Question

why water is a compound but air is a mixture

Answer 1

Air is a mixture because:
1) Its components vary from place to place
2) Its components continue to vary
3) Its components can be seperated by physical means

Water is a compound because:
1) Its components are bound together chemically
2) Its components always occur in a fixed proportion -- be it tap water, sea water, etc.
3) Its components cannot be seperated by physical means.

Answer 2

1. The components of air change from place to place...that of water stays the same (H:O = 1:16 by mass).
2. Water does not show the individual properties of H and O, while the individual parts of air display their own property.
3. You can separate different gases from air through a physical process. The only way to obtain H & O from H2O is to pass electricity through it...
Mixtures have a varying composition, show the individual chemical & physical props. of each component & in most cases, can be separated physically. The opposite is true for compounds