A salt (P) when heated with ${\mathrm{K}}_{\mathrm{2}}{\mathrm{Cr}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ and conc. ${\mathrm{H}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}$ liberates a gas which is absorbed in NaOH solution. The NaOH solution turns yellow. When this solution is acidified with acetic acid and lead acetate solution is added, a yellow precipitate (Q) is formed.
When (P) is mixed with MnO₂ and heated with conc. ${\mathrm{H}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}$ a gas (R) is evolved which turns starch-iodide paper blue.