The molecules are the group of two or more atoms which form the smallest identifying unit into which a pure substance can be divided and still retain the chemical properties and composition of that substance. The molecules are electrically neutral group of two or more atoms held together by chemical bonds.

Compounds are made up of only one kind of molecule. They are homogeneous, have definite physical and chemical properties, fixed melting and boiling point, and characteristic taste and colour. Examples of compounds are hydrochloric acid, nitrogen molecule, sodium hydroxide, etc.

A mixture is a substance made up of two or more than two substances (elements or compounds) mixed together in any proportion under any conditions. Components of mixture can be separated by simple physical methods. In a mixture, properties of its components are retained.