Living Science Solutions for Class 7 Science Chapter 7 Acids, Bases And Salts are provided here with simple step-by-step explanations. These solutions for Acids, Bases And Salts are extremely popular among class 7 students for Science Acids, Bases And Salts Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science Book of class 7 Science Chapter 7 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science Solutions. All Living Science Solutions for class 7 Science are prepared by experts and are 100% accurate.

Page No 75:

Question 1:

Which of the following is not an acid-base indicator?

(a) blue litmus
(b) methyl orange
(c) phenolphthalein
(d) Digene

Answer:

(d) digene
Digene is not an acid–base indicator but an antacid (base). It is used as a medicine to reduce acidity of the stomach by neutralising it.

Page No 75:

Question 2:

Which of the following is a strong acid?

(a) nitric acid
(b) citric acid
(c) acetic acid
(d) tartaric acid

Answer:

(a) nitric acid
Nitric acid is highly corrosive in nature. It can burn our skin severely.

Page No 75:

Question 3:

Which of the following is not a base?

(a) sodium hydroxide
(b) magnesium hydroxide
(c) copper carbonate
(d) ammonium hydroxide

Answer:

(c) copper carbonate
Copper carbonate is not a base but an ionic salt.

Page No 75:

Question 4:

Which of these metals cannot be used to prepare hydrogen by the action of an acid?

(a) sodium
(b) iron
(c) calcium
(d) copper

Answer:

(d) copper
Copper is a less reactive metal; therefore, it cannot displace and liberate hydrogen from acids.

Page No 75:

Question 5:

What is the common name of sodium bicarbonate?

(a) phitkari
(b) caustic soda
(c) baking soda
(d) blue vitriol

Answer:

(c) baking soda
Baking soda
is the common name for sodium bicarbonate.

Page No 75:

Question 6:

Lime juice will turn

(a) blue litmus red
(b) methyl orange yellow
(c) red litmus blue
(d) phenolphthalein pink

Answer:

(a) blue litmus red
Lime juice contains citric acid; therefore, it will turn a blue litmus paper red.

Page No 75:

Question 7:

Sodium chloride will turn

(a) blue litmus red
(b) methyl orange yellow
(c) red litmus blue
(d) none of the above

Answer:

d) none of the above
Sodium chloride is a salt, and salts are neutral in nature. Thus it will induce no change to the acid-base indicators.

Page No 75:

Question 8:

Acids

(a) have sour taste
(b) are corrosive in nature
(c) are soluble in water
(d) have all of these properties

Answer:

(d) have all these properties
Acids have a sour taste and are extremely corrosive in nature. They also dissolve easily in water.

Page No 75:

Question 9:

Alkalis

(a) have sour taste
(b) are soluble in water
(c) turn blue litmus red
(d) have all of these properties

Answer:

(b) are soluble in water
All alkalis are water soluble bases.

Page No 75:

Question 1:

Which of these is neutral-hydrochloric acid, sodium hydroxide, sodium chloride?

Answer:

Sodium chloride is neutral in nature as it is a salt. It does not possess the properties of acids or bases.

Page No 75:

Question 2:

If a liquid turns blue litmus red, is it an acid, base or salt?

Answer:

If a liquid turns blue litmus red, then it is an acid.

Page No 75:

Question 3:

Methyl orange has________colour in an acidic solution

Answer:

 Methyl orange turns red in an acidic solution.



Page No 76:

Question 1:

What is an acid-base indicator? Give one example.

Answer:

Acid-base indicators are special substances which are used to detect the acidity or basicity of  other substances. The change in colour of the  indicators help us in understanding which substance is acidic and and which one is basic.

Example: Litmus paper is a natural acid-base indicator.

Page No 76:

Question 2:

Name one each of acidic, basic and neutral substances.

Answer:

Acidic substance: Lemon juice is an acidic substance, as it contains citric acid.
Basic substance: Baking soda is basic in nature, as it contains sodium bicarbonate.
Neutral substance: Common salt is a neutral substance, as it is made of sodium chloride.

Page No 76:

Question 3:

What happens when dilute sulphuric acid is added to zinc?

Answer:

Zinc will react with dilute sulphuric acid to form zinc sulphate salt and hydrogen gas. The reaction is given below:

              Zn + H2SO4  ZnSO4 + H2
      

Page No 76:

Question 4:

How can carbon dioxide be prepared from an acid?

