Question

Which of the following is a coloured compound:
A.Nitrogen dioxide
B.Nitrous oxide
C.Nitric oxide
D.Ammonia

Answer 1

To answer this question, you should recall the concept of properties of oxides of nitrogen. Use a complementary colour scheme and apply it for the colour of nitrogen compounds. The compound showing colour is an oxide of nitrogen which causes air pollution.

Complete step by step answer:
Nitrogen Oxide molecule absorbs light at a frequency which is not present in the visible spectrum but reflects all colors we can see. This is the reason due to which Nitrogen dioxide appears as a white gas in high concentration due to dimerization.
At low pressures or at high temperatures, $N{O_2}$ has a deep brown colour, but at low temperatures the colour almost completely disappears to form dinitrogen tetroxide​.
At room temperature there is equilibrium between the two gases. Therefore, nitrogen dioxide is the coloured compound i.e. a reddish-brown gas.

Hence the correct answer to this question is option A.

Note:
We know that the majority of all the coloured compounds have transition element(s) ions in them. The colour of transition metal ions is due to the presence of unpaired electrons in it. These electrons absorb radiations of one colour and undergo transition from one level to another within the d-subshell.
Due to this electronic transition, coloured light is emitted which is complementary colour of the light absorbed. Down you'll find the list of absorbed colour and the complementary colour which is given out:
Violet-Yellow
Blue-Orange
Green-Red
Yellow-Violet
Orange-Blue
Red-Green