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

Options

A. They have high melting points in comparison to pure metals
B. They are very hard
C. They retain metallic conductivity
D. The are chemically very reactive

Explanation

To determine which of the given statements is not a characteristic property of interstitial compounds, we will analyze each statement one by one.

### Step-by-step Solution:

1. **Understanding Interstitial Compounds**:
- Interstitial compounds are formed when small atoms (like hydrogen, carbon, or nitrogen) occupy the spaces (interstices) between the larger metal atoms in a crystal lattice.

2. **Analyzing Statement I**:
- **Statement I**: "They have high melting points in comparison to pure metals."
- **Analysis**: This statement is true. Interstitial compounds generally have higher melting points than the pure metals they are derived from due to the strong bonding interactions between the small atoms and the metal lattice.

3. **Analyzing Statement II**:
- **Statement II**: "They are very hard."
- **Analysis**: This statement is also true. Interstitial compounds are known for their hardness because the presence of small atoms in the lattice increases the overall strength of the material.

4. **Analyzing Statement III**:
- **Statement III**: "They retain metallic conductivity."
- **Analysis**: This statement is true as well. Interstitial compounds maintain some degree of metallic conductivity because the metallic character of the host metal is retained in the compound.

5. **Analyzing Statement IV**:
- **Statement IV**: "They are chemically very reactive."
- **Analysis**: This statement is false. Interstitial compounds are typically chemically inert or less reactive compared to the pure metals. They do not readily react with other substances.

6. **Conclusion**:
- The statement that is **not** a characteristic property of interstitial compounds is **Statement IV**: "They are chemically very reactive."

### Final Answer:
The correct answer is **IV. They are chemically very reactive.**