH = 6?

A solution with a pH of 2 is more acidic than a solution with a pH of 6, because an acidic solution has a pH value of less than 7. As the pH value decreases from 7, the acidity of a solution increases because there is an increase in the concentration of hydrogen ions (H+ ions).

#### Question 7:

Which is more basic (or more alkaline) : a solution of pH = 8 or a solution of pH = 11?

A solution with a pH of 11 is more basic than a solution with a pH of 8. This is because a pH value greater than 7 indicates a basic solution. As the pH value increases from 7 to 14, there is an increase in the concentration of OH-, which increases the strength of the solution.

#### Question 8:

Name the scientist who developed the pH scale.

The scientist, Soren Sorenson developed the pH scale.

#### Question 9:

Name the indicator which can give us an idea of how strong or weak an acid or base is.

A universal indicator shows different colours at different concentrations of hydrogen ions in a solution and tells us how strong or weak an acid or a base is.

#### Question 10:

The pH of soil A is 7.5 while that of soil B is 4.5. Which of the two soils, A or B, should be treated with powdered chalk to adjust its pH and why?

Soil B, with a pH of 4.5, is very acidic and is thus not suitable for the normal growth of plants. Therefore, to reduce its acidity, soil B should be treated with powdered chalk (CaCO3), which is a base.

#### Question 11:

What is the name of the indicator which can be used for testing the pH of a solution?

A universal indicator shows different colours at different concentrations of hydrogen ions in a solution and can be used for testing the pH of a solution.

#### Question 12:

What colour will universal indicator show if you add it to the following substances?
(a) potassium hydroxide, pH = 12
(b) soda water, pH = 5
(c) sulphuric acid, pH = 2

 SUBSTANCE COLOUR CHANGE Potassium hydroxide Dark purple Soda water Orange yellow Sulphuric acid Red

#### Question 13:

A beaker of concentrated hydrochloric acid has a pH of 1. What colour will full range universal indicator turn if it is added to this beaker? Is it a strong or a weak acid?

HCl, with a pH of 1, imparts a red colour when treated with a universal indicator. As the pH value decreases from 7, the acidity increases, as there is an increase in the concentration of hydrogen ions (H+). So, the given acid is very strong.

#### Question 14:

Two solutions X and Y are tested with universal indicator. Solution X turns orange whereas solution Y turns red. Which of the solutions is a stronger acid?

Solution Y is stronger because the red colour indicates a pH value of 1, whereas the orange colour indicates a pH value higher than 1 (around 4) .

#### Question 15:

Two solutions A and B have pH values of 3.0 and 9.5 respectively. Which of these will turn litmus solution from blue to red and which will turn phenolphthalein from colourless to pink?

Solution A, being acidic, with a pH value of 3.0, will turn litmus solution from blue to red, and solution B, being basic, with a pH value of 9.5, will turn phenolphthalein from colourless to pink.

#### Question 16:

Two drinks P and Q gave acidic and alkaline reactions, respectively. One has a pH value of 9 and the other has a pH value of 3. Which drink has the pH value of 9?

Drink Q will have a pH value of 9, because basic solutions have a pH greater than 7 on a pH scale.

#### Question 17:

Two solutions X and Y have pH = 4 and pH = 8, respectively. Which solution will give alkaline reaction and which one acidic?

As per the given data, solution X has a pH value of 4 (which is less than 7) and will give an acidic reaction. Solution Y has a pH value of 8 (which is greater than 7) and will give an alkaline reaction.

#### Question 18:

Fill in the following blanks with suitable words:
(a) Acids have a pH...........than 7.
(b) Alkalis have a pH.............. than 7.
(c) Neutral substances have a pH of ................
(d) The more acidic a solution, the ................. the pH.
(e) The more alkaline a solution, the............... the pH.

(a) Acids have a pH less than 7.
(b) Alkalis have a pH greater than 7.
(c) Neutral substances have a pH of 7.
(d) The more acidic a solution, the lesser the pH.
(e) The more alkaline a solution, the greater the pH.

#### Question 19:

Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increase or decrease? Why?

Usually, fresh milk has a pH of 6. When fresh milk changes into curd, its pH value decreases because of an increase in the acidity of the milk. Bacteria usually change fresh milk to curd by producing lactic acid. Fresh milk that has changed into curd is sour, because its acidity has increased. Hence, the pH value of fresh milk becomes less than 6 when it changes into curd.

#### Question 20:

(a) What is a universal indicator? For what purpose is it used?
(b) How does a universal indicator work?
(c) Water is a neutral substance. What colour will you get when you add a few drops of universal indicator to a test-tube containing water?

(a) A universal indicator is a combination of different dyes that show colour change with solutions of different pH values on the complete pH scale.
A universal indicator paper is used to differentiate between acids and bases. It also differentiates between strong acids and bases and between weak acids and bases on the basis of the intensity of colour change.

(b) When one or two drops of an acid or a base solution are added to a universal indicator paper, it generates a new colour. By comparing the colour with the pH scale, the pH of the added solution is known. From the pH value, we can identify the strength of the acid or base solution.

(c) A neutral substance has a pH value of 7. When we add a few drops of a universal indicator to a test tube of water (which is a neutral substance), we get a green colour, indicating the neutrality of water.

#### Question 21:

Which chemical is injected into the skin of a person:
(a) during an an's sting?
(b) during the nettle leaf hair sting?
How can the effect of these stings be neutralised?

(a) Methanoic acid is injected into the skin of a person during an ant sting.
(b) Methanoic acid is injected into the skin during a nettle-leaf-hair sting.

Both an ant sting and a nettle-leaf-hair sting are acidic, as methanoic acid is injected into the body, which causes severe pain and an itching sensation. Applying a baking soda solution (a base) on the skin soothes the pain by neutralising the acid. Thus, the effect of these stings is neutralised.

#### Question 22:

(a) Explain the pH change as the cause of tooth decay. How can tooth decay caused by pH change be prevented?
(b) Explain how pH change in the lake water can endanger the lives of aquatic animals (like fish). What can be done to lessen the danger to the lives of aquatic animals in the lake?

(a) When the pH of the mouth falls to around 5.5, tooth decay begins. As the acidity in the mouth increases, the acid reacts with the tooth enamel and causes tooth decay.
Tooth decay can be prevented by brushing with toothpastes (which are basic and neutralise the acid formed) regularly and by cleaning the mouth carefully after meals.

(b) Because of acid rain (when the pH of rainwater is 5.6), the acidity of river and lake water increases. Aquatic animals find it very difficult to survive in water with low pH levels, and they die.
To neutralise this excess acidity in lakes and rivers caused by acid rain, calcium carbonate, a base, is frequently mixed with the water.

#### Question 23:

(a) What happens during a bee sting? What is its remedy?
(b) What happens during a wasp sting? What is its remedy?

(a) When a bee stings a person, it injects an acidic solution into the person’s skin. This causes severe pain and irritation. The remedy is to apply a baking soda solution (sodium hydrogen carbonate solution, which is a base). The solution soothes the pain by neutralising the acid.

(b) When a wasp stings a person, it inserts an alkaline solution into the person's skin. This causes severe pain and irritation. The remedy is to apply a weak acid, such as vinegar, to the wound. The acid neutralises the alkaline solution and soothes the pain.

#### Question 24:

(a) Why is it wrong to treat a bee sting with vinegar?
(b) Why is it wrong to treat a wasp sting with baking soda solution?

(a) When a bee stings a person, it injects an acidic solution into the person’s skin, which causes severe pain and irritation. This acid has to be neutralised by a base and not an acid. If we apply vinegar, which is a weak acid, the pain worsens. Hence, it is wrong to treat a bee sting with vinegar.

(b) When a wasp stings a person, it inserts an alkaline solution into the person's skin, which causes severe pain and irritation. The alkali has to be neutralised by an acid and not a base. If we apply a baking soda solution, which is a weak base, it worsens the pain. Hence, it is wrong to treat a wasp sting with a baking soda solution.

#### Question 25:

(a) What does the pH of a solution signify? Three solution A, B and C have pH values of 6, 4 and 10 respectively. Which of the solutions is highly acidic?
(b) A farmer has found that the pH of soil in his fields is 4.2. Name any two chemical materials which he can mix with the soil to adjust its pH.

(a) The pH of a solution signifies whether it is an acid or a base and also its strength. Solutions with a pH greater than 7 are considered basic or alkaline, solutions with a pH less than 7 are acidic, and solutions with a pH of 7 are considered neutral.
Solution B with the lower pH value of 4 is highly acidic.

(b) If the pH of the soil is 4.2, then it is highly acidic. Hence, the soil should be treated with chemicals (bases) such as quick lime (calcium oxide) or slaked lime (calcium hydroxide).

#### Question 26:

(a) The pH values of six solutions A to F are given below:

Which of the above solutions are (i) acids (ii) alkalis?
(b) Name the acids or alkalis used to make (i) car batteries (ii) explosives (iii) soaps (iv) fertilisers.

(a) (i) Acidic solutions: A with pH = 0, D with pH = 3, C with pH = 6
(ii) Basic solutions: F with pH = 8, B with pH = 11, E with pH = 13

(b) The acids or alkalis used to make the following products are given against them.
(i) Car batteries – Sulphuric acid
(ii) Explosives – Nitric acid (in explosives such as TNT, or trinitrotoluene)
(iii) Soaps – Sodium hydroxide
(iv) Fertilisers – Nitric acid, sulphuric acids

#### Question 27:

(a) The pH of a cold drink is 5. What will be its action on blue and red litmus solutions?
(b) The pH values of three acids A, B and C having equal molar concentrations are 5.0, 2.8 and 3.5 respectively. Arrange these acids in order of the increasing acid strengths.

a) A cold drink with a pH value of 5 is acidic. Therefore, blue litmus turns red and red litmus remains as it is.

b) Acids have a pH value of less than 7. Lower the pH value, the stronger the acid. Hence, the acids in the increasing order of their strengths are as follows: A < C < B.

