How do I know whether a substance is pure or impure?

Studying the properties of the substance can indicate whether it is pure or impure. A pure substance will have these properties: 

• Homogeneous with a uniform composition throughout its mass
• Definite composition 
• Definite physical and chemical properties
• Sharp melting and boiling points

Elements and compounds are examples of pure substances. 

Impure substances will have these properties: 

• Homogeneous or heterogeneous mixture with a variable composition throughout its mass
• Made up of two or more pure substances mixed together in any ratio
• Variable composition
• Variable melting and boiling points

Solution of salt in water, air, seawater, and smog are common examples of impure substances.