Some reactions do not go to completion. The system still remains as a mixture of products and reactants. These such reactions are reversible reactions.

  • When reactants are mixed together, the forward reaction is initially fast.
  • However, as the concentration of reactants decrease, so does the rate of the forward reaction
  • As the concentration of the products increase, the rate of the reverse reaction also increases from zero initially
  • A stage is finally reached when the rate of the forward reaction is equal to the rate of the reverse reaction, called equilibrium.

When a chemical reaction system is at equilibrium,

  • The amount and concentration of the reactants and products remain constant, and thus, so does its macroscopic properties
  • The rates of the forward and reverse reactions are equal, but the reactions have not stopped.

equilibrium reaction graph

equilibrium concentration graph

Vapour pressure

  • When a liquid is placed in a sealed container, some of the liquid evaporates, producing a vapour above the liquid.
  • This vapour may also condense back into liquid form.
  • After some time, the rate at which the liquid is evaporating and the rate at which the vapour condenses reaches an equilibrium.
  • The pressure exerted by the vapour upon the liquid is called vapour pressure, and is useful for determining the physical properties of the liquid.

Saturated solution

  • When a soluble solid is placed in a liquid, it begins to dissolve
  • The dissolved particles can also crystallise to reform the solid
  • If enough solid is present, the rate at which the solid dissolves and the rate at which the dissolved particles crystallise will reach an equilibrium.
  • This is called a saturated solution.


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