#### Question 28:

Under what soil conditions do you think a farmer would treat the soil of his fields with quicklime (calcium oxide), or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Calcium oxide, calcium hydroxide and calcium carbonate are all bases (pH > 7), and hence, they must be used only in cases where the soil is too acidic (pH < 7). They help in neutralising the acidity of the soil and assist in bringing the pH closer to 7.

#### Question 29:

Which acid is produced in our stomach? What happens if there is an excess of acid in the stomach? How can its effect be cured?

Dilute hydrochloric acid is the acid produced in our stomach. Sometimes, excess acid is produced in the stomach for various reasons such as overeating. When this happens, the excess generation of acid in the stomach causes indigestion with pain and discomfort. This can be cured by the intake of antacids, which are weak bases. They neutralise the excess acid produced in the stomach, providing relief from indigestion and relieving the pain.

#### Question 30:

The soil in a field in highly acidic. Name two materials which can be added to this soil to reduce its acidity. Give the reason for your choice.

The soil can be treated with bases such as quick lime (calcium oxide) or slaked lime (calcium hydroxide) to reduce its acidity.
These bases react with the excess acid present in the soil and neutralise its acidity. This increases the pH of the soil, which makes it suitable for the growth of various crops.

#### Question 31:

What is meant by strong bases and weak bases? Classify the following into strong bases and weak bases:
NH4OH, Ca(OH)2, NaOH, KOH, Mg(OH)2

Strong bases ionise completely in water, producing huge amounts of hydroxide ions (OH ions). Generally, basic or alkali solutions of pH 8, 9 and 10 are considered weak bases or alkalis.
Weak bases ionise partially in water, producing less amounts of hydroxide ions (
OH ions).
Basic or alkali solutions of pH 11,12,13 and 14 are considered strong bases or alkalis.

Let us classify the given bases as strong or weak.

 Strong bases Weak bases Sodium hydroxide, NaOH Potassium hydroxide, KOH Ammonium hydroxide, NH4OH Calcium hydroxide, Ca(OH)2 Magnesium hydroxide, Mg(OH)2

#### Question 32:

What ions are present in the solutions of following substances? (write the symbols only)
(i) Hydrochloric acid
(ii) Nitric acid
(iii) Sulphuric acid
(iv) Sodium hydroxide
(v) Potassium hydroxide
(vi) Magnesium hydroxide

The ions present in the solutions of the following substances are given below.
(i) Hydrochloric acid, HCl = H+ and Cl- ions
(ii) Nitric acid,
HNO3 = H+ and NO3- ions
(iii) Sulphuric acid, H2SO4 2H+ and SO4- ions
(iv) Sodium hydroxide, NaOH = Na+ and OH- ions
(v) Potassium hydroxide, KOH = K+ and OH- ions
(vi) Magnesium hydroxide,  Mg(OH)2 = Mg2+and 2OH- ions

#### Question 33:

(a) What would you expect the pH of pure water to be?
(b) What colour would the universal indicator show in an aqueous solution of sugar? Why?
(c) A sample of rain water turned universal indicator paper yellow. What would you expect its pH to be? Is it a strong or a weak acid?

a. Pure water is a neutral substance with a pH of 7.

b. A universal indicator will show a green colour in a sugar solution (which is the colour for pH 7). An aqueous solution of sugar is also a neutral substance. Hence, its pH value is 7.

c. A universal indicator shows a greenish-yellow colour for pH 6 and orange yellow for pH 5. This means that if the indicator turns yellow, the pH should be between these two values. Hence, it must have a pH value between 5 and 6. As the pH is closer to 7 (between 5 and 6), it is a weak acid.

#### Question 34:

(a) What do you think will be the pH in the stomach of a person suffering from indigestion: less than 7 or more than 7?
(b) What do you think will be the pH of an antacid solution: less than 7 or more than 7?
(c) How does an antacid work?
(d) Name two common antacids.

(a) The pH in the stomach of a person suffering from indigestion would be less than 7, because of excess acid generation in the stomach.

(b) The pH of an antacid solution would  be greater than 7. Antacids are anti-acids; that is, they are bases.

(c) Our stomach produces dilute hydrochloric acid, which helps in the digestion of food. Sometimes, excess acid is produced in the stomach, overeating being one of the causes. This results in indigestion along with pain. For relief, antacids, which are weak bases and neutralise the excess acid, are taken.

(d) Two common antacids are
magnesium hydroxide (also called milk of magnesia) and sodium hydrogen carbonate (also called baking soda).

#### Question 35:

Separate the following into substances having pH values above and below 7. How do these influence litmus paper?
(i) Lemon juice
(ii) Solution of washing soda
(iii) Toothpaste
(iv) Vinegar
(v) Stomach juices

On the pH scale, bases have values greater than 7, whereas acids have values less than 7. A basic solution turns red litmus blue. An acidic solution turns blue litmus red. With this information, let us separate the substances given below into those with pH values above and below 7.

 Substances Acid/base pH value Litmus test result 1. Lemon juice Acid Less than 7 Turns blue litmus red 2. Solution of washing soda Base Greater than 7 Turns red litmus blue 3. Toothpaste Base Greater than 7 Turns red litmus blue 4. Vinegar Acid Less than 7 Turns blue litmus red 5. Stomach juices Acid Less than 7 Turn blue litmus red

#### Question 36:

(a) Do basic solutions also have H+ (aq) ions? If yes, then why are they basic?
(b) When a solution becomes more acidic, does the pH get higher or lower?

(a) Yes, basic solutions also have H+ (aq) ions.
Generally, bases generate hydroxide ions when they are dissolved in water. Basic solutions also have H+ (aq) ions, which are obtained from the ionisation of water. The amount of H+ ions in basic solutions is very less compared with the amount of OH- ions. Hence, they are basic in nature.

(b) When a solution becomes more acidic, the pH becomes lower.

#### Question 37:

(a) Define an acid and a base. Give two examples of each.
(b) Give the names and formulae of two strong bases and two weak bases.
(c) What type of ions are formed:
(i) when an acid is dissolved in water?
(ii) when a base (or alkali) is dissolved in water?
(d) Write the neutralisation reaction between acids and bases in terms of the ions involved.
(e) Write any two important uses of bases.

(a) Acids: Substances that ionise on dissolving in water producing hydrogen ions (H+ions) are called acids. Acids are sour in taste and they turn blue litmus to red.
Examples: Hydrochloric acid, HCl, and sulphuric acid, H2SO4.

Bases: Substances that ionise on dissolving in water producing hydroxide ions (OH- ions) are called bases. Bases are bitter in taste, slippery to touch and they turn red litmus to blue.
Examples: Magnesium hydroxide, Mg(OH)2, and sodium hydroxide, NaOH.

(b) Two strong and two weak bases are the following.

 Strong bases Formula Weak bases Formula 1. Sodium hydroxide NaOH 1. Magnesium hydroxide Mg(OH)2 2. Potassium hydroxide KOH 2. Calcium hydroxide Ca(OH)2

(c) The types of ions formed are explained below.

(i) When an acid is dissolved in water, it ionises to produce hydrogen ions (H+ ions).

(ii) When a base is dissolved in water, it ionises to produce hydroxide ions (OH- ions).

(d) Let us consider an acid-base neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) to give sodium chloride salt (NaCl) and water.

Equations involving ions:
Hydrochloric acid ionises in water as follows,
HCl(aq)  → H+(aq) + Cl- (aq)

Sodium hydroxide ionises in water as
NaOH(aq) → Na+(aq) + OH-(aq)

Let us now combine both the reactions above.
Na+(aq) + Cl-(aq) + H+(aq) + OH-(aq) → NaCl(aq)+ H2O(l)

(e) The two important uses of bases are as follows.
1. Sodium hydroxide is widely used in the manufacture of soaps and detergents.
2. Magnesium hydroxide is used as an antacid, which gives relief from excess acidity in the stomach.

#### Question 38:

(a) What happens when zinc granules are heated with sodium hydroxide solution? Write equation of the reaction which takes place.
(b) What happens when bases react with non-metal oxides? Explain with the help of an example. What does this reaction tell us about the nature of non-metal oxides?

(a) When zinc granules are heated with sodium hydroxide (base), sodium zincate salt is formed with the liberation of hydrogen gas.

(b) Bases react with non-metal oxides to form a salt and water.
Bases + Non-metal oxides  Salt + Water

Example:

Here, sodium hydroxide (base) reacts with carbon dioxide (non-metal oxide) to give sodium carbonate (salt) and water.

This reaction shows that non-metal oxides are acidic in nature.

#### Question 39:

(a) What effect does the concentration of H+ (aq) ions have on the nature of a solution?
(b) What effect does the concentration of OH ions have on the nature of a solution?
(c) Someone put some universal indicator paper into vinegar. The pH is 3. What does this tell you about the vinegar?
(d) Someone put some universal indicator paper onto wet soap. The pH is 8. What does this tell you about the soap?
(e) State whether a solution is acidic, alkaline or neutral if its pH is:
(i) 9
(ii) 4
(iii) 7
(iv) 1
(v) 10
(vi) 3

(a) Acids furnish hydrogen ions (H+ ions) when dissolved in water. The more will be the concentration of H+ ions in  a solution, the lower will be its pH. Thus, an increase in the concentration of H+ ions in a solution leads to increase in the acidity of the solution.

