A reversible reaction is said to be in a state of equilibrium when the rate of the. forward reaction is equal to the rate of backward reaction. At equilibrium, the concentrations of the reactants and products remain constant provided the conditions under which the reaction is performed are not changed. Some observable properties (colour, concentration, partial pressure or density) of the reactants or products may be used to indicate the concentration of the reaction at equilibrium.
For example, the colour of nitrogen dioxide (NO2) may be used in recognising the equilibrium state when the reaction takes place between carbon monoxide and nitrogen dioxide. During the reaction, the reddish brown colour of nitrogen dioxide slowly fades as it is changed to colourless nitric oxide. After some time, the intensity of the colour decreases and there will be no further change in colour. Then we can say that the equilibrium state is reached.
In the case of dissociation of calcium carbonate, the attainment of equilibrium is recognised by observing the constant partial pressure of carbon dioxide in the manometer.