An alloy is a mixture of two or more elements with a fixed composition, in which the main element is a metal. Alloying makes a metal harder and stronger because atoms of a different size disrupt the orderly layers of metal atoms and stop them sliding.
Alloys are also made to resist corrosion and improve appearance.
This page covers one Form 4 content standard from the Manufactured Substances in Industry chapter: alloys and their uses. The exam wants a precise definition of an alloy, the arrangement-of-atoms explanation for why alloys are harder than pure metals, the reasons we make alloys, and a few named alloys with their composition and uses. The atomic explanation is the part that most often decides the marks, so learn it word for word.
What an alloy is
An alloy is a mixture of two or more elements with a fixed composition, in which the main (major) component is a metal. For example, steel is an alloy of iron with a small amount of carbon, and bronze is an alloy of copper and tin. An alloy is a mixture, not a compound: the elements are combined physically, not chemically bonded in fixed formula units.
Why a pure metal is soft
To explain the properties of alloys you first explain pure metals. In a pure metal, the atoms are all the same size and are packed in an orderly arrangement of layers. When a force is applied, these layers can slide over one another easily, so a pure metal is relatively soft and ductile. Pure gold, pure copper and pure aluminium are all quite soft for this reason.
Why an alloy is harder
When a metal is alloyed, atoms of a different element with a different size are added. These foreign atoms disrupt the orderly arrangement of the original metal atoms. The layers can no longer slide over one another easily, because the different-sized atoms get in the way. As a result the alloy is harder and stronger than the pure metal. This single idea, different-sized atoms disturb the layers and stop them sliding, is the standard mark-scoring explanation.
Why we make alloys
We alloy metals for three main reasons:
- To increase hardness and strength, the most important reason, as explained above.
- To improve resistance to corrosion, for example, stainless steel does not rust as ordinary steel does.
- To improve the appearance or lustre of the metal, so it looks more attractive.
Common alloys and their uses
You should be able to name a few alloys, their component elements, and a use:
- Bronze (copper + tin), statues, medals and monuments.
- Brass (copper + zinc), musical instruments, decorative ornaments and kitchenware.
- Steel (iron + carbon), the bodies of buildings, bridges and cars.
- Stainless steel (iron + carbon + chromium + nickel), cutlery and surgical instruments, because it resists rust.
- Duralumin (aluminium + copper + magnesium), aircraft bodies, because it is strong yet light.
- Pewter (tin + antimony + copper), souvenirs and decorative items.
- Cupronickel (copper + nickel), coins.
Worked example
Question. A sample of brass has a total mass of 50 g and contains 35 g of copper, the rest being zinc. Calculate (a) the mass of zinc in the sample and (b) the percentage by mass of zinc in the brass.
Step 1, Find the mass of zinc. Mass of zinc = total mass − mass of copper = 50 g − 35 g = 15 g.
Step 2, Set up the percentage. Percentage by mass of zinc = (mass of zinc ÷ total mass) × 100.
Step 3, Substitute and calculate. = (15 g ÷ 50 g) × 100 = 0.30 × 100 = 30 %.
Answer. (a) The mass of zinc is 15 g. (b) The percentage by mass of zinc in the brass is 30 %.
Practice question
Question. (a) Define an alloy. (b) Using the arrangement of atoms, explain why brass is harder than pure copper. (c) State one reason, other than hardness, for making stainless steel from iron.
Answer. (a) An alloy is a mixture of two or more elements with a fixed composition, in which the main element is a metal. (b) In pure copper the atoms are the same size and arranged in orderly layers that slide over one another easily, so it is soft. In brass, the zinc atoms are a different size from the copper atoms and disrupt the orderly arrangement, so the layers can no longer slide easily and the brass is harder. (c) Stainless steel is made so that the metal resists rusting/corrosion (the chromium and nickel protect the iron).
Exam tip
The atomic explanation must contain three ideas to earn full marks: different-sized atoms, disrupt the orderly layers, and prevent the layers from sliding. Miss any one and you lose part of the explanation mark. When you name an alloy, always give its components and one use, a name on its own is rarely enough. And remember an alloy is a mixture, so never call it a compound. Practise writing “harder and stronger”, not just “stronger”, because both properties are commonly credited.
Choosing the right alloy for a use
Higher-mark questions often describe a use and ask you to choose a suitable alloy and justify it. The logic is always to match the property to the need. Aircraft bodies need a metal that is strong but light, so duralumin is chosen. Cutlery and surgical instruments must not rust and must stay clean, so stainless steel is chosen. Coins must be hard-wearing and not corrode in everyday handling, so cupronickel is used. If you keep asking “what property does this use demand, and which alloy provides it?”, you can answer even unfamiliar examples, because the reasoning, not memorising every alloy, is what the exam rewards.
Where this fits
This is content standard 8.3 of the Manufactured Substances in Industry chapter, and it draws on the metallic-structure ideas you met in earlier chapters. Reinforce it with the chapter revision notes and practise the definition, explanation and composition questions in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, with a paid one-hour trial, our teachers make sure your “different-sized atoms disturb the layers” explanation is complete and precise, because that is the sentence that carries the marks in 4541.
Quick recap
- An alloy is a mixture of elements with a fixed composition, mainly a metal.
- A pure metal is soft because same-size atoms form layers that slide.
- An alloy is harder because different-sized atoms disrupt the layers and stop them sliding.
- We alloy metals to increase hardness/strength, resist corrosion and improve appearance.
- Know bronze, brass, steel, stainless steel, duralumin, pewter, cupronickel with their uses.
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