Introduction to Equilibrium

Equilibrium refers to the state of balance out of various opposing factors. A static stone kept on a table is at an equilibrium as the gravitation force is balanced by the normal force from the table. Various opposing factors counter the effects of each other making net impact as zero on the system.

Consider two glasses of water. One is filled with colored water, say blue. Another is plain clean water. The water of one glass is poured into second one and vice versa. The two water are mixed by repeating this process. At start, when colored water is added to clean water, the latter turns blue. However, the intensity of the blue color is low. But as the mixing is continued, the intensity of blue color in clean water continuously increases. After some time, the intensification of the color reaches a maximum and no further deepening of color takes place even after repeated interchanging of the water between the two glasses. This is equilibrium condition.

Similarly, an equilibrium can be observed in a closed vessel half filled with water. The water evaporates and exists as vapor phase. The vapor particles collide with liquid phase and is trapped there, i.e. convert in to liquid. At a time when the rate of vaporization is equal to rate of condensation, the system is in equilibrium. It can be seen as :
$$ce {H2O (l) <=> H2O(vap)} $$

The double half arrows indicate that two way processes are happening simultaneously. The equal lengths of the half arrows indicate that the rate of two opposing processes are same.

The composition of the system at given equilibrium is called as equilibrium mixture.

For a chemical equilibrium, the mixture of reactants and products at the equilibrium condition is called as equilibrium mixture.