Assertion: `ZnO` becomes yellow when it is heated.
Reason: The anionic sites occupied by unpaired electrons (due to `F-centres`)

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reson are true but reason is not the correct explanation of assertion.
C. if assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "ZnO becomes yellow when it is heated."

- **Explanation**: At room temperature, zinc oxide (ZnO) is white. When heated, it undergoes a change that leads to the formation of zinc ions (Zn²⁺) and free electrons. These free electrons can occupy interstitial sites in the crystal lattice of ZnO. The presence of these electrons allows ZnO to absorb certain wavelengths of light, which results in a yellow coloration.

### Step 2: Analyze the Reason
The reason states that "the anionic sites are occupied by unpaired electrons due to F-centres."

- **Explanation**: F-centres are defects in ionic crystals where an anion vacancy is occupied by an unpaired electron. However, in the case of ZnO when heated, the electrons are not occupying anionic sites but rather interstitial sites due to a metal excess defect. Therefore, the reason provided is incorrect.

### Step 3: Conclusion
- The assertion is **true**: ZnO does become yellow when heated.
- The reason is **false**: The unpaired electrons do not occupy anionic sites due to F-centres; instead, they occupy interstitial sites.

### Final Answer
Since the assertion is true and the reason is false, the correct option is option 3.

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