A metal higher up in the reactivity series will displace any metals lower in the series from their compounds. The reaction
is called a displacement reaction. For example, what happens if an iron nail is placed in a copper sulphate solution? [answer]
On the other hand, a metal of lower reactivity cannot displace a higher metal so there is no reaction.
Consider a competition for oxygen between magnesium and copper oxide. Since magnesium is more reactive than copper, it
displaces copper from its compound, and itself becomes magnesium oxide.
Copper oxide + magnesium —> magnesium oxide + copper
CuO + Mg —> MgO + Cu
In the above reaction, copper is said to be reduced as it loses oxygen (reduction), and magnesium is oxidised as it gains oxygen (oxidation).
However, there is no reaction between magnesium oxide and copper because copper is less reactive than magnesium and cannot displace magnesium from its oxide.
Thus, the reactivity series can be used to predict whether a reaction would occur between metals and metal compounds.