Freshly distilled colourless HI (aqueous solution) gradually turns brown with time.

Answer

**Step-by-Step Solution:**

1. **Understanding the Initial Condition**: We start with a freshly distilled, colorless aqueous solution of hydrogen iodide (HI). This solution is initially clear and has no color.

2. **Exposure to Air**: Over time, this solution is exposed to air, which contains oxygen (O2). The presence of oxygen is crucial for the next step.

3. **Chemical Reaction**: The HI in the solution reacts with the oxygen in the air. The reaction can be represented as follows:
\[ 4 \text{HI} + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} + 2 \text{I}_2 \]
In this reaction, hydrogen iodide (HI) reacts with oxygen to produce water (H2O) and iodine (I2).

4. **Formation of Iodine**: As the reaction proceeds, iodine (I2) is liberated. Iodine is a brown-colored substance, and its presence in the solution is what causes the color change.

5. **Color Change Observation**: Initially, the solution is colorless. As iodine is produced and dissolves in the solution, the color changes from colorless to brown due to the formation of iodine.

6. **Conclusion**: Therefore, the gradual browning of the freshly distilled colorless HI solution over time is due to the liberation of iodine (I2) as a result of its reaction with oxygen in the air.

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