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Acid Rain


Molten sulfur reacts with oxygen in air to form SO2 and SO3 , which is swirled in the headspace above water.  The resulting sulfuric acid dissolves into solution, which changes the universal indicator from green to yellow to orange to red as the pH drops.


Both sulfur dioxide and sulfur trioxide are toxic.

Chemicals and Solutions

  • universal indicator solution
  • powdered sulfur


  • 500 mL flat bottom boiling flask
  • deflagrating spoon
  • bunsen burner and matches
  • universal indicator array display
  • tap water


  1. Fill the flask about half full with tap water and add several drops of universal indicator.  The solution should be green.
  2. Place some sulfur in a deflagrating spoon and heat it in a bunsen burner until it is molten.  Now place the deflagrating spoon in the headspace of the flask such that it is not immersed in the water.
  3. Gently swirl the flask until the indicator changes from green to red.

(Alternatively, the 6000mL florence flask can be used.  The volume of water can be increased to slow down the indicator change)


During combustion sulfur combines with oxygen to form sulfur dioxide and sulfur trioxide:

\( \ce{S_{(s)} + O2_{(g)} -> SO2_{(aq)}} \)

\( \ce{2S_{(s)} + 3O2_{(g)} -> 2SO3_{(aq)}} \)

The sulfur dioxide and sulfur trioxide then combine with water to give acids:

\( \ce{SO2_{(g)} + H2O_{(1)} -> H2SO3_{(aq)}} \)

\( \ce{SO3_{(g)} + H2O_{(1)} -> H2SO4_{(aq)}} \)

The acid formed changes the indicator. 

The burning of fossil fuels in industry, power plants and in homes accounts for most of the SO2 emitted to the atmosphere.

The sulfur dioxide is oxidized by several pathways to sulfur trioxide.

Both sulfur dioxide and sulfur trioxide react with rain water to form acids.

The resulting acids can corrode buildings made of limestone and marble.

In the northeastern U.S. the precipitation has a pH of 4.