Experiment 23: Prepare Sulphur dioxide gas


  • Prepare and study Sulphur dioxide gas


Materials required:

  • Iron fillings/ nails
  • conc. sulphuric acid
  • dil. sulphuric acid
  • potassium permanganate solution
  • potassium dichromate solution
  • red and blue litmus papers
  • test tube
  • delivery tube


  • 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






Look at the colour of the gas liberated

White fumes


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


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