Question

Which of the following turns potassium dichromate paper green?

• A. $N{{O}_{2}}$
• B. $HCl$
• C. $N{{H}_{3}}$
• D. $S{{O}_{2}}$

Answer 1

Answer: (D)

$S{{O}_{2}}$

Answer 2

$SO _{2}$ gas is a reducing agent, hence turns dichromate paper green due to formation of chromium sulphate. Sulphite radical $\left( SO _{3}^{2}\right)$ is identified by this. $K_2Cr_2O_7 + 3SO_2 + H_2SO_4 \to \underset{\text{green}}{Cr_2(SO_4)_2} + K_2SO_4 + H_2O$