Which of the following statements about the interstitial compounds is incorrect?

Options

A. They retain metallic conductivity
B. They are chemically reactive
C. They are much harder than the pure metal
D. They have higher melting points than the pure metals.

Explanation

To determine which statement about interstitial compounds is incorrect, we need to analyze the properties of these compounds based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Definition of Interstitial Compounds**:
- Interstitial compounds are formed when small atoms (impurities) occupy the interstitial sites (spaces) in a metal lattice.

2. **Conductivity**:
- Interstitial compounds retain metallic conductivity. This is true because the presence of small atoms does not significantly disrupt the electron flow in the metallic lattice.

3. **Chemical Reactivity**:
- It is mentioned that interstitial compounds are chemically reactive, but their reactivity decreases due to the presence of impurities occupying the interstitial sites. This suggests that the statement claiming they are chemically reactive is misleading, as they can become less reactive.

4. **Hardness**:
- Interstitial compounds are generally harder than pure metals. The presence of impurities leads to stronger bonding, which increases hardness.

5. **Melting Points**:
- Interstitial compounds have higher melting points than pure metals. The additional bonds formed due to the impurities require more energy to break, thus raising the melting point.

6. **Inertness**:
- The statement that interstitial compounds become inert is incorrect. While their reactivity may decrease, they do not become completely inert.

### Conclusion:
Based on the analysis, the incorrect statement about interstitial compounds is that they are chemically reactive. Instead, they can become less reactive due to the presence of impurities.