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Two exothermic reactions involving granulated sugar (sucrose) are common favorites as lecture demonstrations. The first uses concentrated sulfuric acid to dehydrate sugar (see eq 1), forming an impressive black column of carbon1 that eventually rises well above the top of the reaction vessel (1-4). The second reaction produces a brilliant purplish flame by using potassium chlorate (plus a drop of concentrated sulfuric acid, see eq 2) to oxidize sugar (5).
Although both of these demonstrations are vivid and memorable, they also produce copious quantities of acrid smoke. Aside from innocuous water vapor and CO2, these fumes contain a significant amount of carbon monoxide (1 ); furthermore, fumes from reaction 1 also contain SO2 and H2SO4 vapor (1), and fumes from reaction 2 contain Cl2, CIO, and C1O2 (5). All these gases are toxic; and since most classrooms have relatively poor ventilation, the outcome of these fine demonstrations is often a roomful of choking students. Reaction 1 has the further drawback that it uses a substantial amount of concentrated sulfuric acid, and the highly acidic product must be neutralized before disposal.
We have combined these two demonstrations, and in doing so have devised a reaction that uses no sulfuric acid, yields relatively little smoke, and produces an exciting and unpredictable growing column of black carbon. Reaction 2 calls for 3 g of KClO3 per 1 g of sucrose (8.4 mol KClO3/mol sucrose), plus a drop of concentrated H2SO4 (5); the reaction runs very hot and converts essentially all of the sucrose carbon to CO2. Our modified demonstration calls for only 1 g of KClO3 per 5 g of sucrose (0.56 mol KCLO3/mol sucrose). As a result, the reaction runs at a somewhat cooler temperature, and less than 10% of the sucrose carbon is converted to C02. The remaining sucrose dehydrates according to reaction 1 to produce a twisting black column of solid carbon.
Procedure
On a large (15 cm diameter) watch glass, weigh out 25 g...