Give reasons: (i) transition metals and their compounds act as good catalysts, (ii) they form interstitial compounds, (iii) they form alloys. Give one example of each.

1. 1 · Catalysis. Variable oxidation states and surface bonding lower Ea. Example: V₂O₅ in the Contact process; Fe³⁺ in 2I⁻ + S₂O₈²⁻ → I₂ + 2SO₄²⁻.
2. 2 · Interstitial compounds. Small H, C, N atoms fit in the holes of the metal lattice; e.g. TiC; very hard, high melting, still conducting.
3. 3 · Alloys. Atomic radii are within about 15%, so atoms replace one another; e.g. brass (Cu-Zn), stainless steel.
4. 4 · Link to daily life. Nickel catalyst hardens oil into vanaspati.
5. 5 · Close in one line. So d electrons and similar atomic sizes give transition metals these useful properties.

Scoring tip: Write the two Fe³⁺ catalysis equations if the question says "explain with an example"; they carry the mark.