NCERT Class 7 Science Fifth Chapter Acids, Bases and Salts Exercise Solution

Inside Questions With Answers:

Why Curd, lemon juice, orange juice and vinegar taste sour?

Ans. This is because these substances contain acids.

What is bases?

Ans. Substances which are bitter in taste and feel soapy on touching are known as bases.

What is called indicators? Give Example.

Ans. Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators.

Example: Turmeric, litmus, china rose.

In where Acetic acid is found?

Ans. Vinegar.

In where Formic acid is found?

Ans. Ant’s sting.

In where Citric acid is found?

Ans. Citrus fruits such as oranges, lemons, etc.

In where Lactic acid is found?

Ans. Curd.

In where Oxalic acid is found?

Ans. Spinach.

In where Ascorbic acid is found?

Ans. Amla, Citrus fruits.

In where Tartaric acid is found?

Ans. Tamarind, grapes, unripe mangoes, etc.

In where Calcium hydroxide base is found?

Ans. Lime water.

In where Ammonium hydroxide base is found?

Ans. Window cleaner.

In where Sodium hydroxide/ Potassium hydroxide base is found?

Ans. Soap.

In where Magnesium hydroxide base is found?

Ans. Milk of magnesia.

What is called neutral solutions?

Ans. The solutions which do not change the colour of either red or blue litmus are known as neutral solutions.

Define neutralisation.

Ans. The reaction between an acid and a base is known as neutralisation.

Exercises

(1) State differences between acids and bases.

Acids	Bases
(i) Acids are sour to taste.(ii) Acid turns blue litmus to red.(iii) Acid is a substance which contains    hydrogen ion.	(i) Bases are bitter to taste.(ii) Base turns red litmus to red.(iii) Bases are substances which contain hydroxyl ion.

(2) Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

Ans: Ammonia has basic nature.

(3) Name the source from which litmus solution is obtained. What is the use of this solution?

Ans: Litmus solution is extracted from lichens. It is used to determine whether the given solution is acidic or basic.

(4) Is the distilled water acidic/basic/neutral? How would you verify it?

Ans: Distilled water be natural. We can verify it by showing that neither blue nor red litmus paper changes its colour when dipped in it.

(5) Describe the process of neutralisation with the help of an example.

Ans: The reaction between an acid and a base is known as neutralization. Salt and water are produced in this process with the evolution of heat.

Antacids like milk of magnesia, baking soda, etc which contain a base are used for reducing acidity in stomach when excessive acid released by glands.

(6) Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Nitric acid turn red litmus blue. = F

(ii) Sodium hydroxide turns blue litmus red. = F

(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. = T

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. = T

(v) Tooth decay is caused by the presence of a base. = F

(7) Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?

Ans: Dorji can decide with the help of litmus paper.

(i) The drink which would turn a red litmus blue would be basic.

(ii) If the drink which would turn a red litmus to red would be acidic.

(iii) the drink which would not affect both red and blue litmus would be neutral.

(8) Explain why:

(a) An antacid tablet is taken when you suffer from acidity

Ans: We take an antacid such as milk of magnesia to neutralizes the excessive acid released in stomach.

(b) Calamine solution is applied on the skin when an ant bites.

Ans: Ant injects an acidic liquid into the skin on biting which causes inflammation, to the skin.

(c) Factory waste is neutralised before disposing it into the water bodies.

Ans: The wastes of factories contain acids. If acids are disposed off in the water body, the acids will harm the organisms. So factory wastes are neutralized by adding basic substances.

(9) Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.

Ans: (i) Hydrochloric acid yellow to blue.

(ii) Sodium hydroxide yellow to red.

(iii) Sugar solution no change.

(10) Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

Ans: (i) It can identified on the basis of the following observations: Bases change the color of litmus paper to blue. As the color of blue litmus paper is not affected, the solution must be basic.

(ii) If the solution is neutral, even then color of litmus will not change.

(11) Consider the following statements:

(a) Both acids and bases change colour of all indicators.

(b) If an indicator gives a colour change with an acid, it does not give a change with a base.

(c) If an indicator changes colour with a base, it does not change colour with an acid.

(d) Change of colour in an acid and a base depends on the type of the indicator. Which of these statements are correct?

(i) All four          (ii) a and d              (iii) b, c and d             (iv) only d

Ans: (ii) (a) and (d)