Answer:

Metal carbonates react with dilute acids to form salt, water and carbon dioxide gas. Thus, carbon dioxide can be prepared by reacting any metal carbonate with any dilute acid. 
Example: Carbon dioxide gas is liberated when sodium carbonate reacts with dilute hydrochloric acid.

                             Na2CO3 + 2HCl 2NaCl + H2O + CO2

Page No 76:

Question 5:

Give two examples each of strong acids and weak acids.

Answer:

Examples for strong acids: Sulphuric acid and nitric acid.
Examples for weak acids: Citric acid (present in citrus fruits) and acetic acid (vinegar).

Page No 76:

Question 6:

Why does a base applied to your skin give relief from an ant bite?

Answer:

While biting, insects inject formic acid into the skin. This causes irritation to the skin, and sometimes results into rashes. To get relief from this, a base like baking soda is applied over the bitten area. As bases neutralize acids, the baking soda (being a base) will neutralize the injected formic acid, thereby reducing the irritation. Thus, a base should be applied to the skin to get instant relief from an ant bite.

Page No 76:

Question 1:

Give two main uses each of

(a) sulphuric acid
(b) hydrochloric acid
(c) nitric acid

Answer:


(a) Sulphuric acid (H2SO4):
1. It is used to manufacture fertilizers such as ammonium sulphate and superphosphate.
2. It is also used to manufacture detergents, paints, drugs, plastics, paper, etc.
 
(b) Hydrochloric acid (HCl): 
1. It is widely used in the oil industries to dissolve the rocks containing oil.
2. It is also used to manufacture aqua regia. The aqua regia comprises of 3 parts of hydrochloric acid and 1 part of nitric acid. It is mainly used to dissolve noble metals such as gold and platinum.
 
(c) Nitric acid (HNO3):
1. It is used to manufacture fertilizers such as ammonium nitrate.
2. It is also used to manufacture explosives such as TNT (trinitrotoluene) and nitroglycerine.

Page No 76:

Question 2:

What do you mean by a neutralization reaction? How can a neutralization reaction be used to prepare common salt?

Answer:

Neutralization reaction is the process in which an acid reacts with a base to produce salt and water while evolving a good amount of heat. In this reaction, both the acid and base loose their properties to produce a new substance which is neutral in nature, i.e., the salt formed will neither be acidic nor basic.
Acid + Base  Salt + Water with evolution of heat

The preparation of common salt (sodium chloride) involves the neutralization reaction between hydrochloric acid and sodium hydroxide (base). The acid and base reacts to produce sodium chloride (salt), water and heat.

The reaction is represented by the following equation:
                      HCl + NaOH  NaCl+ H2O

Page No 76:

Question 3:

Some acids are dangerous, other are not. Explain, giving examples.

Answer:

Acids can be classified into mineral acids and organic acids.
Most of the mineral acids are strong and corrosive in nature. They are extremely dangerous to handle. Strong acids (highly concentrated due to less amount of water in them) can easily melt paper, wool, wood and cloth. If they come in contact with our skin, they cause severe burns to it (carbonic acid is an exception in this case). Since srong acids can corrode even metals like iron, aluminium, etc., they are stored in glass containers. Hence, we should be highly cautious while handling strong acids.

Examples: Hydrochloric acid, nitric acid, sulphuric acid, etc.

Organic acids are naturally occurring weak acids. They are non-corrosive in nature, and are safe to handle even in their concentrated state.

Examples: Acetic acid (present in vinegar), Lactic acid (present in milk), Citric acid (present in citrus fruits), etc.

Page No 76:

Question 4:

What are bases? What are their physical properties?

Answer:

Bases are substances that have a soapy feel and bitter taste. Bases help in neutralizing the acidity by forming salt and water.
Like acids, bases can also be strong and weak. Sodium hydroxide and potassium hydroxide are strong bases which can burn our skin; but magnesium hydroxide and copper hydroxides are weak bases, and safe to handle even in their concentrated state.

Some of the properties of bases are listed below:
1. Most of the bases are metal hydroxides.
2. They are bitter in taste.
3. They are soapy in touch.
4. They turn red litmus blue.
5. Their reaction with acids gives salt and water.

