A coloured compound (A) reacts with dilute $H_2SO_4$ to produce a colourless gas (B) and colourless solution (C). The reaction between (B) and the acidified $K_2Cr_2O_7$ solution produces a green solution and a slightly yellowish precipitate (D). The substance (D) burns in air to produce a gas (E) which also can change colour of $K_2Cr_2O_7$ solution.