A pale yellow precipitate and a gas with pungent odour are formed on warming dilute hydrochloric acid with an aqueous solution containing

Options

A. sulphate ion
B. sulphide ion
C. thiosulphate ion
D. sulphite ion

Explanation

To solve the problem, we need to identify which of the given ions will react with dilute hydrochloric acid to produce a pale yellow precipitate and a gas with a pungent odor. Let's analyze each option step by step.

### Step 1: Analyze the Options
We have four options to consider:
1. Sulfate ion (SO4^2-)
2. Sulfide ion (S2-)
3. Thiosulfate ion (S2O3^2-)
4. Sulfite ion (SO3^2-)

### Step 2: Reaction with Hydrochloric Acid
We need to consider how each of these ions reacts with dilute hydrochloric acid (HCl).

- **Sulfate ion (SO4^2-)**:
- When sulfate ions react with HCl, no precipitate or pungent gas is formed. The reaction does not produce any noticeable products.

- **Sulfide ion (S2-)**:
- Sulfide ions react with HCl to produce hydrogen sulfide (H2S), which has a characteristic rotten egg smell, but no pale yellow precipitate is formed.

- **Thiosulfate ion (S2O3^2-)**:
- When thiosulfate ions are heated with hydrochloric acid, the reaction produces sulfur dioxide (SO2), which has a pungent odor, and elemental sulfur (S), which is a pale yellow precipitate.
- The reaction can be represented as:
\[ 2 \, \text{S}_2\text{O}_3^{2-} + 4 \, \text{H}^+ \rightarrow 2 \, \text{SO}_2 + \text{S} + 2 \, \text{H}_2\text{O} \]

- **Sulfite ion (SO3^2-)**:
- Sulfite ions react with HCl to produce sulfur dioxide (SO2) and water, but no pale yellow precipitate is formed.

### Step 3: Conclusion
Based on the analysis:
- The only ion that produces both a pale yellow precipitate (elemental sulfur) and a gas with a pungent odor (sulfur dioxide) when reacted with dilute hydrochloric acid is the **thiosulfate ion (S2O3^2-)**.

Thus, the correct answer is the aqueous solution containing the **thiosulfate ion (S2O3^2-)**.

### Final Answer
The correct choice is the thiosulfate ion (S2O3^2-).

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