Physical and Chemical Characteristics of Interstitial Compounds

Interstitial compounds are formed when small non-metal atoms like $\text{H}$, $\text{C}$, or $\text{N}$ are trapped inside the interstitial sites (voids) of the crystal lattices of transition metals.

The characteristic physical and chemical properties of interstitial compounds as defined by NCERT chemistry principles are:

1. Melting Point: They possess very high melting points, which are significantly higher than those of pure metals due to enhanced lattice strength.
2. Hardness: They are extremely hard; some borides even approach diamond in hardness. Thus, describing them as "very soft" is incorrect.
3. Conductivity: They retain metallic conductivity similar to their parent transition metals.
4. Chemical Nature: They are chemically inert and generally non-stoichiometric in nature (e.g., $\text{TiH}_{1.7}$, $\text{VH}_{0.56}$, $\text{Fe}_3\text{C}$). They are neither typical ionic nor covalent compounds.

Evaluating the given options:

• Option A: Correct characteristic.
• Option B: Incorrect characteristic, because interstitial compounds are extremely hard (not soft) and are neither ionic nor covalent.
• Option C: Correct characteristic.
• Option D: Correct characteristic.

Since the question asks for the option that is NOT a physical or chemical characteristic of interstitial compounds, Option B is the incorrect statement.

Hence, the correct option is B.