Why air is consider as mixture but not a compound and why water is consider as compound but not a mixture.

Air is considered as a mixture because it exhibits following properties:

1. Each component present in air retains its properties.
2. Each component can be separated by simple physical processes.
3. The components do not have any fixed proportion. All gases are present in different amount. Example, in greener area—more oxygen and water vapour is present; near industrial area—air consists of lot of impurities and smoke suspended in it.

1. water is made up of two elements in a fixed ratio
2. water has totally different properties from the two elements
3. its constituents can be separated only by chemical or electrochemical reactions
4. water is formed by reaction between hydrogen and oxygen.

thats why water is a compound not a mixture