iGCSE Chemistry Multiple Choice (Extended) : 12.5 Identification of ions and gases: Exam Style Questions Paper 2

Question

X is a white powder. The following tests are done on X.

When a few drops of aqueous sodium hydroxide are added to a solution of X, no precipitate is seen.
When X is heated with aqueous sodium hydroxide, no gas is formed.
X gives a lilac colour when put into a flame.
When acidified aqueous silver nitrate is added to a solution of X, a yellow precipitate is seen.

What is X?

A) ammonium bromide
B) ammonium iodide
C) potassium bromide
D) potassium iodide

▶️ Answer/Explanation

Solution

Ans: D

Let’s analyze each test result:

1. No precipitate with NaOH – Rules out most metal cations except Group I (like potassium).

2. No gas when heated with NaOH – Rules out ammonium compounds (which would release ammonia gas).

3. Lilac flame test – Characteristic of potassium ions (K⁺).

4. Yellow precipitate with acidified AgNO₃ – Characteristic of iodide ions (I⁻), forming silver iodide (AgI). Bromide would give cream precipitate.

Therefore, X must be potassium iodide (KI).

Option A and B are ammonium salts (ruled out by test 2). Option C is potassium bromide but would give cream precipitate, not yellow.

Question

Which row describes a test and the observation for aqueous sulfate ions?

	test	observation
A	add dilute nitric acid	a gas is produced which turns limewater cloudy
B	add dilute nitric acid and aqueous barium nitrate	white precipitate forms
C	add dilute nitric acid and aqueous potassium manganate(VII)	solution decolourises
D	add dilute nitric acid and aqueous silver nitrate	white precipitate forms

▶️ Answer/Explanation

Solution

Ans: B

The standard test for sulfate ions (SO₄^2-) involves:

First adding dilute nitric acid to remove any carbonate or sulfite ions that might interfere
Then adding barium nitrate solution
A positive result is the formation of a white precipitate of barium sulfate (BaSO₄)

Why other options are incorrect:

A: Gas turning limewater cloudy indicates carbonate, not sulfate
C: Decolorization of manganate(VII) indicates a reducing agent, not specifically sulfate
D: Silver nitrate tests for halides (chloride, bromide, iodide), forming white/slightly colored precipitates

The barium sulfate precipitate is particularly distinctive because it’s insoluble in both water and acids, making this a very reliable test for sulfate ions.

Question

Which statement describes how a flame test is done?

A The tip of a clean wire is dipped into the substance and the wire is placed in a blue Bunsen burner flame.
B The tip of a clean wire is dipped into the substance and the wire is placed in a yellow Bunsen burner flame.
C A wooden splint is lit and is placed above a test-tube containing the gas being tested.
D A wooden splint is lit, blown out and the glowing splint put into a test-tube of the gas being tested.

▶️ Answer/Explanation

Solution

Ans: A

The correct procedure for a flame test is:

1. Use a clean wire loop (usually platinum or nichrome) to avoid contamination

2. Dip the wire into the test substance (often moistened with hydrochloric acid)

3. Place the wire in the blue (hottest) part of a Bunsen flame

4. Observe the characteristic color produced

Option B is incorrect because the yellow flame contains soot particles that would interfere with the test. Options C and D describe gas tests, not flame tests for metal ions.

Question

Which statements about lime are correct?
1 Lime is made by heating calcium carbonate (limestone).
2 Lime is used to desulfurise flue gases.
3 Lime is used to treat alkaline soil.
4 The chemical name for lime is calcium oxide.

A. 1 and 3 B. 1, 2 and 4 C. 1 and 4 only D. 2, 3 and 4

Answer/Explanation

Ans:

iGCSE Chemistry Multiple Choice (Extended) : 12.5 Identification of ions and gases: Exam Style Questions Paper 2

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