give reason.. why air is a mixture and not a compound? and also give reason why tap water is a mixture and not compound?

Air is not a compound because its components are not chemically bonded.

The individual compounds and molecules (O2, N2, CO2, etc...) still stay the same while they are mixed in with each other. For example, For example, pure oxygen is a supporter of combustion. Oxygen present in air also supports combustion. Pure carbon dioxide turns lime water milky. Carbon dioxide present in air also turns lime water milky.

The proportions of the constituents of air vary from place to place and even from time to time. In a compound, the proportions of different constituents always remain fixed.

On distillation of liquefied air, nitrogen distills off first, leaving behind oxygen in the liquid form. This is the characteristic of a mixture, not of a compound.

No thermal change occurs when the constituents of air (oxygen, nitrogen etc.) are mixed together. Liquid air does not have a fixed boiling point. This is the characteristic of a mixture, not of a compound.

Hence air is a mixture of compounds and elements.

Pure water would be a compound as it is made up of 2 atoms of H and 1 atom of oxygen and are chemically bonded. But tap water as a whole is a mixture, because it contains the water and dissolved chemicals such as Calcium Carbonate and magnesium, and even fluoride. A mixture is not chemically bonded together, just different elements or compounds physically mixed and distributed throughout.

So, tap water is a mixture.