It is an experimental fact that most of the process including chemical reactions, when

carried out in a closed vessel, do not go to completion. They proceed to some extent

leaving considerable amounts of reactants & products. When such stage is reached in

a reaction, it is said that the reaction has attained the state of equilibrium. Equilibrium

represents the state of a process in which the properties like temperature, pressure,

concentration etc. of the system do not show any change with passage of time. In all

processes which attain equilibrium, two opposing processes are involved. Equilibrium

is attained when the rates of the two opposing processes become equal.

If the opposing processes involve only physical changes, the equilibrium is called

Physical Equilibrium. If the opposing processes are chemical reactions, the

equilibrium is called Chemical Equilibrium.

If you add more and more salt in water taken in a container of a glass and stirred

with a glass rod, after dissolving of some amount. You will find out no further

salt is going to the solution and it settles down at the bottom. The solution is now

said to be saturated and in a state of equilibrium. At this stage, many molecule of

salt from the undissolved salt go into the solution (dissolution) and same amount

of dissolved salt are deposited back (Precipitation).

Thus, at equilibrium rate of dissolution is equal to rate of precipitation.