Is a substance a mixture a compound or an element?

Liquid air is a mixture not a compound.

As we know that air is a mixture of different gases. It contains 78% nitrogen and 21% oxygen and remaining 1% is comprising various other gases.

The process of preparing liquid air does not involve any chemical reaction and hence no compound is formed. It is basically a physical process involving compression of air as well as cooling simultaneously. Thus liquid air remains a mixture.