Explain why stainless steel is harder and stronger than pure iron.

Cambridge International IGCSE Chemistry (0620) — 9.3 Alloys and their properties · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Stainless steel is an alloy made by mixing iron with chromium and nickel. Pure iron is a relatively soft metal that is rarely used on its own in construction.

Model answer (4 marks)

In pure iron the atoms are all the same size and arranged in regular layers, so the layers can slide over each other easily, making it soft. In stainless steel, chromium and nickel atoms are of different sizes to iron atoms. These different‑sized atoms distort and disrupt the regular layers, preventing them from sliding, which makes the alloy harder and stronger.

Examiner tips

Use the word ‘distort’ or ‘disrupt’ to show understanding of alloying effects
Mention the role of different atomic sizes in blocking layer movement
Keep the answer concise – 4 points can be covered in 3–4 sentences
Use UK spelling (e.g., ‘distort’ not ‘distort’)

Common mistakes

Failing to mention the different atomic sizes of chromium/nickel
Describing alloying as ‘adding strength’ without explaining the mechanism
Using vague terms like ‘more complex’ instead of ‘distort layers’

Mark scheme (4 marks)

In pure iron the atoms are all the same size and arranged in regular layers
The layers can slide over each other easily (in pure iron)
In stainless steel, atoms of different sizes are present (chromium / nickel atoms are different sizes to iron atoms)
The different-sized atoms distort / disrupt the layers, preventing them from sliding, making the alloy harder

Key terms in this question

pure iron · stainless steel