Why does not a wall immediately acquire a white colour when a coating of slaked lime is applied on it?

Options

A. slaked lime reacts with oxygen to form calcium carbonate, which imparts white colour
B. slaked lime reacts with carbon dioxide to form calcium hydroxide which imparts white colour
C. slaked lime reacts with carbon dioxide to form calcium carbonate which imparts white colour
D. slaked lime turns white on solidification

Explanation

### Step-by-Step Solution

1. **Understanding Slaked Lime**:
- Slaked lime is chemically known as calcium hydroxide (Ca(OH)₂). It is a white powder that is produced by adding water to quicklime (calcium oxide, CaO).

2. **Application on Wall**:
- When slaked lime is applied to a wall, it does not immediately appear white. This is because the slaked lime is initially in a wet form and does not have the solid white appearance.

3. **Reaction with Carbon Dioxide**:
- The key reason for the gradual appearance of white color is the reaction of slaked lime with carbon dioxide (CO₂) present in the air.
- The chemical reaction can be represented as:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
- In this reaction, calcium hydroxide reacts with carbon dioxide to form calcium carbonate (CaCO₃), which is a solid and has a white color.

4. **Formation of Calcium Carbonate**:
- Calcium carbonate is commonly known as chalk or limestone, and it is the substance that gives the wall its white color. This process takes time as the carbon dioxide from the air slowly reacts with the slaked lime.

5. **Conclusion**:
- Therefore, the wall does not immediately acquire a white color because it takes time for the slaked lime to react with carbon dioxide in the air to form calcium carbonate, which is responsible for the white appearance.

### Final Answer:
The wall does not immediately acquire a white color when slaked lime is applied because it reacts with carbon dioxide in the air to form calcium carbonate, which gives the white color. This reaction occurs gradually over time.