Edexcel IGCSE Chemistry · Spec 2.25C
Low-Carbon, High-Carbon and Stainless Steel (Paper 2)
Covers compare three steel types, Link composition to properties and Select a steel for a stated use.
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Low-Carbon, High-Carbon and Stainless Steel (Paper 2)
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Explained
Three steels, and choosing between them
Steel is an alloy of iron with carbon. Changing how much carbon it contains, or adding other elements, changes the properties enough to produce materials suited to completely different jobs.
Why an alloy is harder than a pure metal
In pure iron the atoms are all the same size and sit in regular layers, which can slide over one another when a force is applied. That sliding is what makes a pure metal soft and malleable.
Adding carbon introduces atoms of a different size, which disrupt the regular arrangement. The layers can no longer slide easily, so the alloy is harder and stronger but less malleable.
That single explanation covers every alloy question in the course, and the marks are for the disruption and the sliding rather than for the word alloy.
The three types
Low carbon steel contains a small percentage of carbon. It is tough and malleable, so it can be pressed and shaped easily, which suits car body panels, nails, chains and girders. It rusts unless it is protected.
High carbon steel contains more carbon. It is much harder and stronger but more brittle, so it is used where a hard edge or a rigid shape matters: cutting tools, knives, blades, springs, hammers and rail tracks.
Stainless steel contains chromium, and usually nickel, as well as iron and carbon. It resists corrosion, because the chromium forms a thin protective oxide layer, so it is used for cutlery, sinks, surgical instruments and chemical plant. It is more expensive, which is why it is not used for everything.
Hard, strong, tough and brittle
These words are not interchangeable, and questions rely on the difference.
Hard means resistant to scratching or denting. Strong means it takes a large force to break. Tough means it absorbs energy and deforms rather than snapping. Brittle means it shatters instead of bending.
High carbon steel is hard and strong but brittle. Low carbon steel is less hard but tough. Choosing between them is choosing between those two profiles.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for a question comparing pure iron with high carbon steel awards marks across six points: that the steel is an alloy, a comparison of malleability between the two, a use for pure iron, a use for the steel, that the steel contains different sized atoms which disrupt the regular arrangement, and that the layers therefore cannot slide over each other.
Note that a use for each material is credited separately. A question of this kind wants the property, the reason and an application, and naming a real use is the easiest of the three to supply.
On rusting, the same paper is strict about names. It rejects iron oxide written with an incorrect oxidation state, so hydrated iron(III) oxide is what rust is, and iron(II) oxide is a different compound.
And on protecting iron with paint, it credits that the paint acts as a barrier and that this prevents water and oxygen reaching the iron, while rejecting galvanising, coating, covering and sacrificial protection.
Those rejected words either name a different method or restate the question. Barrier is the word that explains how paint works, and the mechanism has to say what is being kept out.
Answering a selection question
If asked which steel suits a stated use, name the steel, give the property that matters, and link it to the job.
A chisel needs high carbon steel, because it is hard, so it keeps a sharp edge and is not blunted by the material it cuts. A car body needs low carbon steel, because it is malleable, so it can be pressed into shape, and tough, so it deforms in a collision rather than shattering.
Spec 2.25C
What you need to know
- Compare three steel types
- Link composition to properties
- Select a steel for a stated use
Active recall
Quick check
Answer each question before opening the answer.
Which steel is easiest to shape?
Low-carbon steel
Why is high-carbon steel used for blades?
It is hard and wear resistant
What gives stainless steel corrosion resistance?
A protective chromium oxide layer
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