2021 O Level Chem P1 Q25 — Metals / Alloys Solution

Answer: B. Pure iron is a regular pattern (lattice) of iron atoms arranged in layers; these layers can slide over each other easily, which makes pure iron soft.

Steel is an alloy made by adding a small amount of carbon. The carbon atoms are smaller than the iron atoms and fit into the gaps between them, while the underlying regular pattern of iron atoms is kept. The carbon atoms stop the iron layers from sliding, making steel harder and stronger than pure iron.

So the correct description is a regular pattern of iron atoms with carbon atoms fitting into the gaps between them. Option A is the wrong way round (it makes carbon the main lattice); options describing an irregular pattern or alternating rows do not match how an alloy is structured.