A chemical reaction alters the distribution or arrangement of the atoms. However, there is no loss of atom because according to the Law of Conversion of Mass, Matter can neither be created nor destroyed.
For example, the makeup of water is two molecules of Hydrogen and One Molecule of Oxygen (that is O). It can change states and still retain its composition (that is, the same number of atoms) however if it comes in contact with Sodium metal (Na), the atoms become re-arranged and new substances are formed. The two substances that are formed are:
molecular Hydrogen (H2) and
Sodium hydroxide (NaOH).
When this happens, we know that a chemical reaction has happened.
