Why Nacl is a substance but aire is not a substance?

Answer 1

They are both matter. NaCl (salt) is a pure chemical (in this case an ionic crystal). Air is a mixture of different gases, N2, O2, CO2, Ar and a few traces of NOx, O3 etc.. The difference is that the gases in air do not form a chemical compound or "substance", they are just mixed together. In comparison, NaCl is a single substance, not a mixture.

Answer 2

Air is by general use a substance though not a chemical substance. It occupies space, has mass, but as a definable gas with constant constitution it is problematic. If air had an invariable constitution no matter the location it would fit the definition of a chemical substance. Air would need to have the same gases in the same ratios no matter where it was sampled but if I took a sample of air over Los Angeles, Mount St. Helen, and in Yellowstone Park I would not find the same gaseous constituents in the same ratios.

http://creationwiki.org/Pure_substance

A substance can be either an element or a compound. A compound substance consists of two or more elements chemically-bonded together.

Answer 3

I suppose by 'substance' you mean 'solid' while air is not solid? Well it all depends on the type of bonding: in solid substances the bonding is 'tight' so that most of the atoms stay close together and so form solids while in air (they are also compounds: for eg. CO2, O2, N2, CO, H2, etc.) they are only loosely bonded and so they drift away from each other.
In liquids the molecules 'slide' past each other, but are still close together to form a substance.
We don't see the individual atoms as such in air and so air is also invisible, but it is still there: you still feel it, due to its density, temperature and so on.

Answer 4

In salt the two elements present bond with one another. In air the elements are mixed together, but do not bond.