Question

Air is regarded as a mixture because:
(A) Its pressure may vary
(B) Its temperature may change
(C) Its volume changes under different conditions
(D) Its composition may vary

Answer 1

Hint: A mixture is formed when two or more different substances combine together such that their individual characteristics are retained. Mixing of substance does not involve any chemical reaction and it is possible to separate the individual components of the mixture by physical separation methods.

Complete answer:
- Air is a homogenous and colourless mixture consisting of gaseous nitrogen, oxygen, carbon dioxide, argon, and some trace amounts of water vapour, methane, ammonia, etc.
- It is possible to separate air into its constituent gases by means of physical methods like fractional distillation.
- Since the boiling point of nitrogen is less than that of oxygen, when liquefied air is distilled, nitrogen is separated out first and oxygen is left in the mixture.
- No energy or heat is either absorbed or evolved while mixing gases.
- The identity of the constituent gases in air are retained. This is the reason why air shows the chemical and physical properties of the gases present in it.
- The composition of air is not fixed but varies with time and place. So, the property of air cannot be defined by a fixed chemical formula.
- Liquid air does not boil at a particular temperature. And thus, mixtures do not have constant boiling and melting points.
- Based on the above statements, air is regarded as a mixture.

Therefore, the answer to the question is (D) Its composition may vary.

Note: Air is a mixture and not a compound. Different substances react chemically to form a compound which has properties different than that of the individual components. Moreover, a compound has a definite formula and cannot be broken into its constituents by physical methods of separation.