Pure solids and liquids are excluded from the equilibrium expression because their concentrations remain constant throughout the reaction.

In the context of chemical equilibrium, the concentrations of reactants and products are essential for determining the position of equilibrium. This concept is often articulated through the law of mass action, which states that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants. However, this law applies exclusively to substances that are in the gaseous state or dissolved in a solution.

Pure solids and liquids do not participate in the equilibrium expression because their concentrations do not change during the reaction. The concentration of a pure solid or liquid can be regarded as its density, which remains constant regardless of the quantity present. This stability arises because the particles in a solid or liquid are closely packed and lack the freedom to move and disperse like those in a gas or solution. Consequently, adding or removing a pure solid or liquid does not influence the equilibrium position.

Additionally, the activity of pure solids and liquids is defined as one. In thermodynamics, the activity of a substance represents its ‘effective concentration’ within a mixture. For pure solids and liquids, this value remains one, independent of the actual quantity present. This characteristic further explains their exclusion from the equilibrium expression.

In contrast, the concentrations of gases and solutions are subject to change during a reaction, as their particles can move freely and disperse. Therefore, these substances are included in the equilibrium expression. The concentrations of gases are typically expressed in terms of partial pressures, while the concentrations of solutions are conveyed in terms of molarity.