(b) Bases furnish hydroxide ions (OH- ions) when dissolved in water. The concentration of hydroxide ions increases as bases dissolve in water. With the increase in the concentration on OH- ions, the pH also increases, thus, strengthening the basic nature of the solution.

(c) Universal indicator paper dipped into vinegar shows pH 3, indicating that vinegar is an acid. Acids show a value below 7 on the pH scale.

(d) Universal indicator paper put onto wet soap shows pH 8, indicating that wet soap is a base. Bases show a value above 7 on the pH scale.

(e) We know that solutions that have pH values less than 7 (from 0) are acidic, those with pH greater than 7 (up to 14) are basic (or alkaline) and those with pH 7 are neutral. The lower the pH, the higher the acidity, and the higher the pH, the higher the basicity. Let us classify solutions with the given pH values as acidic, alkaline or neutral.

 The pH of the solution Nature of the solution (i) 9 Basic (ii) 4 Acidic (iii) 7 Neutral (iv) 1 Acidic (v) 10 Basic (vi) 3 Acidic

#### Question 40:

One of the following is a medicine for indigestion. This is:
(a) sodium hydroxide
(b) manganese hydroxide
(c) magnesium hydroxide
(d) potassium hydroxide

(c) Magnesium hydroxide

Magnesium hydroxide, which is a base, acts as an antacid. It neutralises excess acid present in the stomach and cures indigestion. Hence, it is used as a medicine.

#### Question 41:

Bee sting contains:
(a) an acidic liquid
(b) a salt solution
(c) an alkaline liquid
(d) an alcohol

(a) an acidic solution

A bee sting contains an acidic solution. The name of the acid is methanoic acid.

#### Question 42:

Wasp sting contains:
(a) a sugar solution
(b) an acidic liquid
(c) a salt solution
(d) an alkaline liquid

(d) an alkaline liquid

A wasp sting injects an alkaline solution into the skin, which causes high irritation and pain.

#### Question 43:

One of the following does not inject an acidic liquid into the skin through its sting. This is:
(a) honey bee
(b) ant
(c) wasp
(d) nettle leaf hair

(c) wasp

A honey bee, ant and a nettle leaf hair inject methanoic acid into the skin through their stings. But a wasp injects an alkaline solution into the skin through its sting.

#### Question 44:

A solution turns red litmus blue. Its pH is likely to be:
(a) 1
(b) 4
(c) 5
(d) 10

(d) 10

A basic solution turns red litmus blue. On the pH scale, bases have values greater than 7, whereas acids have values less than 7. Therefore, the pH of the given solution is likely to be 10.

#### Question 45:

A solution turns blue litmus red. Its pH is likely to be:
(a) 7
(b) 5
(c) 8
(d) 14

(b) 5

An acidic solution turns blue litmus red. On the pH scale, acids have values less than 7, whereas bases have values greater than 7. Solutions with pH equal to 7 are neutral in nature. Therefore, the pH of the given solution is likely to be 5.

#### Question 46:

A solution turns phenolphthalein indicator pink. The most likely pH of this solution will be:
(a) 6
(b) 4
(c) 9
(d) 7

(c) 9

Every substance with a pH greater than 7 is considered basic or alkaline. Basic or alkaline solutions turn phenolphthalein indicator pink. Thus, the pH of the solution is most likely 9.

#### Question 47:

The colour of methyl orange indicator in a solution is yellow. The pH of this solution is likely to be:
(a) 7
(b) less than 7
(c) 0
(d) more than 7

(d) more than 7

Basic or alkaline solutions turn a methyl orange indicator yellow. Thus, the pH of the solution is likely to be more than 7.

#### Question 48:

Bee stings can be treated with:
(a) vinegar
(b) sodium hydrogencarbonate
(c) potassium hydroxide
(d) lemon juice

(b) sodium hydrogencarbonate

When a bee stings a person, it injects an acidic solution into the person’s skin, which causes severe pain and an itching sensation. Applying a baking soda solution (sodium hydrogencarbonate solution, which is a base) soothes the pain by neutralising the acid.

#### Question 49:

Wasp stings can be treated with:
(a) baking soda
(b) vinegar
(c) washing soda
(d) milk of magnesia

(b) vinegar

When a wasp stings a person, it inserts an alkaline solution into the person's skin. This causes high irritation and pain. Applying a weak acid, such as vinegar, to the wound neutralises the alkaline solution and soothes the pain.

#### Question 50:

It has been found that rubbing vinegar on the stung area of the skin of a person gives him relief. The person has been stung by:
(a) wasp
(b) ant
(c) honey bee
(d) nettle leaf hair

(a) wasp

When a wasp stings a person, it injects an alkaline solution into the person's skin. This causes high irritation and pain. Applying a weak acid, such as vinegar, to the wound neutralises the alkaline solution and soothes the pain.

#### Question 51:

Fresh milk has a pH of 6. When milk changes into curd, the pH value will:
(a) become 7
(b) become less than 6
(c) become more than 7
(d) remain unchanged

(b) become less than 6

When fresh milk changes into curd, its acidity increases and it turns sour. Therefore, the pH of fresh milk becomes less than 6 when milk changes into curd.

#### Question 52:

The acid produced naturally in our stomach is:
(a) acetic acid
(b) citric acid
(c) hydrochloric acid
(d) sulphuric acid

(c) hydrochloric acid

Our stomach produces dilute hydrochloric acid naturally. The acid that is produced, which has a pH of around 1.4, does not damage our stomach. Rather, it helps in the digestion of food.

#### Question 53:

The daffodil plants grow best in a soil having a pH range of 6.0 to 6.5. If the soil in a garden has a pH of 4.5, which substance needs to be added to the soil in order to grow daffodils?
(a) salt
(b) lime
(c) sand
(d) compost

(b) lime

If the pH of the soil is 4.5, then it is highly acidic. Thus, soil should be treated with a base, such as quick lime (calcium oxide) or slaked lime (calcium hydroxide). These bases react with the excess acid present in the soil and neutralises it. Thus, the pH of the soil is increased, making it suitable for growing daffodils.

#### Question 54:

A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?

(a) Usually, milk is acidic, with a pH of 6. The milkman has added a very small  amount of baking soda to the milk to make it alkaline, so that it will not become sour with the formation of lactic acid. The alkaline medium prevents the milk from changing into curd. This is the reason why the milkman has shifted the pH of fresh milk from 6 to slightly alkaline.

(b) The milkman has made the milk alkaline by adding baking soda. The alkaline medium of milk slows down the transformation of the milk into curd. The lactic acid produced undergoes neutralisation, so the milk takes a long time to set as curd.

#### Question 55:

Which of the following elements would from oxides which would indicate pH values less than seven, using moist pH paper?
Magnesium, Carbon, Sulphur, Hydrogen, Copper

Sulphur  and Carbon burn in the air to form sulphur dioxide and carbon dioxide. Both are acidic oxides that indicate pH values less than 7 when moist pH paper is used.

S + O2 → SO2
C + O2 → CO2

Hydrogen is also a non-metal, but forms water, which has a neutral pH of 7, after reacting with oxygen. The oxides of magnesium and copper do not have pH values less than 7.

#### Question 56:

The pH values of five solutions A, B, C, D and E are given below:

 A 1 B 5 C 7 D 11 E 13

Which solution is (i) weakly alkaline (ii) neutral (iii) strongly acidic (iv) strongly alkaline, and (v) weakly acidic?

(i) Solution D, with a pH value of 11, is weakly alkaline.

(ii) Solution C, with a pH value of 7, is neutral.

(iii) Solution A, with a pH value of 1, is strongly acidic.

(iv) Solution E, with a pH value of 13, is strongly alkaline.

(v) Solution B, with a pH value of 5, is weakly acidic.

#### Question 57:

Potatoes grow well on Anhad's farm which has soil with a pH of 5.5. Anhad decides to add lot of lime to soil so that he can grow broccoli in the same farm:
(a) Do potatoes grow better in acidic or alkaline soil?
(b) Does broccoli grow better in acidic or alkaline soil?

(a) Potatoes grow better in acidic soil.
(b) Broccoli grows better in alkaline soil.

#### Question 58:

Here are some results of solutions tested with universal indicator paper:

 Sulphuric acid :    Red Metal polish :    Dark blue Washing-up liquid :    Yellow Milk of magnesia :    Light blue Oven cleaner :    Purple Car battery acid :    Pink

Arrange the solutions in order of their increasing pH values (starting with the one with the lowest pH).

The solutions have been arranged below in the increasing order of their pH values.

Sulphuric acid (~ pH 1) < Car battery acid (~ pH 2) < Washing-up liquid (~ pH 5) < Milk of magnesia (~ pH 9) < Metal polish (~ pH 12) < Oven cleaner (~ pH 14)

#### Question 59:

Solution A turns universal indicator blue to purple whereas solution B turns universal indicator orange to red.
(a) What will be the action of solution A on litmus?
(b) What will be action of solution B on litmus?
(c) Name any two substances which can give solutions like A.
(d) Name any two substances which can give solutions like B.
(e) What sort of reaction takes place when solution A reacts with solution B?

(a) As solution A is a basic solution, it would turn red litmus blue.

(b) As solution B is an acidic solution, it would turn blue litmus red.

(c) Sodium hydroxide and household ammonia can give solutions similar to solution A.

(d) Dilute hydrochloric acid and citric fruit juice can give solutions similar to solution B.

(e) When solution A (a base) reacts with solution B (an acid), a neutralisation reaction takes place, producing a salt and water.

#### Question 60:

A first-aid manual suggests that vinegar should be used to treat wasp stings and baking soda for bee stings. What does this information tell you about the chemical nature of:
(a) wasp stings?
(b) bee stings?

