What are interstitial compounds?

Interstitial compounds are formed when small atoms like $\mathrm{H}$, $\mathrm{B}$, $\mathrm{C}$, or $\mathrm{N}$ occupy the interstitial spaces in the crystal lattice of transition metals.

Examples: titanium carbide, tungsten carbide, steel-like carbides/nitrides, etc.

General properties of interstitial compounds

These compounds are known to:

• retain metallic conductivity,
• have high melting points,
• be very hard,
• and are generally chemically inert / less reactive.

Check each option

A: They are chemically reactive

• This is incorrect.
• Interstitial compounds are usually chemically inert rather than reactive.

B: They have metallic conductivity

• This is correct.
• Because the metallic lattice character is largely retained.

C: They have high melting points

• This is correct.
• Interstitial compounds are typically refractory in nature.

D: They are very hard

• This is correct.
• This is one of their most important characteristics.

Conclusion

The incorrect statement is: $\boxed{\text{A}}$