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Combustion in Pure Oxygen,
Part 3:
Burning Sulfur — Fire and Brimstone


Sulfur is only mildly flammable under normal atmospheric conditions, but in pure oxygen, it burns with a very nice blue flame.  The products of the combustion are sulfur dioxide and sulfur trioxide:

S(s)  +  O2(g)  ——>  SO2(g)

2S(s)  +  3O2(g)  ——>  2SO3(g)

These substances react with water in the air to produce sulfurous and sulfuric acid, respectively:

SO2(g)  +  H2O(g)  ——>  H2SO3(g)  [sulfurous acid]

SO3(g)  +  H2O(g)  ——>  H2SO4(g)  [sulfuric acid]

This can be demonstrated by quenching the burning sulfur with a small amount of distilled water, and adding an appropriate acid-base indicator, such as methyl orange.


In the following demonstration, a small amount of sulfur is placed in a deflagrating spoon, heated in a Bunsen burner until it begins to burn, and then lowered into a jar of pure oxygen.  The sulfur then flares up into a much brighter blue flame, and eventually begins to throw off fumes of sulfur dioxide and trioxide.


Video Clip:  REAL, 4.20 MB



!!! Hazards !!!

Use a long deflagrating spoon to hold the sulfur in the oxygen jar.

Fumes of sulfur dioxide and sulfur trioxide are produced; these fumes should be avoided as they will hydrolyze to sulfurous and sulfuric acid on exposure to moisture.








John Emsley, The Elements, 3rd ed.  Oxford, Clarendon Press, 1998, p. 148-149, 198-199.

David L. Heiserman, Exploring Chemical Elements and their Compounds.  New York:  TAB Books, 1992, p. 32-36, 65-69.