Experiment 7: Non-metallic oxides are acidic



Show that non-metallic oxides are acidic in nature

Materials required:

  • Powdered Sulphur
  • Red & blue litmus paper
  • Test tubes
  • Dropper
  • Test tube holder
  • Cork with a hole
  • Delivery tube


  • Take 5g sulphur powder in a test tube (A). Set up the experiment as shown. Heat it over a spirit lamp.
  • Collect the gas liberated on heating Sulphur in test tube B
  • Shake test tube B to mix the gas with water
  • Take few drops of the solution from test tube B in a dropper and drop a drop on blue and red litmus paper
  • Observe the change in the colour of the litmus papers


  • There is no change in the colour of red litmus paper.
  • Blue litmus paper turns red


  • The non-metallic oxide after dissolving in water shows acidic character. Sulphur on burning in air forms sulphur dioxide gas, which dissolves in water and form sulphurous acid
  • Sulphur + Oxygen (from air) → Sulphur dioxide
  • Sulphur dioxide + Water → Sulphurous acid
  • Sulphurous acid turns the colour of blue litmus paper red.


  • Sulphur forms two main oxides; the gas sulphur dioxide (SO2) and the liquid sulphur trioxide (SO3).
  • Sulphur dioxide is a dense colourless gas, which is soluble in water, and has a suffocating and unpleasant smell of burnt matches. It has a melting point of -72.7°C, and a boiling point of -10°C.
  • Sulphur dioxide gas can be made directly by heating its constituent elements. Burning molten sulphur in either air or pure oxygen leads to a reaction, which produces a pale blue coloured flame. This looks quite impressive in a darkened room.
  • S8 (l) + 8 O2 (g) → 8 SO2 (g)
  • Sulphur dioxide is an acidic gas and this can easily be demonstrated by adding water and a few drops of universal indicator to a container of the gas. The resulting acid is the weakly dibasic acid sulphurous acid (H2SO3)
  • SO2 (g) → SO2 (aq)
  • SO2 (aq) + H2O (l) → H2SO3 (aq)
  • Sulphur dioxide is a major component of acid rain since it mixes with water vapour in the atmosphere, reacting to produce sulphuric acid (H2SO4). This is possible as UV radiation in the upper atmosphere catalyses the reaction between sulphur dioxide and oxygen to produce sulphur trioxide which goes on to react with water. Much has now been done to reduce SO2 emissions with desulphurisation of fuels to help reduce acid rain.
  • A simple test for sulphur dioxide is to pass the gas over a piece of filter paper soaked in acidified Na2Cr2O7. The paper goes from an orange colour for the Cr6+ to a green colour for Cr3+. As the oxidation number of chromium is reduced from 6+ to 3+ this clearly indicates a useful property of sulphur dioxide, namely it is a reducing agent. It has found use as an antioxidant, which can help prevent food from spoiling.