The phase diagram of water has following features
a) Triple point : this is the point of intersection of three curves and is the point where all the three phases of water exist together
The triple point of water is 273.16K, and 4.58 mm Hg
b) Curves : these represents the presence of two phases together
the curve is shared by any two phases
i) melting curve : shared between liquid and solid phase of water
ii) vapour curve : shared between liquid and gas phase
iii) sublimation curve: shared between solid and gas phase of water
c) area : these are the portions of a phase diagram which represent single phase of water : solid, liquid or gas
Thus answer is :
Curves are used to indicate that the solid and liquid states of water can exist in equilibrium
