Reduction is a chemical reaction that involves addition of hydrogen or removal of oxygen from a substance. When hydrogen is passed over heated black copper oxide, the latter changes to reddish-brown copper metal. Here copper oxide is said to be reduced to copper by loss of oxygen.
H2 + CuO → Cu + H2O
Chlorine reacts with hydrogen in the presence of diffused light to form hydrogen chloride. Chlorine is said to be reduced to hydrogen chloride by gain of hydrogen.
Cl2 + H2 → 2HCl
A reducing agent is one that can reduce other substances by removing oxygen from them or by supplying hydrogen to them. In the reaction between copper oxide and hydrogen, hydrogen is the reducing agent as it removes oxygen from copper oxide. In the reaction between chlorine and hydrogen, hydrogen is the reducing agent as it supplies hydrogen to chlorine.
