UPSC NDA I 2015 General Ability Paper

Concept:The dimerization equilibrium of nitrogen dioxide explains the colour change from reddish-brown to colourless at low temperature.Explanation:Nitrogen dioxide ($\mathrm{NO}_2$) is a reddish-brown gas at higher temperatures.At low temperatures (below $-11.2^\circ\mathrm{C}$), two $\mathrm{NO}_2$ molecules combine to form dinitrogen tetroxide ($\mathrm{N}_2\mathrm{O}_4$).This dimer is colourless.The equilibrium is: $2\,\mathrm{NO}_2 (\text{reddish-brown}) \rightleftharpoons \mathrm{N}_2\mathrm{O}_4 (\text{colourless})$.As temperature decreases, the equilibrium shifts toward the colourless $\mathrm{N}_2\mathrm{O}_4$.Thus Statement I is true: colour of nitrogen dioxide changes to colourless at low temperature.Statement II correctly explains this: at low temperature $\mathrm{N}_2\mathrm{O}_4$ (colourless) is formed.Both statements are individually true, and Statement II is the correct explanation of Statement I.Answer:Option A (Both the statements are individually true and Statement II is the correct explanation of statement I).