The reactivity series
Metals can be ranked by how readily they lose electrons to form positive ions — the reactivity series. A common order, most reactive first: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold. Carbon and hydrogen are non-metals, slotted in as reference points.
The whole topic falls out of one idea: the higher a metal sits, the more eagerly it gives up electrons, so the more vigorously it reacts and the more stable its compounds are.
Reactions that reveal the order
Position predicts behaviour. With water: potassium, sodium and calcium react (with cold water); magnesium to iron react only with steam; copper and below do not. With dilute acid: metals above hydrogen fizz, giving a salt and hydrogen, and the fizzing is more vigorous higher up; metals below hydrogen (copper, silver, gold) do not react with dilute acid at all.
Displacement
A more reactive metal displaces a less reactive one from its compound or solution, because it holds electrons less tightly and forces the other to take them back. Iron added to copper sulfate turns the solution from blue to pale and coats the iron in copper: Fe + CuSO₄ → FeSO₄ + Cu.
Displacement is a direct test of the order: if metal A displaces metal B, A is more reactive than B. It is also the basis of the thermite reaction (aluminium displacing iron from its oxide).
Rusting is a slow reaction of iron
Iron rusts only when BOTH water and oxygen are present — the equation is iron + oxygen + water → hydrated iron(III) oxide. Prevent it by keeping those out (paint, oil, galvanising) or by sacrificial protection: attach a more reactive metal (zinc, magnesium) that corrodes instead of the iron.
Extraction depends on reactivity
How a metal is won from its ore is decided entirely by its place in the series — because extraction is reduction (removing oxygen / adding electrons), and a more reactive metal clings to its compound more stubbornly.
| Reactivity | Extraction method | Examples |
|---|---|---|
| Above carbon (K–Al) | electrolysis of the molten ore | aluminium, sodium |
| Below carbon (Zn–Cu) | reduction by heating with carbon | iron (blast furnace), zinc |
| Very unreactive (Ag, Au) | found native — little or no extraction | gold, silver |
A worked example
Zinc is more reactive than copper. (a) What happens if zinc is placed in copper sulfate solution? (b) Would you extract zinc by electrolysis or by heating with carbon?
- (a) Zinc is more reactive, so it displaces copper from the solution.A more reactive metal displaces a less reactive one from its compound.
- Zn + CuSO₄ → ZnSO₄ + Cu — the blue fades and copper deposits on the zinc.Zinc takes the sulfate; copper is set free as the metal.
- (b) Zinc sits BELOW carbon in the reactivity series.Method is chosen by position relative to carbon.
- So zinc is extracted by heating its ore with carbon (reduction), not electrolysis.Carbon can reduce the oxides of metals less reactive than itself.
(a) Zinc displaces copper (Zn + CuSO₄ → ZnSO₄ + Cu); (b) heat with carbon, since zinc is below carbon.
Metals with water, steam and acid
Where a metal sits in the reactivity series predicts exactly how it behaves. The most reactive metals react with cold water; less reactive ones react only with steam; those below hydrogen do not react with acids at all. Reading these observations backwards is how the series was built in the first place.
| Metal | Cold water | Steam | Dilute acid |
|---|---|---|---|
| Potassium, sodium, calcium | reacts (fizzes) | — | violent — dangerous |
| Magnesium, zinc, iron | very slow / none | reacts | reacts, giving hydrogen |
| Copper, silver, gold | no reaction | no reaction | no reaction |
Rusting and how to stop it
Iron rusts only when both water and oxygen are present; salt speeds it up. Because rust flakes off and exposes fresh metal, it keeps going until the iron is gone. Prevention works in two ways: a barrier (paint, oil, grease or plastic) keeps water and air out, while sacrificial protection attaches a more reactive metal (like zinc or magnesium) that corrodes instead of the iron.
Galvanising — coating iron with zinc — does both: the zinc is a barrier, and even if it is scratched, the zinc still corrodes in preference to the iron.
Sacrificial protection
Bolt blocks of magnesium to a steel ship's hull or an underground pipeline: the magnesium is more reactive, so it corrodes and is 'sacrificed', protecting the iron until the blocks are replaced.
Alloys — why we mix metals
Most metals we use are alloys — mixtures of a metal with other elements. A pure metal is soft because its identical atoms sit in neat layers that slide over each other. Mixing in atoms of a different size distorts those layers so they can no longer slide easily, making the alloy harder and stronger. Steel (iron + carbon), brass (copper + zinc) and bronze (copper + tin) are everyday examples.