The information tells us that the chemical nature of
(a) a wasp sting is alkaline
(b) a bee sting is acidic

#### Question 61:

(a) Explain why the pH in a person's mouth becomes lower after each meal.
(b) What damage could be caused while the pH is low?
(c) How could the person change his eating habits to lessen chances of suffering from tooth decay?

(a) After each meal, sugar present in the food is broken down into acids by the bacteria present in the mouth. Acids have a pH less than 7, and as the acidity increases, the pH decreases. Thus, the pH in a person’s mouth becomes lower after each meal.

(b) When the pH in the mouth falls to around 5.5, tooth decay begins. As the acidity in the mouth increases, the acid reacts with the tooth enamel and causes tooth decay.

(c) A person can prevent tooth decay by consuming less sweet foods, such as ice creams and chocolates. Tooth decay can also be prevented by brushing with toothpastes (which is basic) regularly and by cleaning the mouth carefully after meals.

#### Question 62:

A group of students measured the pH of some substances they found in their homes. Their results are given in the following table:

 Substance pH Substance pH Apples 3.0 Salt 7.0 Baking soda 8.5 Sugar 7.0 Black coffee 5.0 Toothpaste 9.0 Household ammonia 12.0 Vinegar 3.0 Lemon juice 2.5 Washing soda 11.5 Milk 6.5

(a) What would the students have used to measure the pH?
(b) Which solution is the most acidic?
(c) Which solution is the most alkaline?
(d) Which solutions are neutral?
(e) Which solution can be used to treat wasp stings?
(f) Which solutions can be used to treat bee stings?

(a) The students would have used universal indicator paper to measure the pH values of the substances. Universal indicator paper indicates different pH values of solutions with different colour changes. The strength of an acid or a base is known with the help of different colours.

(b) Lemon juice, with pH 2.5, is most acidic.

(c) Household ammonia, with pH 12.0, is most alkaline.

(d) Salt and sugar, with pH 7, are neutral.

(e) Vinegar, with pH 3, can be used to treat wasp stings. As a wasp sting is alkaline (pH > 7), an acid, such as vinegar, is applied to the skin, and it neutralises the base and soothes the pain.

(f) Baking soda, with pH 8.5, can be used to treat bee stings. As a bee sting is acidic (pH < 7), a base, such as baking soda, is applied to the skin, and it neutralises the acid and soothes the pain.

#### Question 63:

Hydrochloric acid reacts with a metal X to form a gas Y which burns with a 'pop' sound. Sodium hydroxide solution also reacts with the same metal X (on heating) to form the same gas Y.
(a) Name X and Y
(b) Write the chemical equation of the reaction of metal X with (i) hydrochloric acid, and (ii) sodium hydroxide solution.

(a) X is the metal zinc (Zn) and Y is hydrogen gas.
Zinc reacts with hydrochloric acid and forms zinc chloride salt with the liberation of hydrogen gas (Y), which burns with a 'pop' sound. Zinc displaces hydrogen from hydrochloric acid.
Hence, X is zinc and Y is hydrogen.

(b) The chemical equation is as follows.

(i) Zn + 2HCl $\to$ZnCl2 + H2

(ii) 2NaOH + Zn $\to$Na2ZnO2 + H2

#### Question 1:

What is the chemical formula of (a) baking soda, and (b) washing soda?

(a) The chemical formula of baking soda is NaHCO3. Its chemical name is sodium hydrogen carbonate.
(b) The chemical formula of washing soda is Na2CO3.10H2O. Its chemical name is sodium carbonate decahydrate.

#### Question 2:

Write the chemical formula of (i) soda ash, and (ii) sodium carbonate decahydrate.

(i) The chemical formula of soda ash is ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}$. Its chemical name is sodium carbonate.
(ii) The chemical formula of sodium carbonate decahydrate is ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}$.${\mathrm{H}}_{2}\mathrm{O}$. Its common name is washing soda.

#### Question 3:

State whether the following statement is true or false:
Copper sulphate crystals are always wet due to the presence of water of crystallisation in them.

False
Water of crystallisation is an integral part of the crystal structure of salts. It is not free or additional water outside the crystal; thus, salts do not appear wet. They seem to be completely dry.

#### Question 4:

Which of the following salt has a blue colour and why?
CuSO4.5H2O    or    CuSO4

CuSO4.H2O has a blue colour.

The blue colour is due to the presence of water molecules in the structure. The colour of a salt crystal is usually given by the water of crystallisation. But anhydrous salts (without the water molecules) lose their colour and shape and become white. Thus, CuSO4, which does not have water molecules, appears white.

#### Question 5:

What would be the colour of litmus in a solution of sodium carbonate?

Litmus turns blue in a solution of sodium carbonate.

#### Question 6:

State the common and chemical names of the compound formed when plaster of Paris is mixed with water.

When plaster of Paris is mixed with water, it settles into a hard mass to form gypsum (common name). The chemical name of gypsum is calcium sulphate dihydrate.

#### Question 7:

With which substance should chlorine be treated to get bleaching powder?

Chlorine should be treated with dry slaked lime (calcium hydroxide) to get bleaching powder (calcium oxychloride).

The reaction can be given as follows.

#### Question 8:

What is the commercial name of calcium sulphate hemihydrate?

The commercial name of calcium sulphate hemihydrate is plaster of Paris. The formula is ${\mathrm{CaSO}}_{4}.\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$.

#### Question 9:

Name the product formed when Cl2 and H2 produced during the electrolysis of brine are made to combine.

When hydrogen and chlorine react with each other, hydrochloric acid (HCl) is formed.

#### Question 10:

Name a calcium compound which hardens on wetting with water.

Calcium sulphate hemihydrate, popularly known as plaster of Paris, is a calcium compound that hardens on wetting with water. Its formula is ${\mathrm{CaSO}}_{4}.\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$.

#### Question 11:

Name a sodium compound which is a constituent of many dry soap powders.

Sodium carbonate decahydrate (${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}.10{\mathrm{H}}_{2}\mathrm{O}$), popularly known as washing soda, is a constituent of many dry soap powders.

#### Question 12:

Name a metal carbonate which is soluble in water.

Washing soda (sodium carbonate decahydrate) is soluble in water. It has the formula .

#### Question 13:

Name an acid which is present in baking powder.

Tartaric acid is present in baking powder, which is a mixture of tartaric acid and sodium hydrogen carbonate.

#### Question 14:

Name the metal whose carbonate is known as washing soda.

Sodium is the metal whose carbonate, sodium carbonate decahydrate, is known as washing soda.

#### Question 15:

Which compound is used as an antacid in medicine : NaHCO3 or Na2CO3?

NaHCO3.
Sodium hydrogen carbonate (baking soda) is used as an antacid, to remove acidity in the stomach. As baking soda is alkaline, it reacts with excess acid and neutralises the acid.

#### Question 16:

What is the common name of (a) NaHCO3 and (b) Na2CO3.10H2O?

a. NaHCO3 is commonly known as baking soda.
b. Na2CO3.10H2O is commonly known as washing soda.

#### Question 17:

Write the chemical name and formula of (a) common salt, and (b) caustic soda.

a. The chemical name of common salt is sodium chloride. Its formula is NaCl.​
b. ​The chemical name of caustic soda is sodium hydroxide. Its formula is NaOH.

#### Question 18:

What are the two main ways in which common salt (sodium chloride) occurs in nature?

1. Common salt occurs naturally in sea water in the dissolved form.
2. Common salt is found in underground deposits as rock salt.

#### Question 19:

Name the major salt present in sea-water.

Common salt (or sodium chloride) is present naturally in sea water in the dissolved form. This is the major salt present in sea water.

#### Question 20:

How is common salt obtained from sea-water?

Common salt is obtained from sea water by the process of evaporation through the sun’s heat, which leaves behind crude common salt. Crude salt is further purified to remove impurities as it has some other salts mixed with it.

#### Question 21:

Why is sodium chloride required in our body?

Sodium chloride is required in our body for the functioning of our nervous system, the movement of muscles, the production of hydrochloric acid in the stomach for digestion of food and various other biochemical processes which occur in our body.

#### Question 22:

Name three chemicals made from common salt (or sodium chloride).

• Sodium hydroxide (NaOH), or caustic soda
• Washing soda (Na2CO3.10H2O), or sodium carbonate decahydrate
• Baking soda (NaHCO3), or sodium hydrogen carbonate

#### Question 23:

Give any two uses of common salt (sodium chloride).

Two uses of common salt are as follows.

1. It is used in the manufacture of soap.
2. It is used to melt ice that collects on the roads in cold countries during winter.

#### Question 24:

What name is given to the common salt which is mined from underground deposits? How was this salt formed?

The common salt mined from underground deposits is called rock salt.
When ancient seas dried up by evaporation thousands of years ago, rock salt, which we dig out today, was formed.

#### Question 25:

Name the salt which is used as preservative in pickles, and in curing meat and fish.

Common salt (NaCl) is used as a preservative in pickles and in the curing (or preservation) of meat and fish.

#### Question 26:

Name the raw material used for the production of caustic soda.

Common salt (or sodium chloride) is the raw material for making caustic soda. The chemical formula of sodium chloride is NaCl.

#### Question 27:

The electrolysis of an aqueous solution of sodium chloride gives us three products. Name them.

Three products obtained by the electrolysis of an aqueous solution of sodium chloride are chlorine gas (formed at the anode), hydrogen gas (formed at the cathode) and sodium hydroxide solution (formed near the cathode).

#### Question 28:

During the electrolysis of a saturated solution of sodium chloride, where is:
(a) chlorine formed?
(b) hydrogen formed?
(c) sodium hydroxide formed?

