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Page No 74:

Question A:

Solve the following crossword puzzle:
figure

Across
2. Bases which are soluble in water
4. This indicator is also used as a spice
5. It is a substance which is neither acidic nor basic

Down
1. It is used to treat indigestion in humans
3. It is the most common indicator

Answer:

Page No 74:

Question B.1:

Which of the following is present in milk of magnesia?
(a) magnesium dioxide
(b) magnesium hydroxide
(c) magnesium chloride
(d) magnesium oxide

Answer:

(b) magnesium hydroxide

Magnesium hydroxide is present in milk of magnesia.

Page No 74:

Question B.2:

Which is a strong acid?
(a) acetic acid
(b) citric acid
(c) sulphuric acid
(d) lactic acid

Answer:

(c) sulphuric acid

Acids like sulphuric acid, nitric acid and hydrochloric acid are strong.

Page No 74:

Question B.3:

Which is a neutral acid?
(a) hydrochloric acid
(b) common salt
(c) sodium hydroxide
(d) calcium hydroxide

Answer:

(b) common salt

Common salt is a neutral substance.

Page No 74:

Question B.4:

Which is not an indicator?
(a) blue litmus
(b) methyl orange
(c) beetroot
(d) antacid

Answer:

(d) antacid

Antacid is not an indicator.

Page No 74:

Question B.5:

The common name of sodium hydroxide is
(a) caustic soda
(b) baking soda
(c) quick lime
(d) slaked lime

Answer:

(a) caustic soda

The common name of sodium hydroxide is caustic soda.

Page No 74:

Question B.6:

Chemical name of slaked lime is
(a) sodium hydroxide
(b) sodium bisulphate
(c) sodium bicarbonate
(d) calcium hydroxide

Answer:

(d) calcium hydroxide

The chemical name of slaked lime is calcium hydroxide.

Page No 74:

Question B.7:

Cold drinks contain
(a) carbonic acid
(b) maleic acid
(c) citric acid
(d) lactic acid

Answer:

(a) carbonic acid

Cold drinks contain carbonic acid.

Page No 74:

Question C:

Fill in the blanks:
1. Acids are .............................. in taste.
2. .............................. is used in petroleum refining.
3. Bases are .............................. to touch.
4. .............................. and .............................. are organic acids.
5. Chemical name of baking soda is .............................. .

Answer:

1. Acids are sour in taste.
2. Sulphuric acid is used in petroleum refining.
3. Bases are soapy to touch.
4. Oxalic acid and formic acid are organic acids.
5. Chemical name of baking soda is sodium bicarbonate.



Page No 75:

Question 1:

Two of the most commonly used salts are sodium chloride (NaCI) and sodium bicarbonate (NaHCO3). Find out which of these is used (i) as baking powder, (ii) as a preservative in pickles and in preserving meat and fish and also think of the respective reasons for doing so.

Answer:

(i) Sodium bicarbonate (NaHCO3) is used as baking powder. 
When added to a batter, sodium bicarbonate reacts with the acidic components present in it and releases carbon dioxide. This causes expansion of the batter, which gives a characteristic texture to it.

(ii) Sodium chloride (NaCl) is used as a preservative in pickles. It is also used in preserving meat and fish. 
Being a neutral substance, sodium chloride does not bring any change, either acidic or basic, when added to a food substance. In this way, the food substance can be preserved for a long period of time.

Page No 75:

Question 2:

Why does the Taj Mahal, a great Indian monument, turn yellow?

Answer:

The Taj Mahal is turning yellow because of acid rain. When excesses of carbon dioxide, sulphur dioxide and nitrogen dioxide are released into the atmosphere, they react with the moisture present in the air forming carbonic acid, sulphuric acid and nitric acid, and fall on the Earth in the form of acid rain. Acid rains not only cause damage to the living but also to the buildings, particularly the ones made of marble, like the Taj Mahal.

