The reactions which can occur in both the directions i.e. forward and backward directions, under similar conditions are called reversible reactions.
e.g. (i) when hydrogen gas is passed over heated iron oxide, iron and water are produced.
Fe3O4 + 4H2 → 3Fe + 4H2O
On the other hand if steam is passed over powdered iron at the same temperature, oxide of iron and hydrogen are formed.
3Fe + 4H2O →Fe3O4 + 4H2
When the reaction is carried out in a closed vessel, both reactions can take place in the vessel.
Thus, reactions which proceed in both the directions and do not reach to completion stage are called reversible reactions.
3Fe + 4H2O ⇔ Fe3O4 + 4H2
