Alloys — mixtures; brass and stainless steel
An alloy is a mixture of a metal with other metals, or with a non-metal such as carbon — the components are not chemically combined and have no fixed ratio, so it is not a compound. The syllabus names two: brass (copper + zinc), used for musical instruments, fittings and coins; and stainless steel (iron + chromium + nickel), used for cutlery, surgical instruments and sinks. Chromium makes stainless steel corrosion-resistant.
(Extended) Why alloys are harder than pure metals
In a pure metal all atoms are the same size and pack into regular layers that slide over each other easily, so pure metals are soft. In an alloy, atoms of different sizes distort the regular layers and stop them sliding past one another. Because the layers can no longer slide freely, more force is needed to deform the metal, so the alloy is harder and stronger. Minimum mark-scheme answer: different-sized atoms [1] prevent layers sliding [1].
Why we use alloys, not pure metals
Pure metals are often too soft or corrode too readily for practical use, so they are alloyed. Alloys are harder than the pure metals they are made from: brass is harder than pure copper or zinc; stainless steel is harder than iron and resists rust. Aluminium alloys such as duralumin are much stronger than pure aluminium yet stay light for aircraft. Asked what type of substance an alloy is, answer mixture — compound or element scores zero.
Drawn from real examiner reports.
Alloy is a mixture, not a compound
Alloys are mixtures, not compounds. A compound has a fixed formula and chemically-bonded atoms; an alloy has neither — its proportions can vary (brass is roughly 60-90% copper) and its metals are only mixed, not combined. Asked what type of substance an alloy is, write mixture; compound or element scores zero.
W22 P32 Q2(d)(iii); S23 P31 Q5(c)(i)
(Extended) Name the hardness mechanism
Many candidates start correctly — atoms of different sizes are present — but stop there and lose the second mark. An Explain question needs the mechanism and result: the different-sized atoms distort the layers and prevent them sliding, so the alloy is harder. Different sizes [1] plus prevent layers sliding [1] are both required.
Brass is copper + zinc, not tin
Brass is copper + zinc. A frequent slip is copper + tin, which is bronze, not brass. Another is putting iron in brass — iron belongs to steel alloys. Learn brass = Cu + Zn and keep it separate from bronze (Cu + Sn). Naming the wrong components scores zero even if the rest of the answer is right.
Stainless steel is not ordinary steel
Stainless steel is iron + chromium + nickel. Writing iron + carbon describes ordinary (carbon) steel, which rusts and is used for construction — not stainless steel. The chromium is what forms a protective oxide layer and stops rusting. Both are alloys, but only stainless steel resists corrosion; give Fe + Cr + Ni when stainless steel is named.
Pick the right alloy diagram
In diagram questions, the alloy is mostly one size of circle with some clearly different-sized circles scattered randomly among them. Do not pick the grid of identical circles (that is the pure metal) or a completely disordered mess (alloys still have structure). The different-sized atoms sit randomly, not in neat separate rows.
W22 P32 Q2(d)(iii); S23 P31 Q5(c)(i)
Explain hardness in two steps
For Explain, using particles, why alloys are harder: (1) atoms of different sizes are present; (2) they distort the layers and stop them sliding, so more force is needed and the alloy is harder. Structure alone earns one mark — always add the sliding mechanism.
Match the command word
Watch the command word. State wants a bare fact (components of brass); Describe wants a feature in words; Explain wants a reason plus mechanism (why the alloy is harder); Suggest applies knowledge to a new context. Describing when asked to explain loses marks.
Learn the two named alloys
State questions here reward exact recall: brass = copper + zinc; stainless steel = iron + chromium + nickel; plus one use each (brass — instruments; stainless steel — cutlery). Give the exact components — a near-miss like copper + tin scores zero.
Suggest-a-use: give two properties
For Suggest why an alloy is used instead of a pure metal, give two distinct properties. For stainless steel cutlery: harder than iron [1] and resists corrosion / does not rust [1]. Two separate properties earn two marks; one point reworded does not.
Alloy — a mixture of a metal with one or more other metals, or with a non-metal such as carbon. The components are NOT chemically combined; there is no fixed formula and no new chemical compound is formed.
Harder — requires greater force to scratch or deform; alloys are mechanically stronger than the pure metals from which they are made.
Pure metal — a substance consisting of only one type of metal atom, arranged in a regular lattice. Pure metals are relatively soft because the uniform layers can slide over one another.
Define the term alloy.
State what is meant by the term alloy.