During the electrolysis of a saturated solution of sodium chloride,

(a) chlorine gas is produced at the anode (positive electrode);
(b) hydrogen gas is produced at the cathode (negative electrode);
(c) sodium hydroxide solution is produced near the cathode.

#### Question 29:

Fill in the following blanks:
(a) Common salt is obtained from sea-water by the process of.............
(b) Rock salt is mined just like ......................
(c) Chemical formula of washing soda is ..................
(d) Sodium hydrogen carbonate is .................. soda whereas sodium carbonate is ................. soda.
(e) The chemical formula of plaster of Paris is ............

(a) Common salt is obtained from sea-water by the process of evaporation.
(b) Rock salt is mined just like coal.
(c) Chemical formula of washing soda is Na2CO3.10H2O.
(d) Sodium hydrogen carbonate is baking soda whereas sodium carbonate is washing soda.
(e) The chemical formula of plaster of Paris is CaSO4.1/2H2O.

#### Question 30:

Complete and balance the following chemical equations:
(a)
(b)
(c)
(d)

#### Question 31:

What is washing soda? State two properties and two uses of washing soda.

Washing soda is sodium carbonate. It contains 10 molecules of water of crystallisation and is also known as sodium carbonate decahydrate. The chemical formula is Na2CO3.10H2O.

Properties:

• Washing soda is a transparent crystalline solid.
• Washing soda is soluble in water. The solution of washing soda is alkaline and turns red litmus paper to blue.
Uses:
• Washing soda is used for removing permanent hardness of water.
• It is used in the manufacture of soap, glass, paper and sodium compounds, such as borax.

#### Question 32:

Write the formulae of sodium chloride and sodium carbonate. Explain why an aqueous solution of sodium chloride is neutral but an aqueous solution of sodium carbonate is basic (or alkaline). Write chemical equations of the reactions involved.

The formula of sodium chloride is NaCl and that of sodium carbonate is Na2CO3.

An aqueous solution of sodium chloride is neutral because sodium chloride is formed from a strong acid, hydrochloric acid (HCl), and a strong base, sodium hydroxide(NaOH). When sodium chloride is dissolved in water, it gets hydrolysed to give equal amounts of hydroxide and hydrogen ions, and this makes its aqueous solution neutral.

NaCl(s) + H2O(l) →  NaOH(aq) + HCl(aq)

An aqueous solution of sodium carbonate (washing soda) is alkaline because it is a salt prepared from the reaction of a weak acid, carbonic acid (H2CO3), and a strong base, sodium hydroxide (NaOH). When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid.

Na2CO3(s) + 2H2O(l)  →  2NaOH(aq) + H2CO3(aq)

Sodium hydroxide, being a strong base, is fully ionised to give a large amount of hydroxide ions (OH-), and carbonic acid, being a weak acid, is slightly ionised to form a small amount of hydrogen (H+) ions. Therefore, an aqueous solution contains more hydroxide ions than hydrogen ions, and this makes the solution basic.

#### Question 33:

Write the chemical formula of ammonium chloride. Explain why an aqueous solution of ammonium chloride is acidic in nature? Illustrate your answer with the help of a chemical equation.

The chemical formula of ammonium chloride is NH4Cl. It is formed from a weak base, ammonium hydroxide (NH4OH), and a strong acid, hydrochloric acid (HCl). On dissolving in water, NH4Cl hydrolyses to some extent to form ammonium hydroxide (NH4OH) and hydrochloric acid (HCl).

Hydrochloric acid is a strong acid that is fully ionised to give a large amount of H+ (hydrogen) ions and ammonium hydroxide is a weak base that is slightly ionised to give a small amount of OH- (hydroxide) ions. As ammonium chloride contains more H+ ions than OH- ions, it is acidic with a pH less than 7.

#### Question 34:

What is baking soda? Write the chemical name of baking soda. Give the important uses of baking soda. How does baking soda differ chemically from washing soda?

Baking soda is a sodium salt used for speeding up the cooking of foodstuff such as various pulses. The chemical name of baking soda is sodium hydrogencarbonate (NaHCO3), also called sodium bicarbonate.

Uses:

• Baking soda is used in fire extinguishers.
• It is an ingredient, used in preparing baking powder, which is used in baking cakes, breads, etc.
• It is used as an antacid to remove acidity in the stomach.
Washing soda, or sodium carbonate (Na2CO3.10H2O), consists of two sodium atoms attached to a carbonate group with 10 molecules of water of crystallisation, whereas baking soda comprises one atom of sodium and one of hydrogen, attached to a carbonate group. Hence, baking soda (NaHCO3) differs from washing soda (Na2CO3) by one hydrogen atom. When baking soda is heated up at high temperatures, it breaks down to form sodium carbonate, water vapour and carbon dioxide.

#### Question 35:

Describe how sodium hydrogencarbonate (baking soda) is produced on a large scale. Write equation of the reaction involved.

Sodium hydrogencarbonate (baking soda) is produced on a large scale by reacting a cold and concentrated solution of sodium chloride called brine with ammonia and carbon dioxide.

NaCl + NH3 + H2O + CO2 →  NaHCO3 +  NH4Cl

#### Question 36:

What happens when a cold and concentrated solution of sodium chloride reacts with ammonia and carbon dioxide? Write the chemical equation of the reaction which takes place.

Sodium hydrogencarbonate (baking soda) is produced when a cold and concentrated solution of sodium chloride (brine) reacts with ammonia and carbon dioxide.

NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl

#### Question 37:

(a) What is meant by "water of crystallisation" in a substance? Explain with an example.
(b) How would you show that blue copper sulphate crystals contain water of crystallisation?
(c) Explain how anhydrous copper sulphate can be used to detect the presence of moisture (water) in a liquid.

(a) The water molecules that form part of the structure of a crystal of a salt are called water of crystallisation. For example, washing soda crystals (or sodium carbonate crystals) contain 10 molecules of water of crystallisation in one formula unit. It is represented as,    Na2CO3.10H2O. It is also called sodium carbonate decahydrate the last part of name coming from ten molecules of water present in the crystal structure.

(b) When copper sulphate crystals (chemical formula CuSO4.5H2O) are heated strongly, they lose all the water of crystallisation and form anhydrous copper sulphate (CuSO4), which is white. As the blue colour of the crystals disappeared on heating, we can conclude that the colour was due to water of crystallisation. This proves that blue copper sulphate crystals contain water of crystallisation.

CuSO4.5H2O →  CuSO4 + 5H2O

(c) A few drops of the liquid to be tested are added to white anhydrous copper sulphate powder. The appearance of a blue colour in anhydrous copper sulphate indicates the presence of moisture or water in the liquid.

#### Question 38:

(a) What is the common name of sodium hydrogencarbonate?
(b) What happens when a solution of sodium hydrogencarbonate is heated? Write equation of the reaction involved.
(c) Explain why, sodium hydrogencarbonate is used as an antacid.

(a) The common name of sodium hydrogencarbonate is baking soda.
(b) When a solution of sodium hydrogen carbonate (NaHCO3) is heated, it decomposes to give sodium carbonate (Na2CO3) with the evolution of carbon dioxide gas.

${2NaHCO}_{3}\stackrel{\mathrm{Heat}}{\to }{\mathrm{Na}}_{2}{\mathrm{CO}}_{3}{+ CO}_{2}{+ H}_{2}\mathrm{O}$

(c) Sodium hydrogen carbonate is used as an antacid because it is alkaline and it neutralises excess acid present in the stomach and relieves indigestion.

#### Question 39:

(a) What will happen if heating is not controlled while preparing plaster of Paris?
(b) Write an equation to show the reaction between plaster of Paris and water.

(a) If gypsum (or calcium sulphate dihydrate) is heated above 100⁰C (373K) while preparing plaster of Paris, then all its water of crystallisation is eliminated and anhydrous calcium sulphate (CaSO4), called dead burnt plaster, is formed. The anhydrous calcium sulphate does not set like plaster of Paris on addition of water.

(b)

#### Question 40:

(a) What happens when copper sulphate crystals are heated strongly? Explain with the help of an equation.
(b) What happens when a few drops of water are added to anhydrous copper sulphate? Explain with the help of an equation.

(a) When copper sulphate (CuSO4.5H2O) crystals are heated strongly, they lose all the water of crystallisation and form anhydrous copper sulphate, which is white.

CuSO4.5H2O → CuSO4 + 5H2O

​(b) Anhydrous copper sulphate gets hydrated and turns blue on addition of a few drops of water.

CuSO4 + 5H2O → CuSO4.5H2

#### Question 41:

(a) Name two constituents of baking powder.
(b) How does baking powder differ from baking soda?
(c) Explain the action of baking powder in the making of cake (or bread). Write equation of the reaction involved.

(a) Two constituents of baking powder are baking soda (sodium hydrogencarbonate) and tartaric acid.

(b) Baking powder is a mixture of baking soda (sodium hydrogencarbonate, NaHCO3) and a solid edible acid such as tartaric acid, whereas baking soda is a single compound (sodium hydrogencarbonate, NaHCO3).

(c) On mixing baking powder with water, which is present in the dough prepared for baking a cake or bread, sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas. The carbon dioxide gas gets trapped in the wet dough and bubbles out slowly, and the cake becomes soft and spongy.

Note: Here, H+ is from tartaric acid and Na+ is from sodium tartrate salt.

#### Question 42:

(a) What is the chemical name of bleaching powder?
(b) What is the chemical formula of bleaching powder?
(c) What are the materials used for the preparation of bleaching powder?
(d) State one use of bleaching powder (other than bleaching).

(a) The chemical name of bleaching powder is calcium oxychloride.

(b) The chemical formula of bleaching powder is CaOCl2.

