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We’re gearing up for Halloween here at hBARSCI, which means a lot of spooky science.
Food Coloring (optional)
1. Carefully add about 15ml of the hydrogen peroxide to a 250ml graduated flask. Place the flask on a large sheet of plastic or newspaper (or perform the demonstration in a large plastic tray or sink).
2. Add about 4-5ml of detergent to the hydrogen peroxide in the cylinder. (For a colorful reaction add a few drops of food coloring also).
3. Add 1-2g of potassium iodide to the peroxide/detergent in the cylinder.
DISPOSAL: The resulting foam from the reaction and any liquid remaining in the cylinder may be flushed down the drain with copious amounts of water.
The reaction taking place is a decomposition of hydrogen peroxide, or H2O2. When broken down, a molecule of hydrogen peroxide will split into a water molecule and an oxygen atom.
While water is a very stable molecule, oxygen ions are not. In order to reach a stable energy state, they must bond to another atom or molecule. During the decomposition of H2O2, there are plenty of oxygen atoms floating around, so they simply bind to each other.
This reaction is favorable from an energy standpoint, and occurs continuously at an imperceptibly slow rate. This is why hydrogen peroxide expires and must be replaced after several months once opened.
Foam is formed when the oxygen gas becomes trapped in water molecules; however, the reaction normally proceeds too slowly to allow this to happen. The oxygen gas forms a single bubble that soon pops and releases the oxygen into the air.
The solution is to add a catalyst to the reaction. A catalyst is any compound that speeds up the reaction taking place. Our catalyst in this case is a solution of potassium iodide. The addition of the iodine ions increases the energy of the system and forms an unstable bond with oxygen atoms, speeding up the reactions taking place.
These two reactions need far less energy to occur than the simple decomposition of H2O2, and proceed far quicker. With the breakdown happening so rapidly, the oxygen gas does not have time to escape the peroxide/water solution, and instead forms a foam.