Page No 75:

Question D:

Match the items in Column A with the items in Column B:

Column A Column B
1. Lemons (a) Sodium hydroxide
2. Slaked lime (b) Maleic acid
3. Apple (c) Sodium carbonate
4. Souring of milk (d) Calcium hydroxide
5. Caustic soda (e) Lactic acid
  (f) Citric acid

Answer:

Column A Column B
1. Lemons (f) Citric acid
2. Slaked lime (d) Calcium hydroxide
3. Apple (b) Maleic acid
4. Souring of milk (e) Lactic acid
5. Caustic soda (a) Sodium hydroxide

Page No 75:

Question E:

Write True (T) or False (F) against the following statements in the given brackets:
1. Acids corrode metals. ( )
2. Distilled water is a neutral substance. ( )
3. Sodium hydroxide turns blue litmus red. ( )
4. The stings of bees contain an acid. ( )
5. Milk of magnesia is used in antacids. ( )

Answer:

1. True

2. True

3. False
    Blue litmus turns red only in acidic conditions. Sodium hydroxide is a base that turns red litmus blue.

4. True

5. True

Page No 75:

Question F:

Complete the following:

figure

Answer:



Page No 76:

Question 3:

How do toothpastes prevent tooth decay?

Answer:

Tooth decay occurs due to bacterial action that produces acids. These acids attack the outer layer of the teeth causing damage. Toothpastes wash off this acidic layer and prevent the teeth from decaying.

Page No 76:

Question 4:

Should acids be stored in metallic containers? Why?

Answer:

Acids should not be stored in metallic containers because they are highly corrosive in nature. They have the capacity to corrode metals and hence can damage the metallic containers if stored in them.



Page No 77:

Question A.1:

What are alkalis?

Answer:

Alkalis are bases which are highly soluble in water. For example, sodium hydroxide and potassium hydroxide are alkalis.

Page No 77:

Question A.2:

Name two mineral acids?

Answer:

Sulphuric acid and nitric acid are examples of mineral acids.

Page No 77:

Question A.3:

How would you define a salt?

Answer:

Salt is a neutral substance that is obtained in a process called neutralisation, where an acid completely reacts with a base.

Page No 77:

Question A.4:

Which acid is used for manufacturing super phosphate fertiliser?

Answer:

Sulphuric acid is used in the manufacture of super phosphate fertiliser.

Page No 77:

Question A.5:

Which acid is present in milk?

Answer:

Lactic acid is present in milk.

Page No 77:

Question A.6:

Name the acid which is used in purification of common salt.

Answer:

Hydrochloric acid is used in the purification of common salt.

Page No 77:

Question A.7:

What are neutral substances? Give examples.

Answer:

Neutral substances are neither acidic nor basic in nature. They do not cause any colour change in the indicators.
Common salt and sugar are examples of neutral substances.

Page No 77:

Question B.1:

Why are calamine lotions applied on insect bites?

Answer:

Insect stings contain acids, which causes pain when an insect bites. Hence, insect stings are treated with a substance that can neutralise this effect. Calamine lotions mainly consists of zinc carbonate that acts as a neutraliser and reduces the pain.

Page No 77:

Question B.2:

What is aqua regia? Which acid is present in it?

Answer:

Aqua regia is an acidic solution that is used to dissolve some of the noble metals like gold and platinum. It mainly consists of hydrochloric acid and nitric acid.

Page No 77:

Question B.3:

Why is milk of magnesia used in antacids?

Answer:

Milk of magnesia is used in antacids because it consists of magnesium hydroxide that neutralises the excess hydrochloric acid present in the stomach, thus reducing acidity. The presence of hydrochloric acid in excessive amounts in the stomach causes acidity, which damages the inner walls of the stomach causing painful wounds. Hence, antacids are used to treat acidity.

Page No 77:

Question B.4:

Which substance is used to differentiate between acids and bases? Define it.

Answer:

The substances used to differentiate between acids and bases are called as indicators. An indicator can be defined as a substance that shows a significant change in colour on coming into contact with acids or bases.

Page No 77:

Question B.5:

Give two differences between an acid and a base.

Answer:

  Acid   Base
Acids are sour in taste. Bases are bitter in taste.
Acids turn blue litmus solution red. Bases turn red litmus solution blue.

