Reactivity series

The reactivity series arranges substances in order of decreasing reactivity. A substance higher in the series is more reactive than one below it.

1. Potassium
2. Sodium
3. Calcium
4. Magnesium
5. Aluminium
6. Carbon
7. Zinc
8. Iron
9. Hydrogen
10. Copper
11. Silver
12. Gold

Carbon and hydrogen are reference points, not metals. Their positions allow their reactivity to be compared with that of the metals in the series.

Hydrogen as a reference point

Metals above hydrogen in the series can react with dilute hydrochloric acid, releasing hydrogen gas. Metals below hydrogen—copper, silver and gold—do not react with dilute hydrochloric acid.

Reactions of metals with water and steam

Some metals react with water to produce hydrogen gas. The other product depends on the metal and the reaction conditions: potassium, sodium and calcium form metal hydroxides with cold water, while magnesium forms magnesium oxide with steam.

Potassium, sodium and calcium with cold water

Potassium and sodium react vigorously with cold water. Calcium also reacts with cold water, but less vigorously. This difference is consistent with their positions in the reactivity series: potassium and sodium are more reactive than calcium.

Each reaction produces hydrogen and the hydroxide of the metal:

Magnesium with steam

Magnesium reacts with steam to form magnesium oxide and hydrogen, rather than the metal hydroxide formed in the cold-water reactions above:

Using steam rather than cold water shows how reaction conditions can help reveal differences in metal reactivity. Magnesium is less reactive than potassium, sodium and calcium.

Reactions of metals with dilute hydrochloric acid

Some metals react with dilute hydrochloric acid to form a metal chloride and hydrogen gas. The hydrogen escapes as bubbles. For example, magnesium forms magnesium chloride:

Zinc and iron also react with dilute hydrochloric acid, forming zinc chloride and iron(II) chloride respectively, along with hydrogen gas.

Predicting reactions using hydrogen's position

Magnesium, zinc and iron are above hydrogen in the reactivity series. They can displace hydrogen from dilute hydrochloric acid, so they react with it. Under comparable conditions, magnesium generally reacts most vigorously, followed by zinc, then iron.

Copper, silver and gold are below hydrogen in the reactivity series. They do not displace hydrogen from dilute hydrochloric acid and do not react with it. Hydrogen's position therefore marks the boundary for predicting which of these metals will react.

Deducing a reactivity order from experimental results

A reactivity order can be worked out by comparing how substances behave in the same test under comparable conditions. A substance that reacts more readily or more vigorously provides evidence that it is more reactive than one that reacts less readily or less vigorously.

For a fair comparison, conditions that affect the observations should be kept similar. For example, when comparing metals in the same acid, the acid concentration, temperature and amount and surface area of each metal should be comparable.

Combining the comparisons

Suppose three metals are tested with the same acid under comparable conditions:

• Metal A produces bubbles rapidly.
• Metal B produces bubbles more slowly.
• Metal C shows no visible reaction during the test.

A reacts more vigorously than B, so the result places A above B. B reacts while C shows no visible reaction, so the result places B above C. Combining the comparisons gives the experimental order:

A > B > C, where $$>$$ means “is more reactive than”.

A lack of visible reaction places C below a substance that does react in that test; it does not prove that C can never react. If two substances both show no visible reaction, that result alone does not establish which is more reactive.

The order should include only substances that can be placed using the evidence given. Do not assume positions for other substances from a full reactivity series when they were not tested.