Potassium chlorate, KClO3, is a white, crystalline or powdery solid which is a very good oxidizing agent; it is used in explosives, fireworks, matches, etc. When it decomposes under heating (especially in the presence of a manganese catalyst), it releases molecular oxygen, O2:
2KClO3(s) —heat—> 2KCl(s) + 3O2(g)
Sugar is, of course, extremely easy to oxidize, and is a good source of energy, as you know if you've ever eaten a candy bar.
When potassium chlorate and ordinary table sugar are combined, and a drop of sulfuric acid is added as a catalyst, the two react violently with each other, releasing large quantities of heat energy, a spectacular purplish flame, and a great deal of smoke. (The purple hue of the flame is presumably due to the heating of the potassium.)
In the following demonstration, roughly equal amounts of solid potassium chlorate and sugar (in the form of finely powdered confectioner's sugar) are placed in a glass jar, and mixed vigorously. Upon the addition of a drop of sulfuric acid, the chemicals react with each other, producing a tall purple flame, a lot of smoke, and a fairly spectacular explosion of the glass jar. The same demonstration is then repeated with a closer magnification. In the second video clip, the same chemicals are combined in a Pyrex beaker, with similar results.
Video Clip 1: REAL, 4.33 MB
Video Clip 2: REAL, 1.10 KB
I generally get the best results with this demo by using ordinary granulated table sugar or powdered confectioner's sugar. Reagent grade sucrose often consists of larger granules, and requires more sulfuric acid to get the reaction to sustain itself. When using confectioner's sugar, it is especially important to get a good mixing between the two chemicals, otherwise the finely powdered sugar can smother the fire.
This reaction often produces a great deal of smoke, and must either be done in a well-ventilated area, or in a (working) fume hood. (I've set off smoke alarms when doing this demo in hoods whose ventilation was under par.)
I've seen videos of this demonstration being done by dropping a source of sugar (such as a piece of candy) into a test tube containing molten potassium chlorate. However, the one time I experimented with such a procedure (using a candy cane for a sugar source), it resulted in a loud explosion which blew the bottom of the test tube to pieces, and I haven't been brave enough to try a repeat performance.
!!! Hazards !!!
Potassium chlorate is a powerful oxidizing agent; do not store the mixture of sugar and potassium chlorate since this can detonate unexpectedly.
Use caution when taking potassium chlorate out of the bottle; scraping the compound roughly can produce sparking, which can cause the bottle to ignite or explode.
This reaction must be done in a well-ventilated area, or in a fume hood.
Reaction of Potassium Chlorate and Sugar: Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. 79-80.
F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. New York: John Wiley & Sons, 1988, p. 152-153.
John Emsley, The Elements, 3rd ed. Oxford, Clarendon Press, 1998, p. 120-121.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992, p. 49-53.
Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.