Class 10 Science

# Acids Bases and Salts

## NCERT Solution

### Part 2

Question 9: Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is

(a) neutral?

Answer: D is neutral, as its pH value is equal to 7

(b) strongly alkaline?

Answer: C is strongly alkaline, as its pH value is 11.

(c) strongly acidic?

Answer: B is strongly acidic, as its pH value is equal to 1

(d) weakly acidic?

Answer: A is weakly acidic, as its pH value is equal to 4 and more than solution B.

(e) weakly alkaline?

Answer: E is weakly alkaline, as its pH value is equal to 9 and less than C.

pH value of the given solution in Increasing order of hydrogen-ion concentration: 11 < 9 < 7 < 4 < 1

Question 10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Answer: In test tube A fizzing occurs more vigorously. Because HCl is stronger acid than acetic acid. Hence, HCl liberates hydrogen gas more vigorously, which causes fizzing more vigorously.

Question 11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer: The value of pH of milk decreases, when it turns into curd. Curd is acidic in nature, because an acidic substance has less value of pH.

Question 12: A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

Answer: By making the milk more alkaline, it takes more time to turn into curd. That&apso;s why milkman shifts the pH of fresh milk to slightly alkaline by adding a very small amount of baking soda to it.

(b) Why does this milk take a long time to set as curd?

Answer: As this milk is slightly more alkaline than other, therefore, acid produced to set into curd will be neutralized by baking soda added by milkman. Hence, this milk takes a longer time to set as curd.

Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer: Plaster of Paris turns into Gypsum after reacting with moisture present in air. That's why Plaster of Paris should be stored in a moisture-proof container.

Question 14: What is a neutralisation reaction? Give two examples.

Answer: The reaction between an acid and a base is called neutralisation reaction. When acid is added to base, both neutralise each other and produce salt and water.

Example 1: When Sodium hydroxide is added to hydrochloric acid. Sodium chloride and water are formed.

NaOH + HCl ⇨ NaCl + H2O

Example 2: Milk of magnesia, which is a base, is given as antacid in the case of indigestion, to neutralize the excess acid produced in stomach.

Mg(OH)2 + 2HCl ⇨ MgCl2 + 2H2O

Question 15: Give two important uses of washing soda and baking soda.

Answer: some important uses of washing soda and baking soda are as follows:

1. Washing soda is used to remove the permanent hardness of water.
2. Washing soda is used in making soap.
3. Baking soda is used as baking powder, in the making of bread, and other many items.
4. Baking soda is used in fire – extinguisher.