(c) The substances used for the preparation of bleaching powder are chlorine (Cl2) and slaked lime [Ca(OH)2].

(d) Bleaching powder is used as a disinfectant. It is used for making drinking water free from germs.

#### Question 43:

What does a soda-acid type fire extinguisher contain? How does it work? Explain the working of a soda-acid fire extinguisher with the help of a labelled diagram.

A soda-acid type of fire extinguisher contains solutions of sulphuric acid and sodium hydrogencarbonate in separate containers.

The working of a soda-acid fire extinguisher is described below:

When the knob of the fire extinguisher is pressed, the sulphuric acid mixes with the sodium hydrogencarbonate solution to produce a substantial quantity of carbon dioxide gas. The carbon dioxide gas forces out as a stream of liquid that falls on the burning substance because of the high pressure within the extinguisher. A blanket of carbon dioxide comes out along with the liquid formed around the burning substance and cuts off the supply of air to the burning substance. When the supply of air is cut off, and the fire gets extinguished.
The reaction involved is given below.

2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2

#### Question 44:

(a) Name a sodium compound used for softening hard water.
(b) Which compound of calcium is used for disinfecting drinking water supply?
(c) Name a metal compound which has detergent properties (cleansing properties).
(d) Name one compound of calcium which is used for removing the colour of a coloured cloth.
(e) State a peculiar (or remarkable) property of plaster of Paris.
(f) Name the substance obtained by the action of chlorine on solid (dry) slaked lime.

(a) Sodium carbonate or washing soda (Na2CO3) is used for softening hard water.
(b) Bleaching powder or calcium oxychloride (CaOCl2) is used for disinfecting drinking water supply.
(c) Sodium carbonate, or washing soda (Na2CO3), has a cleansing property.
(d) Bleaching powder, or calcium oxychloride (CaOCl2), is used for removing the colour of a coloured cloth.
(e) Plaster of Paris has a remarkable property of setting  into a hard mass on addition of water.
(f) Bleaching powder, or calcium oxychloride (CaOCl2), is obtained by the action of chlorine on solid (dry) slaked lime.

#### Question 45:

(a) What is gypsum? What happens when gypsum is heated to 100°C (373 K)?
(b) Name a sodium compound which is used for making borax and glass.
(c) Name the compound which is used in hospitals for setting fractured bones.
(d) Which is the real bleaching agent present in bleaching powder?

(a) Gypsum is calcium sulphate dihydrate (CaSO4.2H2O). When gypsum is heated to a temperature of 100C (373K), it loses three-fourths of its water of crystallisation and forms plaster of Paris (CaSO4.1/2H2O).

(b) Sodium carbonate or washing soda is used for making borax and glass.

(c) Plaster of Paris sets into a hard mass on addition of water. It is easy to mould a layer Plaster of Paris around the damaged part, which later solidifies and provides support to the damaged bone for its quick healing. This is why it is used in hospitals for setting fractured bones.

(d) Chlorine is the real bleaching agent present in bleaching powder.

#### Question 46:

(a) What is "baking powder"? How does it make the cake soft and spongy?
(b) In addition to sodium hydrogencarbonate, baking powders contain a substance X. Name the substance X. What is the role of substance X in the baking powder?

(a) Baking powder is a sodium salt called sodium bicarbonate (NaHCO3). It is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. When baking soda mixes with water, the sodium hydrogencarbonate reacts with the tartaric acid to produce carbon dioxide gas. The carbon dioxide gas produced gets trapped in the wet dough made for baking a cake or bread, and bubbles out slowly, and the cake or bread becomes soft and spongy.

(b) In addition to sodium hydrogencarbonate, baking powder contains tartaric acid (X). Tartaric acid is added to baking powder to neutralise the bitter taste that baking soda produces in the cake. Also, carbon dioxide, which makes the cake fluffy and soft, is produced only when the tartaric acid reacts with the sodium hydrogencarbonate or baking soda, in the presence of water.

#### Question 47:

State two uses each of the following compounds:
(a) Sodium hydroxide
(b) Chlorine
(c) Hydrogen
(d) Hydrochloric acid

(a) Sodium hydroxide:
(i)  It is used for making soaps and detergents.
(ii) It is used for making synthetic textile fibre (such as rayon).

(b) Chlorine:
(i)  It is used for the production of bleaching powder.
(ii) It is used to sterilise drinking water supply and the water in swimming pools, because it is        a disinfectant.

(c) Hydrogen:
(i)  It is used in the hydrogenation of oils to obtain solid fats called vegetable ghee.
(ii) Liquid hydrogen is used as a fuel for rockets.

(d) Hydrochloric acid:
(i)  It is used for cleaning iron sheets before tin plating or galvanisation.
(ii) It is used in medicines and cosmetics.

#### Question 48:

(a) What is the common name of the compound CaOCl2?
(b) Name the raw material used for the preparation of plaster of Paris.
(c) Which property of plaster of Paris is utilised in making casts for broken limbs in hospitals?
(d) Explain why chlorine is used for sterilising drinking water supply.

(a)The common name of the compound CaOCl2 is bleaching powder.

(b) The raw material used for the preparation of plaster of Paris is gypsum (CaSO4.2H2O).

(c) Plaster of Paris has a property of setting into a hard mass in about half an hour after wetting with water. This property is utilised in making casts for broken limbs in hospitals.

(d) Chlorine is a disinfectant used for sterilising drinking water supply in order to kill disease-causing bacteria that the water or water pipes might contain.

#### Question 49:

(a) What happens when a concentrated solution of sodium chloride (brine) is electrolysed? Write the equation of the reaction involved.
(b) Why is the electrolysis of a concentrated solution of sodium chloride known as chlor-alkali process?
(c) Name three products of the chlor-alkali process. State two uses of each of these products.

(a) When electricity is passed through a concentrated solution of sodium chloride called brine, the solution decomposes to form sodium hydroxide, chlorine and hydrogen.

(b) The process of electrolysis of sodium chloride solution is called the chlor-alkali process because 'chlor' (chlorine) and alkali (sodium hydroxide, NaOH) are formed as products.

(c) The three main products of the chlor-alkali process are H2, Cl2 and NaOH.

Uses of sodium hydroxide:

(i)  It is used for making soaps and detergents.
(ii) It is used for making synthetic textile fibre (such as rayon).

Uses of chlorine:

(i)  It is used in the production of bleaching powder.
(ii) It is used to sterilise drinking water supply and the water in swimming pools, because it is a disinfectant.

Uses of hydrogen:

(i)  It is used in the hydrogenation of oils to obtain solid fats called vegetable ghee.
(ii) Liquid hydrogen is used as a fuel for rockets.

#### Question 50:

(a) Describe how washing soda is produced starting from sodium chloride (common salt). Write equations of all the reactions involved.
(b) State whether an aqueous solution of washing soda is acidic or alkaline? Give reason for your answer.
(c) What is meant by saying that washing soda has detergent properties?
(d) Give two important uses of washing soda (or sodium carbonate).

1. Preparation of washing soda:
1. A cold and concentrated solution of sodium chloride is reacted with ammonia (NH3) and carbon dioxide to form sodium hydrogen carbonate (NaHCO3) and ammonium chloride (NH4Cl). Sodium hydrogen carbonate precipitates as a solid as it is slightly soluble in water.
NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl
1. The sodium hydrogen carbonate precipitated is separated by filtration, and then dried and heated. On heating, it decomposes to form sodium carbonate (Na2CO3). The anhydrous sodium carbonate formed here is called soda ash.
2NaHCO3  Na2CO3 + CO2+ H2O
1. The soda ash is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation.
Na2CO3 + 10H2O → Na2CO3.10H2O

1. An aqueous solution of washing soda is alkaline, because sodium carbonate is a salt prepared from the reaction between a weak acid, carbonic acid (H2CO3), and a strong base, sodium hydroxide (NaOH). When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid.
Na2CO3(s) + 2H2O(l)  → 2NaOH(aq) + H2CO3(aq)
Sodium hydroxide, being a strong base, is fully ionised to give a large amount of hydroxide ions (OH-), and carbonic acid, being a weak acid, is slightly ionised to form a small amount of hydrogen (H+) ions. Therefore, an aqueous solution contains more hydroxide ions than hydrogen ions, which makes the solution basic.
1. By saying that washing soda has detergent properties, or cleansing properties, one means that it can remove dirt and grease from dirty clothes, etc. It attacks dirt and grease to form water-soluble products, which are washed away on rinsing with water.

1. Uses of washing soda:
1. It is used for removing permanent hardness of water.
2. It is used in the manufacture of glass, soap, paper and sodium compounds, such as borax.

#### Question 51:

(a) What is bleaching powder? How is bleaching powder prepared? Write chemical equation of the reaction involved in the preparation of bleaching powder.
(b) What happens when bleaching powder reacts with dilute sulphuric acid? Give equation of the reaction involved.
(c) State two important uses of bleaching powder.

1. Bleaching powder is calcium oxychloride (CaOCl2) and is also called chloride of lime.
Preparation:
Bleaching powder is prepared by passing chlorine gas over dry slaked lime.

Ca(OH)2 + Cl2
CaOCl2 + H2O
(Slaked lime)
1. When bleaching powder reacts with dilute sulphuric acid, all the chlorine present in it is liberated. This chlorine acts as a bleaching agent.
​​CaOCl2 + H2SO4 →  CaSO4 + Cl2 + H2

c. Uses:

i. Bleaching powder is used in making chloroform (CHCl3).
ii. It is used as an oxidising agent in many chemical industries.

#### Question 52:

(a) What is plaster of Paris? Write the chemical formula of plaster of Paris.
(b) How is plaster of Parish prepared? Write chemical equation of the reaction involved.
(c) Explain why plaster of Paris should be stored in a moisture-proof container.
(d) State two important uses of plaster of Paris.

