Question

Which of these statements are true about air?
i.Air is a mixture of several types of gases.
ii.Air exerts pressure
iii.The composition of air is constant in the atmosphere.
A.(i) and (ii)
B.(ii) and (iii)
C.(i) and (iii)
D.(i), (ii) and (iii)

Answer 1

Hint:We are aware that air is a mixture of different gases. The air in Earth’s atmosphere is made up of approximately\[78\% \] nitrogen and \[21\% \]oxygen. Air also has small amounts of lots of other gases, too, such as carbon dioxide, neon, and hydrogen.

Complete answer:
Air is a mixture of different gases comprising the Earth’s atmosphere. The mixture contains a group of gases of nearly constant concentrations and a group with concentrations that are variable in both space and time. The atmospheric gases of steady concentration and their proportions in percentage by volume are as follows - nitrogen (\[78.084\% \]), oxygen (\[20.94\% \]), argon (\[0.934\% \]), neon (\[0.0018\% \]), helium, methane, krypton, hydrogen, nitrous oxide, xenon.
Air not only has mass, but it also exerts pressure as the particles of air push in all directions and the force that is exerted is called air pressure.
 So, we can say that air is a mixture of several gases like nitrogen, oxygen and carbon dioxide and as air is like gas it exerts pressure. Hence, statement (i) and (ii) are correct.
But, the statement ‘the composition of air is constant in the atmosphere’ is incorrect as the composition of air varies in the atmosphere, i.e. statement (iii) is incorrect.

Therefore, the correct answer is option (A).
Note:
We should note that although the gases present in the air is relatively in small amounts, but these variable constituents is very important for maintaining life on Earth’s surface such as water vapour is the source for all forms of precipitation, Carbon dioxide, helps in the process of photosynthesis and also an important absorber and emitter of infrared radiation. Ozone is an effective absorber of ultraviolet radiation from the Sun and effectively shields the Earth from all radiation.