Question

What do you observe when $S{O_2}$ is passed through acidified potassium dichromate?

Answer 1

Hint:$S{O_2}$ is produced commercially on a vast scale, some of the production methods are: burning sulphur in air, burning ${H_2}S$ in air or b roasting various metal sulphide ores with air in smelters.

Complete step by step solution:
$S{O_2}$ is a colourless gas which has a choking smell, and is very soluble in water. Sulphur dioxide in solution is almost completely present as various hydrated species and the solution contains a minute amount of sulphurous acid. $S{O_2}$ levels above 5 ppm are poisonous to man, but plants are harmed at appreciably lower levels.
$S{O_2}$ may be detected in the laboratory using following methods which are:
1. By its smell.
2. By passing through acidified potassium dichromate is also one of the methods of detection of sulphur dioxide, on passing $S{O_2}$ gas through acidified potassium dichromate because it changes the filter paper moistened with the potassium dichromate solution green, due to formation of chromium cation $C{r^{3 + }}$. The above process has a reaction like the one given below:
${K_2}C{r_2}{O_7} + 3S{O_2} + {H_2}S{O_4} \to C{r_2}{(S{O_4})_3} + {K_2}S{O_4} + {H_2}O$

Hence from the above we can observe that sulphur dioxide on passing through the solution of potassium dichromate turns the solution green.

Additional Information
$S{O_2}$ also on passing through starch iodate paper changes starch iodate paper into blue colour. These are some of the methods that one can use to confirm the presence of sulphur dioxide.

Note:
$S{O_2}$ gas forms discrete V-shaped molecules, and this structure is retained in the solid state. The bond angle in the molecule is approximately 190 degrees.