<P> A chemical reaction is a process that leads to the transformation of one set of chemical substances to another . Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation . Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur . </P> <P> The substance (or substances) initially involved in a chemical reaction are called reactants or reagents . Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants . Reactions often consist of a sequence of individual sub-steps, the so - called elementary reactions, and the information on the precise course of action is part of the reaction mechanism . Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions . </P> <P> Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration . Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms . </P>

The elements or compounds that go into a chemical reaction are