Cambridge IGCSE Chemistry · Syllabus 0620 · Metals
Alloy
What is Alloy?
A mixture of a metal with one or more other elements. The components keep their own identities and there is no fixed formula, because no chemical reaction has taken place; metallic bonding still holds the structure together, which is why an alloy is still a metal in its behaviour.
This definition is part of the Metals chapter in Cambridge IGCSE Chemistry.
Common mistakes with Alloy
- 3. “An alloy is a compound of two metals.” Repair An alloy is a mixture of a metal with other elements. There is no fixed formula, no chemical reaction has happened, and the metallic bonding is still there. Also note “other elements”, not “other metals” — carbon in steel is not a metal. Where Lesson 9.3A. Core 9.3.1
- 4. “An alloy is a compound formed when two metals are melted together.” Core 9.3.1 Defect Calls a mixture a compound. Reason No reaction occurs; there is no fixed formula, and the proportions can be varied while it remains the same alloy. Repair “An alloy is a mixture of a metal with other elements.” Transfer Give one piece of evidence that brass is a mixture. Its composition can be varied; the copper and zinc are not chemically bonded.
- 5. “An alloy is a mixture of two metals.” Core 9.3.1 Defect Too narrow — and it makes stainless steel impossible to describe. Reason Carbon in stainless steel is a non-metal, and it is still part of the alloy. Repair “A mixture of a metal with one or more other elements.” Transfer Name the elements in stainless steel. Iron, with chromium, nickel and carbon.
- 6. “Alloys are harder because the smaller atoms fill the gaps between the big ones.” Supplement 9.3.5 Defect Invents a mechanism the syllabus does not use, and it is not what the diagram shows. Reason The required mechanism is about layers, not gaps: differently sized particles disrupt the regular layers. Repair “Different-sized particles disrupt the regular layers, so the layers cannot slide over each other easily.” Transfer Does an alloy have to contain smaller particles? No — larger ones disrupt the layers just as well.
- 7. “This diagram shows an alloy because the circles are two different colours.” Core 9.3.4 Defect Uses a drawing convention as physical evidence. Reason Colour is chosen by whoever drew the diagram. Size, or a key naming a second element, is what tells you. Repair “The particles are of different sizes, so this is an alloy.” Transfer A diagram has equal circles in two shades, no key. What can you conclude? Nothing about alloying — the evidence does not support a conclusion either way.
Examiner tips on Alloy
- Where the seven Supplement statements sit. One in 9.3, two in 9.4, two in 9.5 and two in 9.6 — and none at all in 9.1 or 9.2, whose syllabus pages print an empty Supplement column. So properties and uses are identical for both routes, and every tier decision in this topic is about alloy structure, displacement, aluminium, rust protection or the extraction equations.