When a white crystalline compound $\mathrm{x}$ is heated with ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ and concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4,$ a reddish-brown gas $\mathrm{A}$ is evolved. On passing $\mathrm{A}$ into caustic soda solution a yellow coloured solution of $\mathrm{B}$ is obtained. Neutralising the solution of $\mathrm{B}$ with acetic acid and on subsequent addition of lead acetate, a yellow precipitate $\mathrm{C}$ is obtained. When $\mathrm{x}$ is heated with $\mathrm{NaOH}$ solution, a colourless gas is evolved and on passing this gas into ${\mathrm{K}}_2{\mathrm{HgI}}_2$ solution, a reddish-brown precipitate $\mathrm{D}$ is formed. Identify $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}$ and $\mathrm{x}$. Write the equations of all the reactions involved.