We are familiar with many chemical changes occurring around us. In such changes, substances are undergoing permanent changes and new substances are formed. When different substances react and new substances having different nature and identity are formed, we say a chemical reaction has occurred. In the language of science, a chemical reaction is a process which transforms one or more substances into new substances.

During chemical reactions, new substances with new properties are formed. The substances which take part in the chemical reaction are called reactants. The substances which are formed as a result of chemical reactions are called as products. The chemical reactions involve the breaking of old bonds and making of new bonds. During chemical reactions, energy is either evolved or absorbed. Generally, energy is needed to get the reaction going. The energy supplied may be in the form of heat, light, electricity or mechanical work ( in the form of mechanical shaking for proper contact between reactants). The chemical reactions vary depending upon the conditions.

A chemical reaction is expressed in terms of a chemical equation. It is a statement of a chemical reaction in terms of language of chemistry. In other words, it expresses a chemical change in terms of symbol and formulae of various elements and compounds. When chemical reactions occur, a variety of rearrangements of atoms may take place. As a result, the reactions occur in many different ways. The reactions in which two or more substances combine to form a single substance are called as combination reactions. In such reactions either two elements may combine to form a new compound or an element and a compound may combine to form a new compound or even two compounds may combine to form a new compound. In certain reactions a compound may break up into two or more simpler substances. Such reactions are called as decomposition reactions. These reactions generally take place when the energy is supplied in the form of heat, light, electricity or catalysts. Such reactions are reverse of combination reactions.

There are other types of reactions in which one element takes the place of another element in a compound. Such reactions are called as displacement reactions. In these reactions, an atom or a group of atoms in a molecule is replaced by another atom or a group of atoms. These type of reactions are very common in chemistry. These reactions mostly occur in solution form and a more active metal displaces or removes another element from a compound. In addition to above type of reactions, there are other types of reactions also, commonly known as redox reactions. In these reactions oxidation and reduction occur in the same reaction. No matter what type, they are always called as chemical reactions.