Interstitial compounds are formed when small atoms are trapped inside the crystal lattice of metals. Which of the following are not characteristic properties of interstitial compounds?
I. They have high melting points in comparison to pure metals.
II. They are very hard.
III.They retain there metallic conductivity IV. They are chemically reactive

Options

A. I
B. II
C. III.
D. IV

Explanation

To solve the question, we need to evaluate the properties of interstitial compounds and identify which of the given statements is not a characteristic property of these compounds.

### Step-by-Step Solution:

1. **Understanding Interstitial Compounds**:
Interstitial compounds are formed when small atoms (like carbon or nitrogen) occupy the spaces (interstices) between the larger metal atoms in a crystal lattice. This affects the properties of the compound.

2. **Analyzing Statement I**:
- **Statement I**: They have high melting points in comparison to pure metals.
- **Evaluation**: This statement is true. Interstitial compounds typically have higher melting points than the pure metals they are derived from due to the presence of small atoms that strengthen the metallic lattice.

3. **Analyzing Statement II**:
- **Statement II**: They are very hard.
- **Evaluation**: This statement is also true. The presence of small atoms in the lattice increases the hardness of the compound compared to pure metals.

4. **Analyzing Statement III**:
- **Statement III**: They retain their metallic conductivity.
- **Evaluation**: This statement is true. Interstitial compounds often retain some degree of metallic conductivity, as they still contain metallic elements in their structure.

5. **Analyzing Statement IV**:
- **Statement IV**: They are chemically reactive.
- **Evaluation**: This statement is false. Interstitial compounds tend to be more inert and less reactive compared to pure metals. They do not readily react with other substances.

6. **Conclusion**:
- The only statement that is not a characteristic property of interstitial compounds is **Statement IV**. Therefore, the answer to the question is that Statement IV is not a characteristic property of interstitial compounds.

### Final Answer:
**IV. They are chemically reactive.**