In case of the Figure 1, it refers to the scenario where a mass of ice is at -20°C. it is under one atmospheric pressure and now we need to increase the temperature of the ice keeping the pressure constant and observe. The figure will help us in this regard.
In the first process, a slight change in the volume is observed and the temperature tends to increase from -20 degree Celsius to 0. The ice is static at 0°C.
In the next stage at 0°C. The solid ice starts to melt where any change in temperature is not witnessed. It is at this stage a decrease in volume is witnessed.
In the third and fourth stage of the process, the temperature of the liquid water increases to 100°C from 0°C. In the case of the volume it first decreases to 4°C and then goes on to increase up to 4 points.
In the fourth and fifth stage of the process, the water tends to start boiling at 100°C. It goes on to evolve into a gas or vapor at a constant temperature with an increase in volume.
In the fifth stage of the process, both the volume and the temperature tend to increase
A point to be considered here is one of the peculiarity of water is the decrease in volume.