Equilibrium in Physical Processes

State of equilibrium can also be reached in physical processes.

Liquid - Vapour Equilibrium

Take some quantity of a liquid in an empty container and close it. Initially the vapour pressure above the liquid will be zero. The liquid will evaporate and its vapour will fill the empty space above it. The rate of evaporation is maximum in beginning. As vapours build up, its pressure increases and the rate of evaporation slows down. Also the reverse process of condensation begins and its rate gradually increases with the increase in the vapour pressure.

After some time the two rates (of evaporation and condensation) become equal and the following equilibrium is established.

Liquid ⇌ Vapour

At equilibrium, the vapour pressure reaches its maximum value and is known as the saturated vapour pressure. At a fixed temperature, each liquid has its own characteristic vapour pressure. The vapour pressure of a liquid increases with rise in temperature.

Solid - Vapour Equilibrium

Volatile solids sublime to form vapour. The situation is just similar to the liquid vapour system. When kept in a closed container at a constant temperature the following equilibrium is established.

Solid ⇌ Vapour

Such an equilibrium can be established by keeping some solid iodine in a gas jar covered with a lid.

Gradually the purple coloured iodine vapours fill the jar and the following equilibrium is established.

I₂ (s) ⇌ I₂ (g)

Solid - Liquid Equilibrium

Below its freezing point a liquid freezes spontaneously

Liquid → Solid

When heated above its melting point the solid melts spontaneously

Solid → Liquid

At the melting point, the two phases are in equilibrium 

Solid ⇌ Liquid

Because the above two processes occur simultaneously and at the same rate, such an equilibrium is characterized by its temperature i.e. the melting point of the solid.

Solute - Solution Equilibria

When sugar crystals are put in a saturated solution of sugar in water; it will appear that no change is taking place and sugar appears to remain undissolved. Actually, the undissolved sugar does dissolve in the saturated sugar solution and an equal amount of sugar separates out from the solution. The solid sugar and the sugar solution form an equilibrium system which is dynamic in nature.

sugar (s) ⇌ sugar solution (saturated)

The equilibrium is established when the rate of dissolution of sugar becomes equal to the rate of crystallization.

solute (s) ⇌ solution (saturated)

This equilibrium is known as Solubility Equilibrium.