A colourless water soluble solid $A$ on heating gives equimolar quantities of $B$ and $C . B$ gives dense white fumes with $\mathrm{HCl}$ and $C$ does so with $\mathrm{NH}_{3} . B$ gives brown precipitate with Nessler's reagent and $C$ gives white precipitate with nitrates of $\mathrm{Ag}^{+}, \mathrm{Pb}^{+}$and $\mathrm{Hg}^{+} . A$ is