Reactivity Series

In names, this is potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver and gold. Carbon and hydrogen are shown in brackets because they are not metals; they are useful reference points within the series.

A metal's place is determined by comparing its reactions under similar conditions. A faster or more vigorous reaction generally indicates a more reactive metal. No single test separates every metal clearly, so evidence from reactions with water and dilute acids is combined.

Why a metal reacts

Metal atoms react by losing one or more electrons and becoming positive ions, called cations. This can be represented generally as:

Here, $\mathrm{M}$ represents a metal atom and $n$ is the number of electrons lost. A highly reactive metal loses electrons and forms cations readily. A less reactive metal has a smaller tendency to do this. The series therefore compares the relative tendency of metal atoms to form cations, rather than merely being a list to memorise.

Using reactions with water

The metals near the top provide the clearest reactions with cold water. They form a metal hydroxide and hydrogen:

For example:

Hydrogen escaping produces bubbles, while the dissolved metal hydroxide makes the resulting solution alkaline. Potassium reacts more vigorously than sodium, and sodium more vigorously than calcium, supporting their order near the top of the series.

Water is less useful for distinguishing metals further down the list because their reactions with cold water are very slow or absent under ordinary classroom conditions. A lack of visible reaction does not prove that several metals have identical reactivity; a different test is needed to separate them.

Using reactions with dilute acids

Dilute hydrochloric acid or dilute sulfuric acid can distinguish many metals that react little or not at all with cold water. A reacting metal forms a salt and hydrogen:

With dilute hydrochloric acid the salt is a chloride; with dilute sulfuric acid it is a sulfate. The escaping hydrogen causes effervescence. Under the same conditions, magnesium fizzes rapidly, zinc reacts steadily and iron reacts more slowly, giving the order:

Hydrogen acts as the dividing reference point. Metals above hydrogen in the series can form cations by displacing hydrogen from a dilute acid, so hydrogen gas is produced. Metals below hydrogen—copper, silver and gold—do not react with dilute hydrochloric or dilute sulfuric acid in this way. Copper therefore gives no hydrogen bubbles, placing it below iron and hydrogen.

Very reactive metals such as potassium and sodium do not need to be compared using dilute acid: their vigorous reactions with water already show that they belong near the top, and adding them to acid would be unsafe.

Combining the evidence

The two kinds of reaction answer different parts of the comparison:

• reactions with water distinguish the most reactive metals;
• reactions with dilute acid distinguish several metals in the middle of the series;
• failure to displace hydrogen from dilute acid places copper, silver and gold below hydrogen.

The observations are outward evidence of the same particle-level idea. Vigorous fizzing means cations and hydrogen are being formed quickly; slower fizzing means they form more slowly; no acid reaction means that the metal does not displace hydrogen under those conditions. Together, these patterns support the complete order required here.