One of my favourite demos. 

Either use a 500ml measuring cylinder or in this case a 500ml round bottom flask (makes it easier to see what’s happening). Add 100ml of 50% hydrogen peroxide and 10ml washing up liquid and mix thoroughly.
In a separate beaker dissolve 5 grams of potassium iodide in 10ml water. (Really good idea to get the students to feel how endothermic the dissolving process is for potassium iodide).
And add the potassium iodide to the flask and stand well back.
Safety. Hydrogen peroxide is harmful and at this concentration will cause burns. Make sure you and students are wearing goggles. Get students to stand well back from the reaction. Oh and make sure you have loads of sodium thiosulphate solution to help with the clean up. 

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