Question

Which of the following turns potassium dichromate paper green?

• A. $NO_{2}$
• B. $HCl$
• C. $NH_{3}$
• D. $SO_{2}$

Answer 1

Answer: (D)

$SO_{2}$

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:
$K _{2} Cr _{2} O _{7}+3 SO _{2}+ H _{2} SO _{4} \longrightarrow \underset{\text{green}}{Cr _{2}\left( SO _{4}\right)_{2}} + K _{2} SO _{4}+ H _{2} O$