Experiment 23: Prepare Sulphur dioxide gas

Place few pieces of iron filings (about 5 g) in a test tube and arrange the apparatus as shown in Fig
Add 5 - 10 mL of conc. sulphuric acid carefully
Place the cork in its position on the test tube (along with delivery tube) and heat the contents gently. The gas formation starts after some time.
Collect the gas in the gas jar and study its properties as per the steps given in the observation table
Dispose the remaining sulphuric acid in the test tube only after diluting with ~250mL of water

Test	Activity	Observation
Color	Look at the colour of the gas liberated	White fumes
Smell	Fan the gas gently towards your nose	Strong pungent
Litmus test	Bring moist blue and red litmus papers near to the mouth of the test tube	Turn wet blue litmus to red
Reaction with potassium permanganate	Take about 2 mL of potassium permanganate solution in a test tube, add about 1 mL of dilute HSO₄ and pass sulphur dioxide gas in this solution.	Potassium permanganate solution is decolorized
Reaction with potassium dichromate	Pass sulphur dioxide through another test tube containing acidified potassium dichromate Solution	Orange solution turns green

Sulphur dioxide is acidic in nature.
It is a strong reducing agent. It decolourises acidified potassium permanganate (KMnO4) solution.
Acidified potassium dichromate solution (K2Cr2O2) is also turned green by SO2.
The reactions with KMnO2 and K2Cr2O7 are due to the reducing property of SO2 and oxidising nature of acidified KMnO4 and K2Cr2O7.

Sulphur dioxide is prepared by the action of hot concentrated sulphuric acid on iron fillings