What is the difference between a pure substance and a mixture?

Hint:Pure substances and Mixtures are categorization of matter. Pure substances cannot be divided into their constituents but mixtures are made up by mixing two or more pure substances so they can be easily divided into their constituents.

Complete step by step answer:
A pure substance has definite composition. It is made up of a single element or compound. It is further categorized into solid, liquid and gas. Mixtures are formed by combining different elements and compounds. There are two types of mixtures namely- Homogeneous mixture and Heterogeneous Mixture. The homogeneous mixture has uniform composition; the word ‘homo’ means the same. Example of such a type of mixture is salt water because it contains uniform composition of salt throughout water. Heterogeneous Mixtures do not have uniform compositions; they are made up of various component parts. Sand is an example of heterogeneous mixture.
Pure substances have the same physical properties while physical properties of mixture vary due to their varying constituents. Pure substances cannot be separated by any physical processes while we can separate mixtures using the process of evaporation, filtering, etc.
Pure substances have fixed melting and boiling points while that of mixture varies. All molecules are the same in pure substances.

Note:
Elements cannot be separated by any physical or chemical means so they come under pure substances, ex- hydrogen gas is a pure substance. A mixture does not have any unique properties and any fixed ratios of the combining substances. They can be mixed in any amount. In the chromatography technique, mixtures will form several spots whereas pure substances will form a single spot.