Hydrogen peroxide 30% in a large round flask decomposes to boiling water and oxygen when postassium iodide is added.
The 12L Round Bottom Flask is set on white C-Fold towels covering a large cork ring on the lab bench. 100-150 ml of 30% hydrogen peroxide is carefully poured in. The liquid should be visible against the white towels from the perspective of the class, and any camera, if used.
The catalyst is 5 g of potassium iodide in a small plastic weighing boat labeled KI.
Safety goggles and gloves. Raise the projection screen and make sure the flask is stable and pointed directly up. 30% H2O2 is a reactive hazard, and will burn skin. After the reaction, the solution is hot, contains potassium iodide only and is not a hazard.
Add the KI and get back
When the KI hits the hydrogen peroxide, it changes color to brown as the active catalytic form of the iodide is formed. This shows up well against the white towels. The brown region bubbles and spreads, and rapidly the entire volume of hydrogen peroxide is releasing oxygen, and then boiling up to nearly filling the flask. Hot oxygen and steam shoot out of the mouth of the flask, to the ceiling.
If the instructor wishes, another aliquot of 75 - 100 ml of hydrogen peroxide can be added, to show that the catalyst is still present. The solution turns brown and bubbles, but does not react as vigorously as the first time.
Since we use the same 12 L flask, it must be rinsed of all trace of iodide before putting it upside down to drip dry. Store on the large cork ring on the shelf with the neck low to keep dust from settling. A lot of things act as a catalyst for decomposing hydrogen peroxide, so keep that flask clean!