Depending on the extent of reaction, reactions are of two types: reversible and irreversible reactions. Reactants, written on the left hand side of a chemical equation, proceed in their reactions. The products are written on the right hand side of the chemical equation. This is indicated by the direction of the arrow in a chemical equation. This is a direct reaction.
For example when steam is passed over hot iron, hydrogen gas and ferroso‐ferric oxide are produced.
In some chemical reactions, as soon as the products are formed they begin to react among themselves and actually a backward or reverse reaction starts.
It is interesting to note that if carried out in a open vessel, the reaction cannot go in the backward direction because, one of the products of the reaction, the hydrogen gas escapes. But if the reaction is carried out in a closed vessel, the reaction becomes reversible, because when hydrogen gas is passed over hot ferroso‐ferric oxide, steam and iron are produced.
These two reactions may be represented by a single equation as
Thus a chemical reactions that proceeds in two directions is known as a reversible reaction.