A reactant that removing electrons from other reactants in a chemical reaction. oxidising agents cause other substances to be oxidised in chemical reactions while they themselves are reduced. For example, nitrate ion is an oxidising agent in the following reaction:

Cu(s) + 4 H+(aq) + 2 NO3-(aq) Cu2+(aq) + 2 H2O( ) + 2 NO2(g)

Copper gets oxidised (its oxidation number goes from 0 to +2) while the nitrogen gets reduced (from +5 in nitrate to +4 in nitrogen dioxide).