A reversible reaction is one where there is a forward and backward reaction occurring:
The double arrow signifies a reversible reaction.
If in the above reaction the concentrations of A, B, C, D do not change, although the reaction is still in progress, then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.
Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.
The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.
So for the reaction:
The equilibrium constant is given by:
Where  represents the concentration of the species in moldm-3.
For gaseous systems, we use Kp instead of Kc. Here, the species are shownin the equilibrium equation in terms of their partial pressures. (In a mixture of gases, the proportion of the total pressure due to a particular gas is dependant on its mole fraction).