A chemical reaction can be understood in two ways: (1) In the laboratory we know a chemical reaction has occurred because the physical and chemical properties of the matter have changed and (2) form an atomic perspective (e.g. Dalton) a chemical reaction has occurred because the atoms have "rearranged" themselves. both of these perspectives are important to keep in mind whenever dealing with chemical reactions.
The substances that must come in contact before a chemical reaction can occur are known as "Reactants" and the substances that are formed from the reaction are known as "Products". The chemist denotes a chemical reaction with a reaction arrow " ---> ";
Generally, the reactants and products are compounds or elements but more descriptive equations are possible e.g.,
A chemical reaction must obey the overriding principle of conservation of mass. It does so through a balancing of the reactants and products. This principle as well as others are discussed in subsequent pages.