Question

When sulphur dioxide is passed through potassium dichromate solution, the colour changes from to .

• A. green
• B. blue
• C. yellow
• D. orange
• E. pink
• F. white

Answer 1

Answer: (A), (D)

The correct options are
A green
D orange

One of the tests to identify presence of sulphate ion in a solution is potassium dichromate test.
Step 1: Cu is treated with $H_{2}S{O}_4$.
$\mathrm{Cu}\left(s\right)+2H_2{\mathrm{SO}}_4(\mathrm{conc}.)\to {\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+{\mathrm{SO}}_2\left(g\right)+2H_{2}O\left(l\right)$
Step 2: Acidified potassium dichromate (orange in colour) is added to this. It combines with the sulphur dioxide released from the previous reaction.
A colour change - orange to green is observed as chromium sulphate is formed. This colour change indicates the presence of sulphate ion.
$K_2{\mathrm{Cr}}_2{O}_7\left(\mathrm{aq}\right)+H_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+3{\mathrm{SO}}_2\left(g\right)\to {\mathrm{Cr}}_2\left({\mathrm{SO}}_4{)}_3\right(\mathrm{aq})+K_2{\mathrm{SO}}_4(\mathrm{aq})+H_{2}O(l)$

$K_2{\mathrm{Cr}}_2{O}_{7\to }$Orange
${\mathrm{Cr}}_2({\mathrm{SO}}_4{)}_{3\to }$Green