10th Acids, Bases and Salt NCERT Solution

Answers to Science Questions

1. A solution turns red litmus blue. Is pH likely to be

(a) 1 (b) 4 (c) 5 (d) 10

Answer: (d) 10

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

Answer: HCl

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be

(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer: (d) 16 mL

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Answer: (c) Antacid

5. Write word equations and then balanced equations for the reaction taking place when-

(a) dilute sulphuric acid reacts with zinc granules.

Word equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Balanced equation: H₂SO₄ + Zn → ZnSO₄ + H₂

(b) dilute hydrochloric acid reacts with magnesium ribbon.

Word equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

Balanced equation: 2HCl + Mg → MgCl₂ + H₂

(c) dilute hydrochloric acid reacts with aluminium powder.

Word equation: Hydrochloric acid + Aluminium → Aluminium chloride + Hydrogen

Balanced equation: 6HCl + 2Al → 2AlCl₃ + 3H₂

(d) dilute hydrochloric acid reacts with iron filings.

Word equation: Hydrochloric acid + Iron → Iron chloride + Hydrogen

Balanced equation: 2HCl + Fe → FeCl₂ + H₂

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Answer: Take a small amount of alcohol or glucose in a test tube. Add a few drops of blue litmus solution. If the litmus color does not change, it shows they are not acids.

7. Why does distilled water not conduct electricity, whereas rainwater does?

Answer: Distilled water does not conduct electricity because it is pure and does not contain any ions or impurities. Rainwater conducts electricity because it contains dissolved gases like carbon dioxide, which form weak acids, and other impurities that create ions to carry the electric current.

8. Why do acids not show acidic behaviour in the absence of water?

Answer: Acids do not show acidic behaviour in the absence of water because they need water to release H⁺ ions, which are responsible for their acidic properties.

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: (a) D

(b) strongly alkaline?

Answer: (b) C

(c) strongly acidic?

Answer: (c) B

(d) weakly acidic?

Answer: (d) A

(e) weakly alkaline?

Answer: (e) E

10. Arrange the pH in increasing order of hydrogen-ion concentration.

Answer: 11, 9, 7, 4, 1

11. 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: Fizzing will occur more vigorously in test tube A because hydrochloric acid is stronger than acetic acid and releases H⁺ ions more quickly, causing a faster reaction with magnesium.

12. 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 pH will decrease as milk turns into curd because curd is more acidic due to the formation of lactic acid by bacteria.

13. 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: (a) He adds baking soda to shift the pH from 6 to slightly alkaline to prevent the milk from turning sour quickly by slowing the growth of bacteria.

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

Answer: (b) This milk takes a long time to set as curd because the slightly alkaline pH slows down the process of lactic acid formation by bacteria.

14. Plaster of Paris should be stored in a moisture-proof container. Explain why.

Answer: Plaster of Paris should be stored in a moisture-proof container because it reacts with water and hardens, making it unusable.

15. What is a neutralisation reaction? Give two examples.

Answer: A neutralisation reaction is when an acid reacts with a base to form salt and water.

Examples:

1. HCl + NaOH → NaCl + H₂O

2. H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O