iGCSE Chemistry Multiple Choice (Extended) : 7.1 The characteristic properties of acids and bases: Exam Style Questions Paper 2

Question

Which colours are seen when litmus and methyl orange are added to separate samples of aqueous sodium hydroxide?

	litmus	methyl orange
A	blue	orange
B	blue	yellow
C	purple	orange
D	purple	yellow

▶️ Answer/Explanation

Solution

Ans: B

Sodium hydroxide is a strong alkali. Litmus turns blue in alkaline solutions (pH > 8). Methyl orange turns yellow in alkaline solutions (pH > 4.4). Purple is the neutral color for litmus, which would be incorrect for NaOH. Orange for methyl orange would indicate an acidic solution, which is also incorrect for NaOH.

Question

Which row shows the difference between a weak acid and a strong acid?

	weak acid	strong acid
A	fully dissociated	partially dissociated
B	concentrated	dilute
C	dilute	concentrated
D	partially dissociated	fully dissociated

▶️ Answer/Explanation

Solution

Ans: D

1. The key difference between weak and strong acids is their degree of dissociation in water.

2. Strong acids completely dissociate into ions in water (e.g., HCl → H⁺ + Cl^–).

3. Weak acids only partially dissociate, establishing an equilibrium between dissociated and undissociated molecules (e.g., CH₃COOH ⇌ H⁺ + CH₃COO^–).

4. Options B and C confuse strength (degree of dissociation) with concentration (amount of acid per unit volume).

5. Option A reverses the definitions.

Question

Which substance turns methyl orange red?

A) aqueous ammonia
B) dilute hydrochloric acid
C) aqueous sodium hydroxide
D) distilled water

▶️ Answer/Explanation

Solution

Ans: B

1. Methyl orange is a pH indicator that turns red in acidic conditions (pH < 3.1).

2. Dilute hydrochloric acid is strongly acidic (pH ~1-2), so it turns methyl orange red.

3. Aqueous ammonia and sodium hydroxide are alkaline and would turn methyl orange yellow.

4. Distilled water is neutral (pH 7) and would give an intermediate orange color with methyl orange.

Question

Which statement about dilute hydrochloric acid is correct?

A) It is a strong acid as it fully dissociates.
B) It is a strong acid as it partially dissociates.
C) It is a weak acid as it fully dissociates.
D) It is a weak acid as it partially dissociates.

▶️ Answer/Explanation

Solution

Ans: A

Hydrochloric acid is a strong acid, meaning it completely (fully) dissociates in aqueous solution to form H⁺ and Cl⁻ ions. Therefore, option A is correct. The terms “strong” and “weak” refer to the degree of dissociation, not concentration – strong acids fully dissociate regardless of concentration (so B and D are wrong). Option C is contradictory as weak acids don’t fully dissociate.

iGCSE Chemistry Multiple Choice (Extended) : 7.1 The characteristic properties of acids and bases: Exam Style Questions Paper 2

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