Displacement reaction between iron and copper sulphate solution:

This is a classic example of a single (or simple) displacement reaction, in which a more reactive metal displaces a less reactive metal from a solution of its salt. The reactivity series of metals ranks metals in order of their tendency to lose electrons (get oxidised); a metal higher in the series can displace any metal below it from an aqueous solution of its compound. Iron lies above copper in the reactivity series, so this reaction is thermodynamically favourable.

The balanced chemical equation is:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Mechanistically, this is a redox reaction: iron atoms on the nail's surface lose two electrons each (are oxidised) to form Fe²⁺ ions, which go into solution as iron(II) sulphate, while Cu²⁺ ions from the solution simultaneously gain those two electrons (are reduced) and are deposited as neutral copper metal onto the surface of the nail.

• The characteristic blue colour of the copper sulphate solution (due to hydrated Cu²⁺ ions) gradually fades and is replaced by the pale green colour of the newly formed iron(II) sulphate solution (due to hydrated Fe²⁺ ions).
• Simultaneously, a visible reddish-brown coating of metallic copper is deposited on the surface of the iron nail, since the displaced copper has nowhere else to go and plates out onto the only solid surface present.

These two observations together — the colour change from blue to green and the appearance of a brown metallic coating — are direct, visible evidence that a chemical displacement has actually taken place, rather than merely a physical change. A white precipitate is not observed in this reaction; that kind of observation is typical of certain other reactions (such as the formation of insoluble sulphates or chlorides), not of this iron–copper sulphate system. The solution does not turn colourless, since it still contains coloured Fe²⁺ ions after the reaction; a colourless result with no change to the iron would suggest no reaction occurred, which contradicts both the chemistry (iron is indeed more reactive than copper) and the experimentally observed colour change and metal deposition.