A chemical reaction is the transforming at the molecular level of one or more substances, known as reactants, into another substance or substances, referred to as products. Such transformations are usually accompanied by the absorption or emission of energy.
If the reactants are single elements, their product will be one or more compounds. If a reactant is a compound, its products can be its constituent elements and/or one or more new compounds.
A very simple example of a chemical reaction is the splitting of water into its constituent elements hydrogen and oxygen by passing an electric current through it. Another example is the creation of water when a mixture of hydrogen and oxygen is exposed to a spark or heat. The former requires an input of energy to break the bonds between the two gases and the latter releases energy with the creation of the bonds.
Chemical reactions are fundamental to a wide range of processes in nature and to life itself. They are also the basis for a vast number of industrial and household activities, ranging from the burning of fuels to produce electric power and heat to the transformation of raw ingredients into appetizing meals through the creation or modification of textures, flavors and appearances.
Chemical reactions differ from physical changes, which are alterations in physical properties without changing the chemical composition. Examples of types of physical changes include changes of state (i.e., changes between solid, liquid and gas phases), the creation or separation of mixtures, the creation of solutions and magnetization.
Whereas chemical reactions involve the rearrangement of electrons surrounding the nuclei of atoms and do not affect the nuclei, nuclear reactions involve changes in the nuclei themselves, which can result in the transformation of one element into another. Nuclear reactions involve vastly larger energy changes than chemical reactions. Also in contrast to chemical reactions, the rates of nuclear reactions are generally independent of external conditions such as temperature, pressure and the presence of catalysts, and such reactions are generally irreversible.