Acids, Bases and Salts - Q&A

1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d)
Explanation:
Litmus turns blue in basic solutions. Bases have a pH value greater than 7.
(a) pH 1 is acidic.
(b) pH 4 is acidic.
(c) pH 5 is acidic.
(d) pH 10 is basic.
Therefore, the solution with pH 10 will turn red litmus blue.

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: (b)
Explanation:
Egg-shells are made of calcium carbonate (CaCO₃). When a metal carbonate reacts with an acid, it produces salt, water, and carbon dioxide gas (CO₂). Carbon dioxide gas turns lime water milky.
Among the options, HCl is the only acid.
Reaction: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

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 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)
Explanation:
10 mL of NaOH requires 8 mL of HCl.
20 mL of NaOH is double the initial amount (2 × 10 mL).
Therefore, it will require double the amount of HCl.
Required HCl = 2 × 8 mL = 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)
Explanation:
Indigestion is often caused by excess acid in the stomach. Antacids are mild bases that neutralise the excess acid, providing relief.

5. Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Answer:
(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen
3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Hydrochloric acid + Iron → Ferrous chloride + Hydrogen
2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer:
Activity:
1. Take solutions of glucose, alcohol, hydrochloric acid (HCl), and sulphuric acid (H₂SO₄).
2. Fix two nails on a cork and place the cork in a 100 mL beaker.
3. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.
4. Pour some dilute HCl in the beaker and switch on the current. Observe the bulb.
5. Repeat the experiment separately with glucose and alcohol solutions.

Observation:
- The bulb glows in the case of acids (HCl and H₂SO₄) because they dissociate into ions (H⁺) which conduct electricity.
- The bulb does not glow in the case of glucose and alcohol solutions because they do not dissociate into ions, even though they contain hydrogen.

Conclusion:
This proves that not all compounds containing hydrogen dissociate to produce H⁺ ions in solution, and therefore, they are not categorised as acids.

7. Why does distilled water not conduct electricity, whereas rain water does?
Answer:
Distilled water is a pure form of water and does not contain any dissolved salts or impurities. Therefore, it does not contain ions to conduct electricity.

Rain water, while falling to earth, dissolves acidic gases like carbon dioxide (CO₂), sulphur dioxide (SO₂), etc., from the atmosphere. These gases form mild acids (like carbonic acid) which dissociate into ions (like H⁺ and CO₃^2-). The presence of these free ions allows rain water to conduct electricity.

8. Why do acids not show acidic behaviour in the absence of water?
Answer:
Acids show acidic behaviour only due to the presence of hydrogen ions (H⁺). Acids produce H⁺ ions only in the presence of water through a process called dissociation or ionization.
Example: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl^-(aq)
In the absence of water, ionization does not occur, no H⁺ ions are formed, and thus acidic behaviour is not shown.

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?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Answer:
(a) Neutral: Solution D (pH 7)
(b) Strongly alkaline: Solution C (pH 11)
(c) Strongly acidic: Solution B (pH 1)
(d) Weakly acidic: Solution A (pH 4)
(e) Weakly alkaline: Solution E (pH 9)

Increasing order of hydrogen-ion concentration:
Hydrogen ion concentration is inversely proportional to pH. Lower pH means higher H⁺ concentration.
Order: pH 11 < pH 9 < pH 7 < pH 4 < pH 1
Solution Sequence: C < E < D < A < B

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:
The fizzing will occur more vigorously in test tube A containing Hydrochloric acid (HCl).

Reason:
Fizzing is due to the evolution of hydrogen gas (H₂) produced by the reaction between the metal and acid. HCl is a strong acid and dissociates completely to produce a high concentration of H⁺ ions. Acetic acid (CH₃COOH) is a weak acid and dissociates partially, producing a lower concentration of H⁺ ions. Higher H⁺ concentration leads to a faster reaction rate and more vigorous fizzing.

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:
As fresh milk turns into curd, its pH will decrease (become more acidic).

Explanation:
When milk turns into curd, bacteria (Lactobacillus) convert the lactose sugar present in milk into lactic acid. The formation of acid increases the concentration of H⁺ ions, which lowers the pH value below 6.

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?
(b) Why does this milk take a long time to set as curd?
Answer:
(a) He shifts the pH to slightly alkaline to prevent the milk from spoiling (turning sour) quickly. In alkaline conditions, the bacteria cannot easily produce acid, thus keeping the milk fresh for longer.

(b) This milk takes a longer time to set as curd because the added baking soda (a base) first neutralises the lactic acid produced by the bacteria. Only after the base is completely neutralised can the milk become acidic enough to turn into curd.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:
Plaster of Paris (CaSO₄.½H₂O) absorbs moisture from the air and reacts with water to form a hard solid mass called Gypsum (CaSO₄.2H₂O).
Reaction: CaSO₄.½H₂O + 1½H₂O → CaSO₄.2H₂O (Hard mass)
To prevent this setting and hardening, it must be stored in a moisture-proof container.

14. What is a neutralisation reaction? Give two examples.
Answer:
A reaction between an acid and a base to form salt and water is called a neutralisation reaction.
General Equation: Acid + Base → Salt + Water

Examples:
1. Sodium hydroxide reacting with Hydrochloric acid:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

2. Potassium hydroxide reacting with Nitric acid:
KOH(aq) + HNO₃(aq) → KNO₃(aq) + H₂O(l)

15. Give two important uses of washing soda and baking soda.
Answer:
Uses of Washing Soda (Sodium Carbonate Decahydrate):
1. It is used in glass, soap, and paper industries.
2. It is used for removing permanent hardness of water.

Uses of Baking Soda (Sodium Hydrogen Carbonate):
1. It is used as an ingredient in antacids to neutralise excess stomach acid.
2. It is used in making baking powder (a mixture of baking soda and a mild edible acid like tartaric acid), which is used to make cakes and bread soft and spongy.