Drop a coiled copper wire into a clear solution of silver nitrate and, within minutes, glittering silver crystals start growing on the wire while the solution slowly turns blue. Put an iron nail into blue copper(II) sulfate and the nail soon wears a coat of reddish copper. In both cases a free element has pushed another element out of a compound and taken its place. In a single displacement reaction, a free element reacts with a compound, replacing ("displacing") one of the elements already in that compound. Whether this actually happens depends on which element is more reactive — and that's exactly what the activity series tells you.
Why reactivity decides: an electron tug-of-war
Metals react by losing electrons to become cations. A more reactive metal gives up its electrons more easily. When zinc metal sits in a solution containing ions, each zinc atom hands two electrons to a copper ion: zinc becomes and dissolves, and copper becomes a neutral atom that coats the zinc as a solid. The reverse cannot happen, because copper holds its electrons more tightly than zinc does, so copper metal has no way to force electrons onto . The evidence you can see: the blue colour of fades, reddish-brown copper appears, and the zinc strip slowly wears away.
The activity series ranks metals from most to least reactive, based on experiments like the ones above. A metal can only displace another metal ion from solution if it sits above that metal on the list — a more reactive metal 'wins' and takes its place, releasing the less reactive metal as a free element. The series also tells you how fiercely a metal reacts with water and acids:
| Metals (most → least reactive) | Reacts with | Example |
|---|---|---|
| Li, K, Ba, Ca, Na | cold water, steam and acids | |
| Mg, Al, Zn, Fe | steam and acids; with cold water, only a very slow reaction (Mg) or none | |
| Ni, Sn, Pb | acids only, slowly | |
| (H) | the dividing line | metals above H can release from acids |
| Cu, Ag, Pt, Au | none of water, steam or ordinary acids | NR |
Four versions of the same pattern
- Metal + ionic compound in solution: the metal replaces the metal cation.
- Very active metal + water: the metal replaces hydrogen in water (think of water as H–OH), giving a metal hydroxide + .
- Metal above H + acid: the metal replaces hydrogen, giving a salt + .
- Halogen + halide salt: a more reactive halogen replaces a less reactive halide ion.
Diatomic elements: HOFBrINCl
Seven elements never exist alone as single atoms in their natural, elemental state — they always pair up into diatomic molecules: — remembered with the mnemonic "HOFBrINCl". Whenever one of these seven elements appears alone as a reactant or product, write it with a subscript 2.
Halogens (Group 17) can also displace each other from a compound, following their own separate reactivity order: — reactivity decreases going down the group, the opposite trend from metals. The reason is the same tug-of-war seen from the other side: halogens react by GAINING electrons. A small fluorine atom has its outer electrons close to the nucleus and pulls an extra electron in strongly; a large iodine atom has its outer shell far from the nucleus, shielded by many inner electrons, so it attracts an extra electron much more weakly. When chlorine water is added to colourless potassium bromide solution, the mixture turns orange-brown as forms.
- Identify the free element and the element it would replace (metal replaces metal or hydrogen; halogen replaces halogen).
- Compare their positions on the correct series. If the free element is lower, write NR and stop.
- Write the new compound from ion charges (a free metal usually forms its common ion: Zn²⁺, Al³⁺, and Fe²⁺ in these reactions).
- Write the displaced element as a free element, diatomic if it is one of HOFBrINCl.
- Add state symbols and balance.