Everything in the world undergoes change. The change may be physical or chemical. A change is said to be physical when it does not involve change in chemical composition of the substance. But when the chemical composition of a substance is changed, the substance is said to undergo a chemical change or it is said that a chemical reaction takes place. Thus during a chemical reaction, the chemical composition of the substance is changed and new substances having new properties are formed. Thus a chemical reaction may be defined as the process in which original substances disappear and new substances are formed. In fact, during a chemical reaction, old bonds are broken and new bonds are formed. In other words, atoms are rearranged to form new substances.

Chemical reaction :- The processes, in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, are known as chemical reaction.

The nature and identity of products totally changes from the reactants. Observations which determines whether the chemical reaction has taken place or not.

(a) Chemical reaction must be associated with change in temperature i.e. Heat should be either evolved or absorbed.

(b)The reaction must occur between fixed quantities of the reactants.