ZnO shows yellow colour on heating due to

Options

A. `Zn^(2+)` ions and electrons move to interstitial sites and F-centres are called
B. oxygen and electrons move out of the crystal and ions become yellow
C. `Zn^(2+)` again combine with oxygen to give yellow oxide
D. `Zn^(2+)` are replaced by oxygen

Answer

To understand why ZnO (zinc oxide) shows a yellow color upon heating, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding the Initial State of ZnO**:
- At room temperature, ZnO is a white solid. This is due to its crystal structure and the presence of both zinc ions (Zn²⁺) and oxide ions (O²⁻) in a stoichiometric ratio.

**Hint**: Remember that the color of a compound can change based on its structure and the presence of defects.

2. **Heating ZnO**:
- When ZnO is heated, it undergoes a change. The heat causes some of the oxide ions (O²⁻) to leave the crystal lattice, resulting in a deficiency of oxygen.

**Hint**: Consider what happens to the ions in a crystal lattice when energy is added through heating.

3. **Formation of Defects**:
- The removal of oxygen creates vacancies in the lattice. This type of defect is known as a metal excess defect, where there are more metal ions (Zn²⁺) than oxide ions (O²⁻).

**Hint**: Think about how the balance of ions in a crystal can affect its properties.

4. **Creation of F-Centers**:
- The electrons that were associated with the removed oxide ions occupy these vacancies. These negatively charged electrons are referred to as F-centers. They can absorb visible light.

**Hint**: F-centers are crucial in understanding how color changes occur in ionic compounds.

5. **Absorption of Light**:
- The electrons in the F-centers absorb certain wavelengths of visible light, which leads to the appearance of a yellow color in the heated ZnO.

**Hint**: Light absorption by electrons can lead to color changes; think about how different colors correspond to different wavelengths of light.

6. **Conclusion**:
- Therefore, the yellow color of ZnO when heated is due to the formation of F-centers, where zinc ions and electrons move to interstitial sites, leading to the absorption of visible light.

**Hint**: Summarize the process: heating leads to oxygen loss, formation of defects (F-centers), and light absorption resulting in color change.

### Final Answer:
ZnO shows yellow color on heating due to the movement of zinc ions and electrons to interstitial sites, forming F-centers that absorb visible light.