Acids Bases and Salts

NCERT Exemplar Problems

MCQs Part 1

Question 1: What happens when a solution of an acid is mixed with a solution of a base in a test tube?

1. The temperature of the solution increases.
2. The temperature of the solution decreases.
3. The temperature of the solution remains the same.
4. Salt formation takes place

1. (i) only
2. (i) and (iii)
3. (ii) and (iii)
4. (i) and (iv)
   Answer: (d) (i) and (iv)

Explanation: When aqueous solution an acid is mixed with aqueous solution of a base, then neutralization reaction takes place. Neutralization reaction is exothermic in nature and results in formation of salt.

Question 2: An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

1. Baking powder
2. Lime
3. Ammonium hydroxide solution
4. Hydrochloric acid
   Answer: (d) Hydrochloric acid

Explanation: Since the given solution is turning red litmus solution to blue, it is a basic solution. So, addition of an acid would reverse the change.

Question 3: During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

1. absorb the evolved gas
2. moisten the gas
3. absorb moisture from the gas
4. absorb Cl⁻ ions from the evolved gas
   Answer: (c) absorb moisture from the gas

Explanation: The humid day in first sentence gives the clue. Had it been some other option then guard tube should have been used in all cases, whether the day was humid or dry.

Question 4: Which of the following salts does not contain water of crystallisation?

1. Blue vitriol
2. Baking soda
3. Washing soda
4. Gypsum
   Answer: (b) Baking soda

Explanation: Baking soda is anhydrous form of sodium bicarbonate. Blue vitriol gets its characteristic blue color due to water of crystallization. Presence of two molecules of water of crystallization in one molecule of Gypsum makes it a hard substance, while removal of 1.5 molecules of water of crystallization makes it Plaster of Paris. Washing soda is prepared by hydrating soda ash.

Question 5: Sodium carbonate is a basic salt because it is a salt of

1. strong acid and strong base
2. weak acid and weak base
3. strong acid and weak base
4. weak acid and strong base
   Answer: (d) Weak acid and strong base

Explanation: Options a and b would give neutral salts, while option c would give acidic salt.

Question 6: Calcium phosphate is present in tooth enamel. Its nature is

1. basic
2. acidic
3. neutral
4. amphoteric
   Answer: (a) basic

Explanation: Calcium phosphate is formed after neutralization reaction between calcium hydroxide (strong base) and phosphoric acid (weak acid). So, it is a basic salt.

Question 7: A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?

1. Lemon juice
2. Vinegar
3. Common salt
4. An antacid
   Answer: (d) An antacid

Explanation: Yellowish-orange colour of pH paper indicates that soil sample is acidic. So, we need to add a base (antacid) to change the colour of pH paper to greenish blue. Another hint to solve this question can be, options a and b show acids, so they are incorrect options. Common salt cannot be an answer because common salt is neutral.

Question 8: Which of the following gives the correct increasing order of acidic strength?

1. Water < Acetic acid < Hydrochloric acid
2. Water < Hydrochloric acid < Acetic acid
3. Acetic acid < Water < Hydrochloric acid
4. Hydrochloric acid < Water < Acetic acid
   Answer: (a) Water < Acetic acid < Hydrochloric acid

Explanation: Water is neutral and acidic acid (organic acid) is a weak acid. Hydrochloric acid is a strong acid because it is inorganic acid, unlike acetic acid.

Question 9: If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

1. Wash the hand with saline solution
2. Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate
3. After washing with plenty of water apply solution of sodium hydroxide on the hand
4. Neutralise the acid with a strong alkali
   Answer: (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate.

Explanation: Washing with plenty of water will help in cooling down the burning sensation which happened due to acid. Paste of sodium hydrogen carbonate will help neutralize the effect of acid. A paste is always better than solution, in terms of sticking for considerable time. Hence, option c is incorrect.

Question 10: Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved?

1. It turns lime water milky
2. It extinguishes a burning splinter
3. It dissolves in a solution of sodium hydroxide
4. It has a pungent odour

1. (i) and (ii)
2. (i), (ii) and (iii)
3. (ii), (iii) and (iv)
4. (i) and (iv)
   Answer: (a) (i) and (ii)

Explanation: When sodium hydrogen carbonate reacts with an acid, it gives carbon dioxide gas. Carbon dioxide turns lime water milky. Carbon dioxide does not support combustion, so it extinguishes burning splinter.