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.