A chemical reaction is a process involving one, two, or more substances, such as compounds or atoms. The substance(s) initially involved in a chemical reaction are called reactants.
Chemical reaction is characterized by a chemical change and it yields one or more product(s) which are different from the reactants.
Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction (in particular the notion of a chemical equation) is applicable to transformations of elementary particles, as well as nuclear reactions.
Many different chemical reactions are used in combinations in chemical synthesis in order to get the desired product. In biochemistry, series of chemical reactions form metabolic pathways, since straight synthesis of a product would be energetically impossible in conditions within a cell.