Why is air considered a mixture and not a compound? List two reasons.

Air is considered a mixture and not a compound for the following reasons: 

1. Variable Composition: The composition of air is not fixed; the proportion of its constituent gases (nitrogen, oxygen, argon, carbon dioxide, etc.) can vary slightly from place to place and time to time. In a compound, elements are combined in a fixed ratio. 

2. Retention of Properties: The components of air (like oxygen and nitrogen) retain their individual properties. For example, oxygen in the air still supports combustion, and nitrogen still acts as a diluent. In a compound, the properties of the constituent elements are lost, and the compound has entirely new properties.