Physical Equilibrium

Physical equilibrium can be defined as the equilibrium which develops between different phases or physical properties. There is no change in chemical composition in these processes. It constitutes the existence of the same substance in two dissimilar physical states. The physical equilibrium can be:

• Phase equilibrium
• Solute-solid equilibrium
• Gas-liquid equilibrium

Types of Physical Equilibrium

Phase Equilibrium:

At 100°C, the number of ice molecules becoming water is equal to the number of water molecules becoming ice, as ice freezing from water to liquid. The rate of melting of ice is the same as the rate of freezing of water. Thus, there is a state of equilibrium between solid ice and liquid water.

Ice (s) ⇌ Water (l)

The number of molecules of a liquid getting vapour will be equal to the number of molecules condensing into liquid in a unrestricted vessel. The rate of evaporation of liquid water is same to the rate of condensation of water vapour. The vapour phase is in equilibrium with its own liquid phase.

Water (l) ⇌ Water (g)

Solute-Solid Equilibrium:

When a solute is dissolved in a saturated solution and it is in a contact with an undissolved solute, the number of molecules going  out of the solution is same as the number of molecules entering from the solid into the liquid. Consequently, the solute in a solution is in equilibrium with the undissolved solid.

Solute (aq) ⇌ Solute (s)

Gas-Liquid Equilibrium:

Gases that do not participate in any reaction with liquid may dissolve directly related to the pressure in the liquid. In a closed vessel, there is an equilibrium between the gas inside the liquid and the gas present above the liquid. For example, in soft drinks, carbon dioxide gas in the liquid is in equilibrium with the gas in the empty space of the vessel.

Gas (solution) ⇌ Gas (g)

Examples of Solid-Liquid Equilibria:

Suppose, ice and water in a perfectly enclosed thermos flask at 00C in an open atmosphere. No change will be witnessed in the level of water and quantity of ice, which implies that the rate of transfer of molecules from water to ice is same to the rate of transfer of molecules from water to ice.

Therefore, we can come to an end that this system is in a stable state. The following equation can be shown to demonstrate the same:

H2O (s)⇌ H2O (l)

The rate of melting is same as the rate of freezing

Examples of Liquid-Gas Equilibria:

Take filtered water in a closed vessel and start heating, the water changes to vapour. After a certain period of time, we will notice that the level of water becomes constant implying that there is no more conversion of water to vapour and vice-versa.

We can technically say that the rate of evaporation (liquid to vapour) is same as the rate of condensation (vapour to liquid) thus achieving a equilibrium state. Below is the following equation:

H2O (l) ⇌ H2O (g)

Rate of evaporation is same as the rate of condensation

Examples of Solid-Vapor Equilibria:

Sublimates (solid directly converts to vapour) can only show this kind of an equilibrium. Suppose, heating of solid iodine in a closed vessel, slowly the container is filled with violet coloured vapour and the intensity of colour increases with time.

After a certain time, the intensity of colour will not change with time. This inferred a steady state is attained, where the rate of sublimation of solid iodine is same as the rate of deposition of iodine vapour.

I2 (s) ⇌ I2 (vapor)

Rate of sublimation is same as the rate of deposition.

CONCLUSION: –

At a given temperature, equilibrium in physical processes is achieved only in a closed system. The opposing processes occur at the same rate and there exists a dynamic but stable condition during equilibrium in physical processes. All properties of the system that are measurable remain constant. Therefore, the physical equilibrium can be defined as the equilibrium which develops between different phases or physical properties. There is no change in chemical composition in these processes. It constitutes the existence of the same substance in two dissimilar physical states. The physical equilibrium can be: Phase equilibrium, solute-solid equilibrium and Gas-liquid equilibrium.