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# A red solid is insoluble in water. However, it becomes soluble if some KI is added to water. Heating red solid in a test tube produces violet coloured fumes and droplets of metal appear on the cooler parts of test tube. The red solid is

(a) $(NH_4)_2Cr_2O_7$
(b) $HgI_2$
(c) $HgO$
(d) $Pb_3O_4$

Answer: $HgI_2$

$HgI_2$ is a scarlet red compound which is insoluble in water
$HgI_2 + 2KI \longrightarrow K_2(HgI_4)$ (soluble)
$HgI_2 \xrightarrow{\Delta} Hg + I_2$ (violet vapour droplet)