Entropy of solid, liquid, and gas at triple point of water

Triple point defines a situation of simultaneous equilibrium between the solid, the liquid and the gas phases.

For such an equilibrium, you simply write:

$$ \Delta S= \frac{\Delta H}{T} $$

that rises from the fact that $\Delta G=0$.

Considering water molar ($m$) enthalpies for each phase transition at $273\,K$:

$$ \Delta H_{melting,m}=6.01\,kJ\,mol^{-1} $$ $$ \Delta H_{vaporisation,m}=45.05\,kJ\,mol^{-1} $$ $$ \Delta H_{sublimation,m}=51.06\,kJ\,mol^{-1} $$

one realises that

• the entropy of the gas phase is higher than the entropy of the liquid phase.
• the entropy of the liquid phase is higher than the entropy of the solid phase

In this respect the triple point has no peculiar behaviour compared to other points where a two-phase equilibrium is established.