HALLOWEEN COLORS

Sometimes chemical reactions occur slowly and we try to speed them up. We can do this by adding a catalyst. To a chemist, a catalyst is a compound that enhances the rate of a chemical reaction without being consumed by that reaction. We take advantage of a catalyst to do some colorful chemistry in this experiment.

Hydrogen peroxide is a fairly common chemical, though the type you can purchase at the pharmacy is rather dilute. In concenratrated form it is not stable and it slowly falls apart into water and oxygen, while releasing heat. The rate of this decomposition can be increased by adding iron(II) ions (species with a charge of +3).

We actually start this demonstration with the catalyst in the large test tube. It is an orange liquid. When we add concentrated peroxide, the solution turns black and seems to foam. The foams arises from the release of oxygen gas. The dark color arises from a reaction between parts of the peroxide with the iron (it forms a complex ion). As all of the peroxide reacts, however, the orange color returns. More peroxide can be added and the color change cycles again. The continual return to the orange iron(III) solution is an important observation. Aside from the colorful show, it indicates that we have a catalyst. It makes the point that the catalyst speeds the reaction but doesn't get used up (or consumed).