Question
What is a physical equilibrium? What type of equilibria are involved in physical equilibrium?
Solution
Physical equilibrium: When two or more than two phases of a substance exist in equilibrium with each other, it is called physical equilibrium. In such equilibrium, there are only physical changes in the system. For example:
(i) Solid-liquid equilibrium: Here, solid form of a substance remains in equilibrium with its liquid form.
A solid and liquid form of a pure substance remain in equilibrium only at a fixed temperature which is called melting point of solid or freezing point of a liquid form of the substance.
(ii) Liquid-gas equilibrium: Here liquid and gaseous phase of the substance are in equilibrium.
The equilibrium state is reached when the rate of vaporisation becomes equal to the rate of condensation of vapour to liquid in a closed container. At equilibrium state, even though molecules still move from liquid to vapour and from vapour to liquid state, there is no change in the quantity of the substance in each state because the two processes viz evaporation and condensation take place at the same rate. Thus, physical equilibrium is dynamic in nature.
(iii) Solid-vapour equilibrium : Solid and gaseous forms of a substance remain in equilibrium.
When solid is changed directly into vapour form, the process is called sublimation.
(i) Solid-liquid equilibrium: Here, solid form of a substance remains in equilibrium with its liquid form.
A solid and liquid form of a pure substance remain in equilibrium only at a fixed temperature which is called melting point of solid or freezing point of a liquid form of the substance.
(ii) Liquid-gas equilibrium: Here liquid and gaseous phase of the substance are in equilibrium.
The equilibrium state is reached when the rate of vaporisation becomes equal to the rate of condensation of vapour to liquid in a closed container. At equilibrium state, even though molecules still move from liquid to vapour and from vapour to liquid state, there is no change in the quantity of the substance in each state because the two processes viz evaporation and condensation take place at the same rate. Thus, physical equilibrium is dynamic in nature.
(iii) Solid-vapour equilibrium : Solid and gaseous forms of a substance remain in equilibrium.
When solid is changed directly into vapour form, the process is called sublimation.
