Question

explain why air is considered as a mixture and not a compound

Answer 1

Air is a mixture for of the following reasons: 
1. Air does not have a fixed composition. Its composition varies from place to place. 
2. The components of air can be separated by a physical method such as fractional distillation of liquid air. 
3. Liquid air does not have a definite boiling point. 
4. We can prepare air artificially by mixing the various components of air in the same proportions in which they occur at a place. This is a physical process because no energy changes occur.

Answer 2

air is not a compound because its components are not chemically bonded
air is mixture because individual mixture still stya the same when they are mixed