What makes a metal a metal

Compared with non-metals, metals share a set of physical properties:

• good conductors of heat and electricity
• malleable (can be hammered into shape) and ductile (can be drawn into wires)
• generally high melting and boiling points, shiny and dense

Their general chemical properties are limited by the spec to reactions with three things:

• with water / steam → a hydroxide (or oxide) + hydrogen, e.g. reactive metals fizz in cold water
• with oxygen → a metal oxide (most metal oxides are basic), e.g. 2Mg + O₂ → 2MgO

Watch out: only the reactive metals do all three readily. Copper, silver and gold barely react with acid or water at all — that low reactivity is exactly why they are used for coins and jewellery.

Acid + metal

Choosing metals — and improving them

We pick a metal for a job because of its physical properties:

• Aluminium in aircraft (low density), in overhead power cables (low density + good conductor) and in food cans (resists corrosion)
• Copper in electrical wiring (excellent conductor, ductile)
• Stainless steel in cutlery (hard and resists rusting)

An alloy is a mixture of a metal with other elements. Two you must know:

• Brass = copper + zinc
• Stainless steel = iron + chromium, nickel, carbon

Alloys are harder and stronger than the pure metals.

Misconception: an alloy is not a compound and the atoms are not bonded in a new way — it is simply a mixture. It is harder only because the layers can't slip.

Why is an alloy harder?

Ranking the metals

The reactivity series lists metals from most to least reactive. Carbon and hydrogen are slotted in as reference points (they aren't metals):

You can place a metal by watching how vigorously it reacts:

• K, Na, Ca react with cold water
• Mg, Zn, Fe react with steam (Mg reacts only very slowly with cold water)
• reaction with dilute hydrochloric acid gets gentler down the series; Cu, Ag, Au don't react with it at all

Supplement Reactivity is the tendency of an atom to form a positive ion by losing electrons. The more easily it loses electrons, the more reactive — and the more vigorous (faster, more energetic) its reactions. Aluminium seems unreactive because a thin, tough oxide layer on its surface protects it.

Displacement reactions Supplement

A more reactive metal will displace (push out) a less reactive metal from a solution of its compound. This is the experimental way to order two metals.

Misconception: displacement only works one way — copper will not displace iron, because copper is less reactive. If nothing happens, the added metal is the less reactive one.

Will it react?

Order the series

Build the reactivity series from most reactive (top) down. Tap the metal that comes next each time.

What single property ranks every metal?

Why let one metal corrode to save another?

How iron rusts

Rusting is the corrosion of iron. It needs both oxygen (from the air) and water:

We stop rust by keeping out air and water (barrier methods) or by using a more reactive metal:

• Barrier methods: painting, greasing/oiling, coating with plastic — keep oxygen and water off the iron
• Galvanising: coating iron with zinc — a barrier and a sacrificial metal Supplement

Misconception: rusting needs BOTH oxygen and water — water with no dissolved air, or dry air alone, will not rust iron. Salt and acids only speed it up.

Sacrificing a metal Supplement

In sacrificial protection a more reactive metal (like zinc or magnesium) is attached to the iron. Because it is more reactive, the attached metal corrodes instead of the iron — it is "sacrificed".

Misconception: the sacrificial metal must be MORE reactive than iron, not less. Galvanising works even if the zinc is scratched, because zinc still corrodes in preference to the exposed iron.

Stopping the rust

Why let one metal corrode to save another?

Why does carbon work for iron but not aluminium?

Reactivity decides the method

Most metals are found combined in ores. The extraction method depends on the metal's position relative to carbon:

• More reactive than carbon (e.g. aluminium) → must be extracted by electrolysis
• Less reactive than carbon (e.g. iron, zinc) → can be reduced by carbon (cheaper)
• Very unreactive (gold) → found native (uncombined), little extraction needed

Misconception: the dividing line is carbon, not "expensive vs cheap". A metal above carbon cannot be displaced by it, so carbon reduction fails and electrolysis is required — which is why aluminium is costly.

Which method?

Extracting iron

Iron is below carbon, so it is reduced by carbon (as carbon monoxide) in the blast furnace. The raw materials are iron ore (hematite, Fe₂O₃), coke (carbon) and limestone, with hot air blasted in.

Key reactions: coke burns (C + O₂ → CO₂); this makes the reducing agent (CO₂ + C → 2CO); CO reduces the ore (Fe₂O₃ + 3CO → 2Fe + 3CO₂). Supplement learn these equations.

The reducing agent

Why does carbon work for iron but not aluminium?

Extracting aluminium

Aluminium is above carbon, so it must be extracted by electrolysis of its ore (bauxite, giving purified aluminium oxide, Al₂O₃).

Supplement The aluminium oxide is dissolved in molten cryolite to lower the melting point (saving energy), then electrolysed with carbon electrodes:

Misconception: cryolite is not the ore — it is a solvent that lowers the melting point of the aluminium oxide so less energy is needed.

Why cryolite?

Pick the extraction method

For each metal, tap the method the spec says to use, based on its position relative to carbon.

Why recycle metals?

Ores are finite and extraction is energy-hungry — especially the electrolysis of aluminium. Recycling metals brings big benefits:

• conserves limited ore reserves
• uses much less energy than extracting from ore (recycling aluminium needs only a small fraction)
• reduces mining, waste and CO₂

Tie-it-together: aluminium's costly electrolytic extraction is exactly why recycling it saves so much — you skip the whole energy-hungry electrolysis step.

The recycling win

Topic 9 in one place

You've covered the key ideas in Cambridge IGCSE Topic 9 — Metals, with the Supplement points flagged. Press Finish to see your score.