Steel – IGCSE Chemistry Definition

Steel is the most important alloy in the IGCSE syllabus. It connects the metals topic (extraction of iron in the blast furnace) to the concept of alloys (why mixed-atom structures are harder). Examiners test both the composition and the structural explanation.

The 0620 definition

Steel is an alloy of iron with carbon (typically 0.1–1.5% carbon) and sometimes other elements. It is harder and stronger than pure iron.

Types of steel

Type	Composition	Properties	Uses
Mild steel	Iron + 0.1–0.3% carbon	Strong, malleable, can rust	Car bodies, nails, girders
High-carbon steel	Iron + 0.6–1.5% carbon	Very hard but brittle	Cutting tools, drill bits
Stainless steel	Iron + chromium + nickel	Corrosion-resistant, hard	Cutlery, surgical instruments, sinks

Why steel is harder than pure iron

In pure iron:

• All atoms are the same size (iron atoms)
• They form regular layers
• Layers slide over each other under force → soft, malleable

In steel:

• Carbon atoms are smaller than iron atoms
• They fit into gaps between iron atoms, disrupting the regular layers
• Layers cannot slide → harder and stronger

This is the same explanation used for all alloys — different-sized atoms prevent layer sliding.

From blast furnace to steel

The iron from the blast furnace is about 96% iron with ~4% carbon. This is cast iron — hard but very brittle.

To make steel, most of the carbon is removed by blowing oxygen through the molten iron. The oxygen reacts with the carbon:

C + O₂ → CO₂

The amount of carbon left is carefully controlled to produce the desired type of steel.

Worked exam question

Explain why stainless steel is used for surgical instruments rather than mild steel. [2]

Mark scheme

Stainless steel does not corrode / rust / is corrosion-resistant [1]; because it contains chromium (which forms a protective oxide layer) / mild steel would rust when exposed to blood and body fluids [1]

Common exam mistakes

• Saying steel is a compound of iron and carbon — it is a mixture (an alloy), not a compound.
• Claiming that adding carbon makes steel softer — carbon makes steel harder. Too much carbon makes it brittle, but that is not the same as soft.
• Forgetting to mention chromium when explaining why stainless steel resists corrosion — saying “it’s stainless” is circular reasoning and does not earn the mark.