For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal …(X)… or normal …(Y)… of the substance.

For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance. The system here is in dynamic equilibrium and we can infer the following (i) Both the opposing processes occur simultaneously. (ii) Both the processes occur at the same rate so that the amount of ice and water remains constant.