Interstitial compounds are formed when small atoms are trapped inside the crystal lattice of metals. Which of the following are 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. I, II, III
B. I, III
C. II, IV
D. IV only

Answer

To determine the characteristic properties of interstitial compounds, we will analyze each of the given statements one by one.

### Step 1: Analyze Statement I
**Statement I:** They have high melting points in comparison to pure metals.

**Explanation:** Interstitial compounds are formed when small atoms (like hydrogen, nitrogen, or carbon) occupy the voids in the crystal lattice of transition metals. The presence of these small atoms creates stronger bonding interactions within the lattice, which leads to higher melting points compared to the pure metals.

**Conclusion:** This statement is **true**.

### Step 2: Analyze Statement II
**Statement II:** They are very hard.

**Explanation:** The incorporation of small atoms into the metal lattice increases the density and enhances the hardness of the material. The strong bonding and the structural integrity provided by the small atoms contribute to the overall hardness of interstitial compounds.

**Conclusion:** This statement is **true**.

### Step 3: Analyze Statement III
**Statement III:** They retain metallic conductivity.

**Explanation:** Interstitial compounds generally retain the metallic properties of the parent metal, including electrical conductivity. The presence of small atoms does not significantly disrupt the metallic bonding, allowing these compounds to conduct electricity.

**Conclusion:** This statement is **true**.

### Step 4: Analyze Statement IV
**Statement IV:** They are chemically very reactive.

**Explanation:** Interstitial compounds are typically more stable than their parent metals due to the strong bonding interactions. As a result, they tend to be less chemically reactive than the pure metals. Therefore, this statement is incorrect.

**Conclusion:** This statement is **false**.

### Final Conclusion
The correct properties of interstitial compounds are:
- I. High melting points in comparison to pure metals (True)
- II. Very hard (True)
- III. Retain metallic conductivity (True)
- IV. Chemically very reactive (False)

Thus, the correct answer is that statements I, II, and III are characteristic properties of interstitial compounds.

### Summary of Answers:
- I: True
- II: True
- III: True
- IV: False