(a) Plaster of Paris is calcium sulphate hemi-hydrate (calcium sulphate half hydrate). The chemical formula of plaster of Paris is CaSO4.1/2H2O.

(b) When gypsum (CaSO4.2H2O) is heated to a temperature of 100C (373K), it loses three-fourths of its water of crystallisation and forms plaster of Paris. ​

CaSO4.2H2$\stackrel{∆}{\to }$  CaSO4.1/2H2O + 3/2 H2O
Gypsum           Plaster of Paris

(c) Plaster of Paris should be stored in a moisture-proof container because the presence of moisture can cause its slow setting by the addition of water of hydration, and it would be useless after some time.

(d) Uses:
(i) Plaster of Paris is used in making decorative materials, toys, cosmetics, cheap ornaments, black-board chalks and casts for fractured bone and statues.
(ii) It is used in making surfaces such as walls of houses smooth before they are painted and for creating ornamental designs on the ceilings of houses and other buildings.

#### Question 53:

(a) What is a salt? Give the names and formulae of any two salts. Also name the acids and bases from which these salt may be obtained.
(b) What is meant by 'a family of salts'? Explain with examples.
(c) What is meant by 'hydrated' and 'anhydrous' salts? Explain with examples.
(d) Write the names, formulae and colours of any two hydrated salts.
(e) What will be the colour of litmus in an aqueous solution of ammonium chloride salt?

1. Salt is a compound formed when a metal replaces the hydrogen atom in an acid. The names and formulae of two salts are sodium chloride, NaCl, and ammonium chloride, NH4Cl, respectively. Sodium chloride salt (NaCl) is formed from an acid, hydrochloric acid (HCl), and a base (NaOH). Ammonium chloride (NH4Cl) is formed from an acid, hydrochloric acid (HCl), and a base, ammonium hydroxide (NH4OH).

2. Salts having the same positive ions or the same negative ions belong to one family of salts. For example, calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to the same family of salts called ‘calcium salts’ because they both contain the same positively charged ions, calcium ions (Ca2+). Similarly, zinc sulphate (ZnSO4) and copper sulphate (CuSO4) belong to the same family of salts called ‘sulphate salts’ because they both contain the same negatively charged ions, sulphate ions (SO42-).

3. Salts that contain water of crystallisation are called ‘hydrated salts’. For example, copper sulphate crystals contain five molecules of water of crystallisation in one formula unit and hence is written as CuSO4.5H2O. It is also called copper sulphate pentahydrate.

Salts that have lost their water of crystallisation on heating are called ‘anhydrous salts’. For example, when copper sulphate crystals (CuSO4.5H2O), which are blue, are heated strongly, they lose all the water of crystallisation and form anhydrous copper sulphate (CuSO4), which is white.

1. Hydrated salts:

1.  Copper sulphate pentahydrate crystals, CuSO4.5H2O, are blue.

2. Iron sulphate heptahydrate crystals, FeSO4.7H2O, are green.

1. Blue litmus turns red. NH4Cl is formed from a weak base, ammonium hydroxide (NH4OH), and a strong acid, hydrochloric acid (HCl). So, on dissolving in water, NH4Cl will hydrolyse to form more H+ ions than OH- ions, and thus it is acidic.

#### Question 54:

The salt which will give an acidic solution on dissolving in water is:
(a) KCl
(b) NH4Cl
(c) Na2CO3
(d) CH3COONa

b) NH4Cl

NH4Cl is formed with a weak base, ammonium hydroxide (NH4OH), and a strong acid, hydrochloric acid (HCl). On dissolving in water, NH4Cl hydrolyses to form more H+ ions than OH- ions. Thus, it is acidic with a pH less than 7.

#### Question 55:

One of the following salts will give an alkaline solution on dissolving in water. This is:
(a) Na2CO3
(b) Na2SO4
(c) NaCl
(d) (NH4)2 SO4

(a) Na2CO3

Na2CO3 is formed from a weak acid, carbonic acid (H2CO3), and a strong base, sodium hydroxide (NaOH). On dissolving in water, Na2CO3 hydrolyses to form more OH- ions than H+ ions. Therefore, its solution in water is basic, or alkaline, with a pH more than 7.

#### Question 56:

The salt which will give a neutral solution on dissolving in water will be:
(a) CH3COONa
(b) NH4Cl
(c) KCl
(d) Na2CO3

(c) KCl

KCl is formed from a strong acid, hydrochloric acid (HCl), and a strong base, potassium hydroxide (KOH). On dissolving in water, KCl hydrolyses to form equal amounts of H+ and OH- ions. This is why an aqueous solution of KCl is neutral with a pH of almost 7.

#### Question 57:

The products of chlor-alkali process are:
(a) NaCl, Cl2 and H2
(b) H2, Cl2 and NaOH
(c) Cl2, Na2CO3 and H2O
(d) NaOH, Cl2 and HCl

b) H2, Cl2 and NaOH

The process of electrolysis of sodium chloride solution is called the chlor-alkali process because of the products formed, i.e., H2, Cl2 and NaOH. 'Chlor' stands for chlorine and 'alkali' for sodium hydroxide (NaOH).

#### Question 58:

The number of molecules of water of crystallisation present in washing soda crystals is:
(a) five
(b) two
(c) ten
(d) seven

c) ten

The chemical formula of washing soda is Na2CO3.10H2O. The water molecules that form part of the structure of a salt are called its water of crystallisation. Hence, the number of molecules of water of crystallisation present in washing soda crystals is 10.

#### Question 59:

The salt whose aqueous solution will turn blue litmus to red is:
(a) ammonium sulphate
(b) sodium acetate
(c) sodium chloride
(d) potassium carbonate

a). ammonium sulphate

Ammonium sulphate [(NH4)2SO4] is formed from a weak base, ammonium hydroxide (NH4OH), and a strong acid, sulphuric acid (H2SO4). On dissolving in water, (NH4)2SO4 hydrolyses to form more H+ ions than OH- ions. As the aqueous solution of the salt (NH4)2SO4 is acidic, it turns blue litmus red.

#### Question 60:

The aqueous solution of one of the following salts will turn red litmus to blue. This salt is:
(a) potassium sulphate
(b) sodium sulphate
(c) sodium chloride
(d) potassium carbonate

(d) potassium carbonate

Potassium carbonate (K2CO3) is formed from a weak acid, carbonic acid (H2CO3), and a strong base, potassium hydroxide (KOH). On hydrolysis, K2CO3 forms more OH- ions than H+ ions. Thus, it is basic and therefore turns red litmus blue.

#### Question 61:

The salt whose aqueous solution will have no effect on either red litmus or blue litmus is
(a) potassium sulphate
(b) sodium carbonate
(c) ammonium sulphate
(d) sodium acetate

a) potassium sulphate

Potassium sulphate (K2SO4) is formed from a strong acid, sulphuric acid (H2SO4), and a strong base, potassium hydroxide (KOH). On dissolving in water, K2SO4 hydrolyses to form equal amounts of  H+ and OH- ions. As the salt solution of K2SO4 is neutral, it has no effect on red or blue litmus paper.

#### Question 62:

The aqueous solution of one of the following salts will turn phenolphthalein indicator pink. This salt is:
(a) KCl
(b) K2SO4
(c) K2CO3
(d) KNO3

(c) K2CO3

K2CO3 is formed from a weak acid, carbonic acid (H2CO3), and a strong base, potassium hydroxide (KOH). On hydrolysis, K2CO3 forms more OH- ions than H+ ions, and thus, it is basic. As the aqueous solution of the salt is basic, it turns a phenolphthalein indicator pink.

#### Question 63:

The formula of baking soda is:
(a) K2CO3
(b) KHCO3
(c) NaHCO3
(d) Na2CO3

(c) NaHCO3

The chemical name of baking soda is sodium hydrogen carbonate, and its chemical formula is NaHCO3.

#### Question 64:

Which of the following is treated with chlorine to obtain bleaching powder?
(a) CaSO4
(b) Ca(OH)2
(c) Mg(OH)2
(d) KOH

(b) Ca(OH)2

Bleaching powder (CaOCl2) is formed by passing chlorine gas over slaked lime [Ca(OH)2].

#### Question 65:

Plaster of Paris is prepared by heating one of the following to a temperature of 100°C. This is:
(a) CaSO3.2H2O
(b) CaCl2.2H2O
(c) CaCO3.2H2O
(d) CaSO4.2H2O

(d) CaSO4.2H2O

When CaSO4.2H2O (gypsum) is heated to a temperature of 100⁰C (373K), it loses three-fourths of its water of crystallisation and forms plaster of Paris (CaSO4.1/2H2O).

#### Question 66:

A salt whose aqueous solution will have a pH of more than 7 will be:
(a) K2CO3
(b) K2SO4
(c) NaCl
(d) NH4Cl

(a) K2CO3

K2CO3 is formed from a weak acid, carbonic acid (H2CO3), and a strong base, potassium hydroxide (KOH). On hydrolysis, K2CO3 forms more OH- ions than H+ ions. Thus, it is basic, with a pH more than 7.

#### Question 67:

A salt is dissolved in water and the pH of this salt solution is measured with a universal indicator paper. If the pH of solution is less than 7, the salt is most likely to be:
(a) CH3COONa
(b) Na2CO3
(c) KCl
(d) NH4Cl

(d) NH4Cl

NH4Cl is formed from a weak base, ammonium hydroxide (NH4OH), and a strong acid, hydrochloric acid (HCl). On hydrolysis, NH4Cl forms more H+ ions than OH- ions. Thus, it is acidic, with a pH less than 7.