Page No 77:

Question B.6:

Give two used of each:
(a) Nitric acid
(b) Sulphuric acid

Answer:

(a) Uses of nitric acid:
(i) It is most commonly used in the manufacture of fertilizers and explosives.
(ii) It is also used in the extraction of metals from their ores.

(b) Uses of sulphuric acid:
(i) It is extensively used in the manufacture of paints, drugs, detergents, etc.
(ii) It is also used in petroleum refining.

Page No 77:

Question B.7:

What is neutralisation reaction? Give an example from everyday life.

Answer:

It is a reaction in which an acid reacts completely with a base producing salt and water. This reaction nullifies the effects of acids and bases and hence it is called neutralisation reaction.

Example:     HCl    +     NaOH      →     NaCl     +     H2O

Page No 77:

Question B.8:

Which alkali is used to reduce the acidity of soil?

Answer:

Slaked lime (calcium hydroxide) or quick lime (calcium oxide) is used to reduce the acidity of soil.

Page No 77:

Question C.1:

How can factory wastes be treated before being discharged into water bodies?

Answer:

Liquid wastes from factories and industries contain acids, which when directly led into the water resources cause water pollution, resulting in the deaths of fishes and other aquatic lives. To prevent this, factory wastes should to be treated with a base like slaked lime, which neutralises the acidic effects.

Page No 77:

Question C.2:

What do you mean by indigestion? How can indigestion be cured?

Answer:

Indigestion is caused by acidity, which in turn is caused due to the excess secretion of hydrochloric acid in the human stomach. Acidity damages the inner walls of the stomach causing painful wounds. This can be cured using antacids. Antacids mainly consist of milk of magnesia or magnesium hydroxide that neutralises the excess hydrochloric acid present in the stomach, thus reducing the acidity.

Page No 77:

Question C.3:

Is the soap solution acidic, basic or neutral? How would you verify it?

Answer:

Generally, soaps and, therefore, soap solutions are basic in nature. This can be easily identified by using the natural indicator turmeric. Bases turn turmeric red.
Prepare a turmeric paste by adding a few drops of water to turmeric powder. Now, take a piece of paper and dip it into the turmeric paste. Allow it to dry in the sunlight. When the paper is completely dried up, add a drop of soap solution on to the paper and observe. The region containing the soap solution turns red, thereby indicating the basic nature of the soap solution.

Page No 77:

Question C.4:

List four properties of acids and bases.

Answer:

Properties of acids:
(i) Acids are sour in taste.
(ii) They are soluble in water.
(iii) Acids turn blue litmus paper red.
(iv) They are good conductors of electricity.

Properties of bases:
(i) Bases are bitter in taste.
(ii) They are soapy to touch.
(iii) Some of them are soluble (alkalis), while some are insoluble in water.
(iv) Bases turn red litmus paper blue.



Page No 78:

Question C.5:

Explain the following:
(a) A person suffering from acidity takes antacid tablets.
(b) Slaked lime is added to the industrial waste before it is discharged into the river.

Answer:

(a) Antacid tablets mainly consist of milk of magnesia or magnesium hydroxide, which neutralises the excess hydrochloric acid present in the stomach, thus reducing acidity. Hence, a person suffering from acidity takes antacid tablets.

(b) Liquid wastes from factories and industries contain acids, which when directly led into the water resources cause water pollution resulting in the death of fishes and other aquatic lives. So, to prevent this, factory wastes have to be treated with a base, like slaked lime, which neutralises the acidic effects.

Page No 78:

Question C.6:

What are indicators? With the help of an activity show that turmeric acts as an indicator.

Answer:

Indicators are substances that show a change in colour on coming into contact with acids or bases.
Prepare a turmeric paste by adding a few drops of water to turmeric powder. Now, take a piece of paper and dip it into the turmeric paste and allow it to dry in the sunlight. When the paper is completely dried up, add a drop of soap solution on to the paper and observe. The region containing the soap solution turns red, indicating the basic nature of the soap solution. Hence, turmeric acts as an indicator.



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