Page No 76:

Question 5:

Give two main uses each of

(a) calcium hydroxide
(b) ammonium hydroxide
(c) sodium hydroxide

Answer:

Two important uses of each base are stated below:

(a) Calcium hydroxide [
Ca(OH)2]:
1. It is used to whitewash buildings; it is also used as an alternative to cement in low-budgeted construction.
2. It is also used to neutralise the acidity of soil. Because of the over usage of chemical fertilisers, soil becomes acidic and hence largely affects the growth of crops. Thus, calcium hydroxide is added to the soil to neutralise its acidity, thereby ensuring the healthy growth of the crops.
 
(b) Ammonium hydroxide (
NH4OH):
1. It is widely used to produce fertiliser like ammonium nitrate.
2. It is also used to produce nylons, plastics, dyes, etc.
 
(c) Sodium hydroxide (NaOH):
1. It is extensively used in the production of soaps.
2. It is used to produce various medicines, rayon, paper, etc.

Page No 76:

Question 6:

State two methods by which salts can be prepared. Give one example of each.

Answer:

Salts which are neutral substances can be prepared from the following methods:

1. Neutralization Reaction – It is the reaction between an acid and a base to produce salt, water and heat.

Example: Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride (common salt), water and heat.

The reaction is represented by teh following equation:

                         HCl +NaOH  NaCl + H2OHCl +NaOH  NaCl + H2O

2. Reaction of metals with acids - Metals react with acids to replace hydrogen from the acids, thereby producing salts and liberating hydrogen gas.

Example: Zinc reacts with dilute sulphuric acid to form zinc sulphate salt liberating hydrogen gas.

The reaction is given below:

             
Zn + dilH2SO4  ZnSO4 + H2O

Page No 76:

Question 7:

What is soap? How can you make soap in the laboratory?

Answer:

The sodium salts of acids are called soaps. A soap is prepared by boiling vegetable oil or fats from animals along with sodium hydroxide (NaOH).

We can prepare a soap in the laboratory by following the below mentioned procedure.

Required materials:

20ml of castor oil (we can also use coconut oil or any other oil)
Half teaspoon of sodium hydroxide pellets
1 teaspoon of common salt (sodium chloride), and
Glass rod for stirring

Procedure :

  • Take the 20ml castor oil in a beaker.
  • Mix ½ teaspoon of sodium hydroxide pellets with about 20ml of water to prepare the sodium hydroxide solution.
  • Now, mix the oil with the sodium hydroxide solution.
  • Heat the mixture and allow it to boil for about 5 to 10 minutes by constantly stirring it with the glass rod.

The oil will react with the sodium hydroxide solution to form soap and glycerine.
 
                      Oil + Sodium hydroxide  Soap + Glycerine
  • The soap formed can be separated from the mixture by adding a tea spoon of common salt (sodium chloride) in the beaker.
  • After cooling, the solid soap gets separated from the mixture by floating on the top. Thus, a soap is prepared in the laboratory.

Page No 76:

Question 8:

How are salts named? Give two examples.

Answer:

The name of a salt is derived on the basis of the reactants involved in its preparation. The salts are named after the metal supplied by the base and the radical supplied by the acid.

Example 1: Sodium chloride (common salt) is formed from hydrochloric acid and sodium hydroxide. The name ‘sodium’ is supplied by the metal of the sodium hydroxide, and the name ‘chloride’ is supplied by the acid, which is a negative radical.

                    

                                 HCl + NaOH  NaCl + H2O
 
Example 2: Ammonium sulphate (salt) is formed from sulphuric acid and ammonium hydroxide. The name ‘ammonium’ is supplied by the ammonium radical of the base, while the name ‘sulphate’ is supplied by the negative radical of sulphuric acid.
                                       

H2SO4 + NH4OH  (NH4)2SO4 + H2O

Page No 76:

Question 4:

When water is added to a strong acid, it becomes dilute. True or false?

Answer:

False.
When water is added to a strong acid, the reaction will evolve heat which is extremely dangerous. Hence, to dilute a strong acid, the acid should be slowly added to the water followed by regular stirring.

Page No 76:

Question 5:

Name one mineral acid.

Answer:

Sulphuric acid is a mineral acid.

Page No 76:

Question 6:

Milk contains________acid.

Answer:

Milk contains lactic acid.

