Question

When $Hg{I_2}$ (yellow) will be turned to $Hg{I_2}$ (red) variety on?
A. Heating
B. Cooling
C. Application of mechanical stress
D. Subliming

Answer 1

Mercurous iodide shows a certain property known as thermochromism, by which a compound exhibits different colours at different temperatures. Thus, by examining the colours of this compound at various temperatures and other conditions, we can determine what factor makes it change its colour.

Complete step by step answer:
Mercurous iodide exists in two forms, the $\alpha$ form and the $\beta$ form, and both are stable at different temperatures. Let us now look at how this is possible and the conditions in which both states exist:
Normally, this compound exists in its $\alpha$ form as a red crystalline solid. But when it is heated beyond the temperature of $400K$, it undergoes a phase transition. This phase transition results in the formation of the $\beta$ form, in which the compound is in the form of a pale yellow crystalline solid. The phenomenon in which compounds change colour due to a temperature difference is known as thermochromism.
Thus, as we can see, the compound acquires yellow colour on heating. Hence, on cooling it, mercurous iodide gradually changes into its natural red colour.

So, the correct answer is Option A.

Note: Apart from the red and yellow states, mercurous iodide can also exhibit a state in which it is present as an orange crystalline solid. This can be obtained by recrystallization of the yellow form and is metastable, meaning that although it is not as stable as the red form, it is stable enough to retain its orange colour under the right conditions. Note that mercurous iodide is used in the manufacture of the Nessler’s reagent, used for the detection of the presence of ammonia.