Alloy — a mixture, not a compound
An alloy is a mixture of a metal with one or more other elements, which may be metals or non-metals. The elements are melted and mixed physically, not chemically combined, so the proportions are not fixed and vary between samples — different grades of stainless steel carry different percentages of chromium, nickel and carbon. The two named examples: brass = copper and zinc; stainless steel = iron with other elements such as chromium, nickel and carbon.
Harder and stronger than the pure metal
Alloys can be harder and stronger than the pure metals they are made from, and that is what makes them more useful. (Extended) The reason is structural: in a pure metal every atom is the same size and the atoms sit in regular layers that slide over each other easily under a force, so the metal is soft. An alloy contains atoms of a different size, which distort those layers, so they can no longer slide as easily and more force is needed to deform the metal.
Uses — property linked to application
A use is justified by naming a physical property and linking it to the job. Stainless steel cutlery: it is hard, so it resists scratching and wear and keeps a sharp edge through repeated use and washing; and it resists rusting, because chromium gives a thin, unreactive, self-repairing oxide layer, so cutlery stays clean and needs no paint or coating. Brass (copper and zinc) is used for decorative and musical items and for fittings.
Drawn from real examiner reports.
An alloy is a mixture, not a compound
An alloy is a mixture — the elements are physically mixed with no new chemical bonds between them, so the proportions are not fixed and vary between samples. A compound is elements chemically combined in fixed proportions. A definition that does not contain the word "mixture" is not credited.
Flagged Jun 2022 P31 Q5ci; Jun 2023 P31 Q1di, P32 Q1cii
(Extended) Naming different-sized atoms is not enough
"An alloy has different atoms" is an observation, not a mechanism, and it stops short of the marks. Finish the chain: the different-sized atoms distort the regular layers, so the layers can no longer slide over each other as easily, so more force is needed to deform the alloy and it is harder and stronger.
Core states it, Extended explains it
Core asks you to state that alloys can be harder and stronger than the pure metal and are therefore more useful — no mechanism required. Extended asks you to explain in terms of structure, which needs the different-sized atoms and the layers that can no longer slide. Giving the Core statement to an "explain" question loses the Extended marks.
Metallic bonding is not a definition
Metallic bonding — a lattice of positive ions in a sea of delocalised electrons — describes the bonding inside any metal, pure or alloyed. It is not an answer to "what is an alloy?", which asks about composition: a mixture of a metal with other elements. Examiners flagged both "metallic bonding" and "compound" as non-credited responses.
Flagged Jun 2022 P31 Q5ci; Jun 2023 P31 Q1di, P32 Q1cii
Alloys melt over a range, not sharply
Only a pure substance melts at one precise, sharp, fixed temperature. An alloy is a mixture, so it melts and solidifies over a range of temperatures. Judge purity by the melting point — candidates who reached for the boiling point instead were marked wrong. A sample melting over a range is a mixture, which includes any alloy.
Flagged Jun 2022 P31 Q5ci; Jun 2023 P31 Q1di, P32 Q1cii · purity judged by melting point, not boiling point: Jun 2023 P42 Q2c
Alloy diagrams need two atom sizes
A pure-metal diagram shows circles that are all the same size in regular, ordered rows. An alloy diagram must show circles of at least two different sizes, with the odd-sized atoms breaking up those rows. Picking the diagram that simply "looks more complicated" is not a justification — say which atom sizes you can see.
(Extended) The four-step structure answer
Link every property to the use
"Describe the use in terms of physical properties" needs both halves: name the property, then say what it lets the object do. For stainless-steel cutlery give two — hardness and rust resistance — and link each. A property listed on its own drops the mark.
Percentage by mass: subtract, then divide
If one component's mass is missing, subtract every given mass from the total first. Then divide that component's mass by the total sample mass, never by another component, and multiply by 100. Reversed: mass = (percentage ÷ 100) × total. Always write the unit.
Write Zn, not ZN
Element symbols take a capital first letter and a lower-case second letter: Zn, Cu, Cr, Ni. Examiners flagged capitalised second letters in a metals question, and this topic is full of two-letter symbols.
Cambridge 0654 spec reference: Section C9 "Metals", sub-topic C9.3 "Alloys and their properties". Core: describe alloys as mixtures of a metal with other elements (brass = copper and zinc; stainless steel = iron and other elements such as chromium, nickel and carbon); state alloys can be harder/stronger and more useful than the pure metals; describe uses of alloys linked to physical properties (stainless steel cutlery -- hardness, rust resistance); identify representations of alloys from diagrams of structure. Supplement (Extended): explain, in terms of structure, why alloys can be harder/stronger than pure metals -- different-sized atoms in the alloy stop the layers sliding over each other.
| Term | Mark-scheme-precise meaning |
|---|---|
| Alloy | A mixture of a metal with one or more other elements (metals or non-metals) -- NOT chemically combined in fixed proportions like a compound |
| Brass | An alloy that is a mixture of copper and zinc |
| Stainless steel | An alloy that is a mixture of iron with other elements such as chromium, nickel and carbon |
| Pure substance | Has one precise, sharp, fixed melting/boiling point |
| Mixture (incl. any alloy) | Melts/boils over a range of temperatures, not at one sharp value |
Composition table:
| Alloy | Made from | Key use in this topic |
|---|---|---|
| Brass | Copper + zinc | Decorative/musical items, fittings (not a named 0654 use statement, but a named composition example) |
| Stainless steel | Iron + chromium, nickel, carbon (and other elements) | Cutlery -- hard (resists wear), resists rusting/corrosion |
What two elements is brass a mixture of?
A sample of brass has a total mass of 15.0 g. It contains 10.5 g of copper, with the rest zinc.
Calculate the percentage by mass of zinc in this sample. (2 marks)
