Question

6. What is metastable equilibrium? Explain with reference to the water system.

Answer 1

The figure above represents the vapour pressure curve( P Vs T) of water system and we can see that the curve OA that represents the water and vapour equilibrium can be extrapolated beyond the triple point 'O' (given by the dotted LINE OD which is the Continuum of OA) that is it is possible to cool water below its freezing point without solidification as it is the extrapolation of the water and vapour equilibrium curve . And it is also below the freezing point of water and therefore the liquid below its freezing point is called in a super cooled state, the supercooled water has its Vapour pressure and that changes with temperature. The supercooled liquid is not quite stable and described as metastable state. Any point on the line represents the equilibrium between water and vapour below the triple point and is called the metastable equilibrium. Here from the curve we can see that the line OD is above the line OB that is the vapour pressure curve of Ice and it is indicating that the metastable supercooled water has a higher vapour pressure than the stable solid state at a given temperature and it is the general principles that vapour pressure of the metastable phase is greater than that of the stable phase.