A red solid is insoluble in water. However, it becomes soluble if some KI is added to water. Heating the red solid in a test tube results in liberation of some violet coloured fumes and droplets of a metal appearing on the cooler parts of the test tube. The red solid is _____.
(A) ${(N{H_4})_2}C{r_2}{O_7}$
(B) $Hg{I_2}$
(C) HgO
(D) $P{b_3}{O_4}$

Hint: If an inorganic compound gives violet fumes upon heating, then there is a strong possibility of iodide ions in the compound which gets released in the form of iodine gas. The metal that gives droplets is liquid at room temperature.

Complete step by step solution:
We will see each property given in the question and then find the compound.
- It is given that the compound is insoluble in water. From the compounds given in the options ${(N{H_4})_2}C{r_2}{O_7}$ is easily soluble in water. So, this cannot be a correct answer. The other three compounds given are insoluble in water.
- It is given that the solid on heating gives violet fumes and the metal gets deposited on the cooler parts of the test tube. Now, if violet fumes are evolved on heating any inorganic compound, then there is a strong possibility that this compound has iodide ions and the violet gas is due to iodine gas.
- Alongside that if the metal gets deposited on the cooler side of the test tube on heating the compound, then the metal in the compound can be Pb, Hg, Au, Ag.
- So, taking into consideration the both points we can say that this compound is $Hg{I_2}$. Its reaction can be shown as below.
\[Hg{I_2}\xrightarrow{\Delta }H{g_{(l)}} + {I_{2(g)}}\]
\[Hg{I_2} + 2KI \to {K_2}Hg{I_4}\]
- It also gets dissolved in water upon addition of KI in the solution which is shown in the above reaction.
- $Hg{I_2}$ is also a scarlet red coloured compound. Thus, it is the correct answer.

Therefore, the correct answer is (B).

Note: Note that KI is added to increase the solubility of many compounds as it can form new ionic compounds with the substance and hence makes it soluble. Generally the substance in KI is added is covalent in nature. i.e. we also add KI to dissolve solid ${I_2}$ in water.