In general substances show a continuous increase of volume with increase in temperature (Fig. 11.3). The path is reversed when the substance is cooled slowly.
The states indicated by points 2, 3, 4 and 5 are known as saturation states. A saturation state is a state from which a change of phase may occur without changing temperature (pressure is also kept constant). Thus point 2 indicates saturated solid state. The point 3 is a saturation state because change of phase from liquid to solid may occur upon cooling. State 1 is not a saturated state because temperature will change before melting occurs.