Alloys as useful materials — AQA GCSE Chemistry

An alloy is a mixture of a metal with one or more other elements, usually other metals. Pure metals have regular layers of identical atoms that slide over one another, so they are soft and malleable. In an alloy, atoms of different sizes disrupt these layers, preventing them from sliding easily. This makes the alloy harder and stronger than the pure metal. Most metals used in everyday life are alloys because their properties are better suited to particular uses. Steel is an alloy of iron with carbon and sometimes other elements; stainless steel also contains chromium. Bronze, brass and solder are further examples. The composition and structure of an alloy determine its properties and therefore its uses.

Bronze and brass are both alloys based on copper, but they contain different second metals. Bronze is a mixture of copper and tin. It is harder than pure copper and resists corrosion, so it is used for statues, medals and bearings. Brass is a mixture of copper and zinc. It is also harder than pure copper, can be shaped easily and has a gold-like appearance, so it is used for musical instruments, door fittings and decorative items. Because alloys are mixtures, their composition can be varied to adjust properties. Knowing the exact metals in each alloy allows you to compare their properties and suggest suitable uses. Both are examples of how alloying changes the properties of a metal.

Pure gold is a soft, dense metal that is too soft to wear well in jewellery, so it is usually alloyed with other metals such as silver, copper and zinc. These added metals make the gold harder and more durable while keeping its attractive appearance. The proportion of gold in the alloy is measured in carats: 24-carat gold is pure gold, while 18-carat gold contains 18 parts gold out of 24 by mass, and the rest is other metals. Different added metals also affect colour, giving yellow, white or rose gold. Because the alloy is a mixture, its composition can be adjusted to produce the desired hardness, colour and cost. This is an everyday example of how alloying changes the properties of a metal.

Carat is a measure of the purity of gold in an alloy, expressed as parts in 24. Pure gold is 24 carat, so it is 100% gold. An 18 carat gold alloy is 18/24 gold, which simplifies to 3/4, or 75% gold; the remaining 25% is other metals added to harden it. To convert carat to percentage, divide the carat value by 24 and multiply by 100. For example, 9 carat gold is 9 ÷ 24 × 100 = 37.5% gold. Pure gold is too soft for most jewellery, so alloying it with metals such as copper, silver or zinc makes it harder and more durable while the carat value records how much gold remains.

Steel is an alloy based on iron. It always contains iron as the main metal, together with a controlled, specific amount of carbon and often other metals added for particular properties. The carbon content is chosen deliberately: low carbon steel and high carbon steel have different properties because their carbon percentages differ. Other metals may be added to make alloy steels with extra strength, hardness or resistance to corrosion; stainless steel, for example, contains chromium. Because the amounts are specific, steel is not a single substance but a family of alloys whose composition is adjusted for a purpose.

High carbon steel contains a relatively large proportion of carbon within its iron alloy. The carbon atoms disrupt the regular layers of iron atoms, so the layers cannot slide over one another easily. This makes the steel strong and hard, able to resist forces without deforming. However, the same disruption means the material cannot absorb much energy by bending, so it tends to crack or shatter rather than deform when struck or overloaded; this is brittleness. High carbon steel is therefore chosen where hardness and strength matter more than flexibility, such as cutting tools, blades and some springs.

Low carbon steel contains only a small proportion of carbon in its iron alloy. With fewer carbon atoms disrupting the structure, the layers of iron atoms can slide over one another more readily when a force is applied. This makes the steel softer and more malleable, so it can be bent, pressed or rolled into shape without cracking. It is therefore useful for applications such as car bodies, nails, wire and structural beams, where the material must be shaped or withstand bending rather than hold a sharp cutting edge. The contrast with high carbon steel shows how carbon content controls properties.

Steel is an alloy of iron with carbon. Adding chromium and nickel produces stainless steel, which is harder than pure iron and resists corrosion. Chromium forms a thin, adherent oxide layer on the surface that seals the metal beneath from water and oxygen, so rusting is prevented; nickel adds toughness and helps the alloy keep its protective surface. Because stainless steel does not flake away like rusted iron, it is chosen for cutlery, sinks, surgical instruments and chemical plant. In an exam, link composition to property and then to a named use: chromium and nickel give hardness and corrosion resistance, so the alloy suits wet, hygienic or outdoor applications.

Pure aluminium is already a light metal, and alloying it with elements such as magnesium, manganese, silicon or copper produces materials that keep a low density while gaining strength. Density is mass per unit volume, so a low-density alloy gives a strong but light component. This matters in aircraft bodies, bicycle frames, car wheels, drink cans and window frames, where reducing mass saves fuel or makes handling easier. In an exam, compare the alloy with steel: for the same volume, an aluminium alloy has much less mass, so it is chosen where low weight is more important than the very high strength of steel.

This outcome has two parts. First, recall a use for each alloy named in the specification, such as stainless steel for cutlery and aluminium alloys for aircraft. Second, when given data about an unfamiliar alloy, interpret its composition and evaluate whether its properties suit a stated use. Interpreting means reading the composition and identifying the elements and their proportions; evaluating means weighing advantages such as strength, density, corrosion resistance and cost against disadvantages. For example, given an alloy of copper and zinc, you might identify it as brass and judge it suitable for a door handle because it is hard, conducts heat and resists corrosion.