Equation (2) is called the law of mass action. The concentrations of a pure liquid or a pure solid are constant, regardless of whether any of the solid or liquid is used in the reaction. Therefore, these values can be combined with the equilibrium constant and eliminated from the right side of equation (2). Since water is a pure liquid, the concentration of water can be eliminated from equations of reactions occurring in aqueous solution. Likewise, the concentration of a solid dissolving in a liquid can also be eliminated from the law of mass action.
If >1, the equilibrium rests in the direction of the products. If <1, the equilibrium rests in the direction of the reactants. If >> 1 then their will be virtually no reactants present when equilibrium is reached.
At any given stage in the reaction, the concentrations of the reactants and products can be measured to see where the current state of the reaction is in relation to equilibrium. When these non equilibrium values are substituted into the fraction of equation (5), the result is the reaction quotient:
