Fig. 11.1 refers unit mass of ice at- 20°C under one atmospheric pressure. Let us increase the temperature of the ice under constant pressure and observe. We see the following (Refer Fig. 11.2)
Process 1-2 : The temperature of ice increases from- 20°C to 0oc and there is a small change of volume. The ice is now at 0°C.
Process 2-3 : The solid ice at 0oc starts melting to liquid without any change in temperature. There is a decrease in volume.
Process 3-4: The temperature of the liquid water at 0°C increases to 100°C. The volume decreases to a minimum at 4°C and then increases upto point 4.
Process 4-5 : The liquid water at l00°C starts boiling to a vapour or gas at constant temperature with a large increase in volume.
Process 5 onwards : Both the temperature and volume of the vapour increases.
Note. The decrease in volume during melting is a peculiarity of water.