What colour is observed when ZnO is heated ?

Options

A. yellow
B. Violet
C. Violet
D. Blue

Explanation

To determine the color observed when ZnO (zinc oxide) is heated, we can follow these steps:

### Step 1: Understand the initial state of ZnO
- At room temperature, zinc oxide (ZnO) is white in color. This is its stable form.

**Hint:** Remember that the color of a substance can change with temperature due to structural or electronic changes.

### Step 2: Heating ZnO
- When ZnO is heated, it undergoes a change. Specifically, it can lose some oxygen atoms from its structure.

**Hint:** Consider what happens to the crystal structure of a compound when it is subjected to high temperatures.

### Step 3: Formation of defects
- The loss of oxygen leads to the formation of interstitial defects in the crystal lattice. This means that there are extra cations (Zn²⁺ ions) occupying interstitial sites, and electrons are also present in the lattice to maintain electrical neutrality.

**Hint:** Think about how defects in a crystal lattice can affect its optical properties, such as color.

### Step 4: Color change due to electron transitions
- The presence of these extra electrons can lead to electronic transitions that absorb certain wavelengths of light. As a result, the color of ZnO changes from white to yellow upon heating.

**Hint:** Consider how the absorption of light in specific regions of the spectrum can lead to the perception of different colors.

### Step 5: Conclusion
- Therefore, when ZnO is heated, the color observed is yellow.

**Final Answer:** The color observed when ZnO is heated is yellow.