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The foam jet happens when we add hydrogen peroxide to a cylinder flask containing potassium iodide dissolved in soapy water (The violet color comes from a little quantity of iodine) Two reactions occur: $H_2O_2$ $+$ $I^-$ $\longrightarrow$ $H_2O$ $+$ $IO^-$ $IO^-$ $+$ $H_2O_2$ $\longrightarrow$ $H_2O$ $+$ $O_2$ $+$ $I^-$
What is the role of the iodide ion ?
It is returned after those reactions, so it acts as a catalyst.
Summarise the two reactions to make only one!
$2H_2O_2$ $\longrightarrow$ $2H_2O$ $+$ $O_2$
Where does the foam come from?
The gas oxygen ($O_2$) produces the foam !