Elephant's toothpaste. This is a demonstration of the rapid decomposition of hydrogen peroxide. A solution of hydrogen peroxide is placed in a measuring cylinder, along with some liquid soap (such as washing-up liquid) and some food colouring. When a catalyst such as potassium iodide is added, the hydrogen peroxide rapidly decomposes to water and oxygen. The oxygen is trapped as bubbles in the liquid soap and foams out of the flask. The reaction is exothermic, the foam can get very hot.