#### Question 68:

Which of the following salts will give an aqueous solution having pH of almost 7?
(a) NH4NO3
(b) NH4Cl
(c) CaCl2
(d) KCl

(d) KCl

KCl is formed from a strong acid, hydrochloric acid (HCl), and a strong base, potassium hydroxide (KOH). Thus, KCl, on hydrolysis, forms equal amounts of H+ and OH- ions. This is why an aqueous solution of KCl shows a pH of almost 7.

#### Question 69:

P and Q are aqueous solutions of sodium chloride and sodium hydroxide, respectively. Which of these will turn:
(a) blue litmus red?
(b) red litmus blue?

a) Neither solution will turn blue litmus red.
Solution P is sodium chloride, formed from a strong acid, hydrochloric acid (HCl), and a strong base, sodium hydroxide (NaOH). When sodium chloride is dissolved in water, it gets hydrolysed to give equal amounts of hydroxide and hydrogen ions, which makes its aqueous solution neutral. Therefore, the solution of sodium chloride will have no effect on red or blue litmus paper.

b) Sodium hydroxide solution (Q) will turn red litmus blue, because solution Q is basic.

#### Question 70:

The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. What could be A, B, C and D?

Copper sulphate crystals (A) are blue and have the chemical formula CuSO4.5H2O. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation (B) and form anhydrous copper sulphate (C), which is white.
A                        C            B
CuSO4.5H2O  →  CuSO4  + 5H2O

Anhydrous copper sulphate turns blue on addition of a few drops of water (D) because it gets hydrated again.
C              D
CuSO4 + 5H2O → CuSO4.5H2O

#### Question 71:

When the concentrated aqueous solution of substance X is electrolysed, then NaOH, Cl2 and H2 are produced. Name the substance X. What is the special name of this process?

Substance X, which gives sodium hydroxide, Cl2 and H2 upon electrolysis, is NaCl. The electrolysis of sodium chloride solution is called the chlor-alkali process because of the products formed, 'chlor' for chlorine and 'alkali' for sodium hydroxide (NaOH).

#### Question 72:

Consider the following substances:
NaCl, Ca(OH)2, NaHCO3, NH3, Na2CO3, H2O, Cl2, CO2, CaSO4.2H2O, 2CaSO4.H2O, CaOCl2
(a) Which two substance combine to form bleaching powder?
(b) Which four substances are utilised in the production of washing soda?
(c) Which compound represents plaster of Paris?
(d) Which compound is a part of baking powder?
(e) Which compound is used as an antacid?

a) Ca(OH)2 combines with Cl2 to give bleaching powder (CaOCl2 ).
The chemical reaction involved is as follows.
2Ca(OH)2 + CaCl2 → CaOCl2 + 2H2O

b) NaCl, NH3, H2O, CO2 are utilised in the production of washing soda.
The chemical reaction involved can be shown in three steps:
i) Production of sodium hydrogen carbonate:
NaCl + NH3 + H2O + CO2 → NaHCO3 +NH4Cl
ii) Production of anhydrous sodium carbonate upon heating sodium hydrogen carbonate:
2NaHCO3 → Na2CO3 + CO2 + H2O
iii) Production of hydrated sodium carbonate when anhydrous sodium carbonate is dissolved in water:
Na2CO3 + 10H2O → Na2CO3.10H2O

c) 2CaSO4.H2O represents plaster of Paris as its chemical formula is CaSO4.1/2H2O.

d) NaHCO3 is part of baking powder, as baking powder is a mixture of  NaHCO3 and tartaric acid.

e) NaHCO3 is used as an antacid, as it is alkaline, thus it neutralises excess acid in the stomach.

#### Question 73:

Give one example each of a salt which gives an aqueous solution having:
(a) pH less than 7
(b) pH equal to 7
(c) pH more than 7

a) NH4Cl is formed from a weak base, ammonium hydroxide (NH4OH), and a strong acid, hydrochloric acid (HCl). On hydrolysis, NH4Cl forms more H+ ions than OH- ions. Thus, it is acidic, with a pH less than 7.

b) KCl is formed from a strong acid, hydrochloric acid (HCl), and a strong base, potassium hydroxide (KOH). Thus, KCl, on hydrolysis, forms equal amounts of H+ and OH- ions. Therefore, an aqueous solution of KCl is neutral, with a pH of almost 7.

c) K2CO3 is formed from a weak acid, carbonic acid (H2CO3), and a strong base, potassium hydroxide (KOH). On hydrolysis, K2CO3 forms more OH- ions than H+ ions. Thus, it is basic, with a pH more than 7.

#### Question 74:

A compound X which is prepared from gypsum has the property of hardening when mixed with a proper quantity of water.
(a) Identify the compound X
(b) Write the chemical equation for its preparation
(c) For what purpose is it used in hospitals?

a) Compound X is plaster of Paris, or calcium sulphate half hydrate (CaSO4.1/2H2O)

b) CaSO4.2H2O → CaSO4.1/2H2O + 3/2 H2O

c) Plaster of Paris is used in hospitals for setting fractured bones in the right position to ensure correct healing. Casts made of plaster of Paris help keep the fractured bone straight.

#### Question 75:

Consider the following salts:
Na2CO3, NaCl, NH4Cl, CH3COONa, K2SO4, (NH4)2SO4
Which of these salts will give:
(a) acidic solutions?
(b) neutral solutions?
(c) basic solutions (or alkaline solutions)?

(a) The salt solutions of NH4Cl and (NH4)2SO4 give acidic solutions, because these salts are each formed from a weak base and a strong acid. The ionisation of a weak base is not complete, whereas a strong acid ionises completely. Hence, there are more H+ ions than OH-  ions, making the solutions acidic.

(b) The salt solutions of NaCl and K2SO4 are neutral, because these salts are formed from a strong base and a strong acid, and on ionisation, there is an equal number of  H+ and OH- ions, making the solutions neutral.

(c) The salt solutions of CH3COONa and Na2CO3 are basic, because these salts are each formed from a weak acid and a strong base. The ionisation of a weak acid is not complete, whereas a strong base ionises completely. Hence, there are more OH- ions than H+ ions, making the solutions basic.

#### Question 76:

A white powdery substance having strong smell of chlorine is used for disinfecting drinking water supply at waterworks. Identify the substance. Give its chemical name and write the chemical reaction for its preparation.

Bleaching powder is a white powder that emits a strong smell of chlorine and is used for disinfecting drinking water supply at waterworks.
The chemical name of bleaching powder is calcium oxychloride (CaOCl2).

The chemical reaction that takes place in its preparation is as follows.
Ca(OH)2 +  Cl2 →   CaOCl2  +  H2O

#### Question 77:

A salt X when dissolved in distilled water gives a clear solution which turns red litmus blue. Explain the phenomenon.

As the aqueous solution of salt X turns red litmus blue, it must be basic in nature.
For example, an aqueous solution of sodium acetate salt is basic. Sodium acetate (CH3COONa) is a salt prepared from the reaction between a weak acid, acetic acid (CH3COOH), and a strong base, sodium hydroxide (NaOH). When sodium acetate is dissolved in water, it gets hydrolysed to some extent to form sodium hydroxide and acetic acid.
CH3COONa (s) + H2O(l) → NaOH(aq) + CH3COOH(aq)
Sodium hydroxide, being a strong base, is fully ionised to give a large amount of hydroxide ions (OH-), and acetic acid, being a weak acid, is slightly ionised to form a small amount of hydrogen (H+) ions. Therefore, the aqueous solution contains more hydroxide ions than hydrogen ions, which makes the solution basic.

#### Question 78:

A person found that the cake prepared by him is hard and small in size. Which ingredient has he forgotten to add that would have caused the cake to rise and become light? Explain your answer.

The person forgot to add baking powder.
Baking powder is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid.
When baking powder mixes with water, the sodium hydrogen carbonate reacts with the tartaric acid to emit carbon dioxide gas. The carbon dioxide gas produced gets trapped in the wet dough made for baking a cake or bread, and bubbles out slowly. Thus, the cake becomes soft and spongy.

#### Question 79:

A white chemical compound becomes hard on mixing with proper quantity of water. It is also used in surgery to maintain joints in a fixed position. Name the chemical compound.

Plaster of Paris, or calcium sulphate hemi hydrate (CaSO4.1/2H2O), is a white compound that sets into a hard mass in about half an hour after wetting with water. This property of plaster of Paris is used in making casts for broken limbs in hospitals.

#### Question 80:

When chlorine and sodium hydroxide being produced during the electrolysis of brine are allowed to mix, a new chemical is formed. Name the chemical and write its uses.

When chlorine and sodium hydroxide produced during the electrolysis of brine are allowed to mix, a new chemical called sodium hypochlorite (NaClO) is formed.
Sodium hypochlorite is a bleaching agent that is used in making household bleaches and for bleaching fabrics.

#### Question 81:

Write the name and formula of one salt each which contains:
(a) two molecules of water of crystallisation
(b) five molecules of water of crystallisation
(c) ten molecules of water of crystallisation

1. Gypsum (calcium sulphate crystals) contains two molecules of crystallisation in one formula unit and hence is written as CaSO4.2H2O. It is also called calcium sulphate dihydrate.
2. Copper sulphate crystals contain five molecules of crystallisation in one formula unit. The chemical formula is CuSO4.5H2O (copper sulphate pentahydrate).
3. Washing soda crystals (sodium carbonate crystals) contain 10 molecules of crystallisation in one formula unit. The chemical formula is Na2CO3.10H2O (sodium carbonate decahydrate).

#### Question 82:

How many molecules of water of crystallisation (per formula unit) are present in:
(a) copper sulphate crystals?
(b) washing soda?
(c) gypsum?