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Acids Bases and Salts (Chemistry) Class 7 - NCERT Questions
Q 1.

State differences between acids and bases.

SOLUTION:

Q 2.

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

SOLUTION:

Ammonia is basic in nature. Products like window cleaner contains ammonium hydroxide, which is a base.

Q 3.

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

SOLUTION:

Litmus solution is extracted from lichens. It is used as an indicator. When extracted, it has a light purple colour. When added to acids it turns red while with bases it turns blue.

Q 4.

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

SOLUTION:

Distilled water is neutral in nature. To verify it, take a small amount of distilled water in two test tubes. Add blue litmus paper to one test tube and red litmus paper to the other test tube. It is observed that there is no change in colour of either blue or red litmus paper which shows distilled water is neither acidic nor basic, hence neutral.

Q 5.

Describe the process of neutralisation with the help of an example.

SOLUTION:

Neutralisation is a process in which an acid reacts with a base to produce salt and water. Take some dilute hydrochloric acid in a test tube and add 2-3 drops of phenolphthalein. The solution will remain colourless. Add to this acidic solution, sodium hydroxide solution with the help of a dropper and shake the test tube after adding each drop. The pink colour which appears keeps disappearing on shaking. Stop adding sodium hydroxide drop when the pink colour does not disappear. This is the point where neutralisation reaction has taken place. After this, if you keep on adding sodium hydroxide the solution will remain pink since it is basic in nature.

Q 6.

Mark ‘T’ if the statement is true and ‘F’ if it is false.
(i) Nitric acid turns red litmus blue. (T/F)
(ii) Sodium hydroxide turns blue litmus red. (T/F)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions.(T/F)
(v) Tooth decay is caused by the presence of a base. (T/F)

SOLUTION:

(i) F, Nitric acid turns blue litmus red.
(ii) F, Sodium hydroxide turns red litmus blue.
(iii) T
(iv) T
(v) F, Tooth decay is caused by the presence of an acid.

Q 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?

SOLUTION:

Dorji can test a small amount of drink with litmus paper. The drink which turns blue litmus red is acidic, the drink which turns red litmus blue is basic and the drink which does not change the colour of the litmus paper is neutral drink.

Q 8.

Explain why:
(A) An antacid tablet is taken when you suffer from acidity.
(B) Calamine solution is applied on the skin when an ant bites.
(C) Factory waste is neutralised before disposing it into water bodies.

SOLUTION:

(A) An antacid tablet contains a mild base like magnesium hydroxide which neutralises the excess of hydrochloric acid present in the stomach.
(B) The sting of an ant is acidic in nature since it contains formic acid. This is neutralised by applying calamine solution which contains zinc carbonate.
(C) Factory waste contains harmful acids and if it is disposed off in water bodies without neutralising, it will be harmful for aquatic plants and animals.

Q 9.

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

SOLUTION:

Take small amount of the liquids and add turmeric indicator. The solution which gives red colour with turmeric is sodium hydroxide. Now add small amount of the remaining solutions to water. The solution which becomes warm on adding water is hydrochloric acid and the third liquid is sugar solution.

Q 10.

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

SOLUTION:

The solution can be either a base or a neutral solution. Acidic solutions change blue litmus red, hence it is not acidic. Bases change red litmus to blue hence there is no change in colour of blue litmus paper. Neutral solutions have no effect on litmus paper.

Q 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 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 indicator.
Which of these statements are correct?
(i) All four (ii) a and d (iii) b and c (iv) only d

SOLUTION:

(iv)