Chemical reactions are reversible and may reach a dynamic equilibriumA dynamic equilibrium is one in which the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the reactants and products remain constant.. The direction of reversible reactions can be altered by changing the reaction conditions.
Reversible reactions
Many chemical reactions are reversible reactions. This means that the productsSubstances formed in a chemical reaction. can be changed back into the original reactantsSubstances present at the start of a chemical reaction.. This is not obvious when a reaction 'goes to completion', where very little or no reactants are left.
Examples of reactions that go to completion are:
complete combustionBurning in a plentiful supply of oxygen or air. Complete combustion of a hydrocarbon produces water vapour and carbon dioxide. of a fuel
many precipitation reactionA reaction in which an insoluble solid is formed when certain solutions are mixed.
effervescenceThe formation of bubbles of gas produced by a chemical reaction in a liquid. in which a gas escapes
It is more obvious in reactions that do not go to completion that the reaction is reversible. This is the case when the reaction mixture contains both reactants and products.