Question

Which of the following gases, in pure form, has a slightly blue colour?
(A) Ozone
(B) Oxygen
(C) Carbon dioxide
(D) Hydrogen

Answer 1

Hint: When a gas absorbs a part of the visible light, it transmits the remaining portion of visible spectrum. Due to this, the gas appears coloured. The colour observed is complementary to colour absorbed.

Complete answer:
The chemical formula of ozone, oxygen, carbon dioxide and hydrogen are $${\text{O}}_{\text{3}},{\text{O}}_{\text{2}},\text{C}{\text{O}}_{\text{2}}$$ and $${\text{H}}_{\text{2}}$$ respectively. Ozone and carbon dioxide are triatomic gases and oxygen and hydrogen are diatomic gases. Oxygen, ozone and hydrogen contain atoms of only one element whereas carbon dioxide contains atoms from two different elements.
Oxygen gas is colourless. Solid, liquid and plasma states of oxygen are pale blue in colour. Carbon dioxide gas is colourless. Hydrogen gas is also colourless.
In a liquid state, ozone has dark blue colour. In solid state, ozone has purple-black colour.
Ozone absorbs all wavelengths of light except blue light.
When physical state changes from liquid to solid, ozone molecules come close to each other. This changes the area of absorption and changes the colour.
Ozone gases, in pure form, have a slightly blue colour.

Hence, the correct option is the option (A).

Note: Ozone is an allotrope of oxygen. Ozone contains three oxygen atoms whereas an oxygen molecule contains two oxygen atoms. The physical and chemical properties of ozone are different from that of oxygen.