A chemical reaction is a transformation resulting from the interaction between two atoms or molecules. Reactions involve the breakage and reformation of the chemical bonds which hold atoms together and can therefore cause changes in the structure or composition of substances.

A reaction may involve two or more reactants that combine to form a reaction product, it may involve a compound that breaks down to create individual reactants. In biological systems, chemical reactions are responsible for processes such as metabolism, respiration, and digestion.

There are four basic categories of chemical reactions. These are known as combination, decomposition, single replacement, and double replacement reactions. Specific types of reactions in these categories include oxidation, reduction, ionization, combustion, polymerization, hydrolysis, condensation, and rearrangement reactions. Regardless of the type, reactions always involve an exchange of energy. If the energy requirements of the reaction product are less than that of the...