Page No 76:

Question 7:

Which gas is produced when sodium carbonate reacts with dilute hydrochloric acid?

Answer:

Carbon dioxide is released when sodium carbonate reacts with dilute hydrochloric acid.
               

NaCO3 + dilHCl  NaCl + H2O + CO2

 

Page No 76:

Question 8:

Name the reaction between an acid and a base which forms a salt and water.

Answer:

The reaction between an acid and a base which results into salt and water is called the neutralization reaction.

Page No 76:

Question 9:

Which acid is used in the manufacture of the fertilizer superphosphate?

Answer:

Sulphuric acid is used to manufacture superphosphate. It is done by reacting sulphuric acid with powdered rock phosphate.

Page No 76:

Question 10:

An_________is a base that is solube in water.

Answer:

An alkali is a base that is soluble in water.

Page No 76:

Question 11:

Which base does not have a metal atom in its molecule?

Answer:

Ammonia is a base which has no metal atom in its molecule. As an alternative of metal, ammonia has the ammonium radical NH4+ in it.

Page No 76:

Question 12:

Is sodium hydroxide an acid or a base?

Answer:

Sodium hydroxide is a strong base, and not an acid.

Page No 76:

Question 13:

Turmeric turns_________in acidic solutions.

Answer:

Turmeric turns pale yellow in an acidic solution.

Page No 76:

Question 14:

If you are a manufacture of soap, which base would you require?

Answer:

Sodium hydroxide is the base which is used to manufacture soaps.

Page No 76:

Question 15:

Which base will you take as medicine, if you are having a stomachache due to indigestion?

Answer:

Magnesium hydroxide is the base (antacid) which can be used as a medicine to cure stomach pain due to indigestion. It is also called the milk of magnesia.

Page No 76:

Question 16:

A salt is formed when a metal displaces hydrogen from an acid. True of false?

Answer:

True
When a metal reacts with an acid, it replaces and liberates hydrogen from the acid to form salts.

Example: Magnesium + Sulphuric acid  Magnesium sulphate + Hydrogen gas

Page No 76:

Question 17:

What is the colour of litmus paper in an alkali?

Answer:

Alkali is a base; therefore, it will turn a red litmus paper blue.

Page No 76:

Question 1:

If you get a turmeric (haldi) stain on your clothes while eating food, and try to wash it off with soap, the stain becomes red. Why? Find out how to remove haldi stains from clothes.

Answer:

Turmeric is a natural acid-base indicator. It turns red in a basic solution and pale yellow in an acidic solution.
Therefore, the turmeric stain will turn red after coming in contact with the soap, indicating the basic nature of the soap (generally soaps are bases – containing sodium hydroxide).
Turmeric contains a type of yellow pigment which is responsible for the staining the clothes.
These stains can be removed using the following methods:
1. Fresh stains can be removed easily by immediately washing them with a soap. However, if there is a delay in washing, the stains will be hard to remove in a single wash. In this case, the stains need to be washed repeatedly using a soap.
2. By applying lemon juice or vinegar on these stains along with a dish soap can help remove the turmeric stains in a better way. 
3. By soaking the stained part in the soda water (baking soda + water) for half an hour, will help in removing the turmeric stains easily.

Page No 76:

Question 2:

Is the meaning of 'weak acid' the same as that of 'dilute acid'?

Answer:

Weak acids are completely different from dilute acids. Weak acids are non-corrosive in nature even in their highly concentrated state. Generally, all organic acids like citric acid, lactic acid, oxalic acid, etc., are weak acids. However, dilute acids are also non-corrosive in nature but are formed by mixing concentrated acids with a large amount of water. For example, dilute hydrochloric acid, dilute sulphuric acid, etc. Dilute acids can include both strong and weak acids.



Page No 77:

Question 3:

The wastes of many factories contain acids. Why is it considered necessary to neutralize them before allowing the wastes to flow into water bodies?

Answer:

Acids are extremely corrosive in nature; therefore, they can cause extensive damage to the living organisms present in water. Also, if the wastes are allowed to flow into the water bodies without treatment, they can pollute the water, making it unfit for drinking. Hence, the wastes should be neutralised by bases before releasing them into water bodies. The base calcium hydroxide (Ca(OH)2) is widely used to treat wastes, containing acids, from the industries.



View NCERT Solutions for all chapters of Class 7