The active mass represents the molar concentration of the reactants (or) products.
\(Active \ mass=\cfrac { n }{ v } =\cfrac { number\ of\ moles }{ volume\ in\ litre } \)
unit of active mass = mol dm^-3 ( or) mol L^-1

When a gas dissolves in a liquid under a given pressure, there will be an equilibrium between gas molecules in the gaseous state and those dissolved in the liquid.
Example: In carbonate beverages the following equilibrium exists.
CO₂(g) ⇌ CO₂ (solution)

When sugar is added to water at a particular temperature, it dissolves to form sugar solution. When more sugar is added to that solution, a particular stage sugar remains as solid and results in the formation of saturated solution. Here a dynamic equilibrium is established between the solute molecules in the solid phase and in the solution phase.
Sugar_(solid) \(\rightarrow\) Suga_(solution)

(i) Physical equilibrium: A system in which the amount of matter constituting different phases does not change with time is said to be in physical equilibrium. H₂O(s) ⇄ H₂O(1). Solid-liquid equilibrium
(ii)  Chemical equilibrium: Chemical reactions in which the forward and backward reactions are proceeding at the same rate and no macroscopic change is observed is said to be in chemical equilibrium. H₂(g) + I₂(g)⇄2HI(g)

At a particular stage, the rate of the reverse reaction is equal to that of the forward reaction indicating a state of equilibrium.