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Q: What’s trapped in the bottle?Only three things are needed for this demonstration: hydrogen peroxide, a catalyst, and flask with a narrow neck. (OK, maybe more than three things. Those involved with running this kind of demo <a name=\'more\'></a>should really wear safety glasses and gloves.) First, about 50 milliliters of 30% hydrogen peroxide is added to a flask (such as an Erlenmeyer flask or rounded boiling flask with a flat bottom). Then, a catalyst is dropped in. Finally, a genie (or puff of water vapor) emerges from the bottle.
No magic is necessary- just a bit of science.
The chemical compound hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>
is often used as a bleaching agent or disinfectant. Over time, hydrogen peroxide will break down or decompose into water and oxygen.2 H<sub>2</sub>O<sub>2</sub> (aq) ⟶ O<sub>2</sub> (g) + 2 H<sub>2</sub>O (l)
(The abbreviation “aq” stands for “aqueous.” This means that the substance has been dissolved in water. The other abbreviations used are for liquid (l) and gas (g).)Because hydrogen peroxide decomposes slowly on its own, the genie in a bottle demonstration requires a catalyst to speed the reaction up. A catalyst lowers the activation energy, or the energy required to start a reaction. The lower activation energy makes it “easier” to start the reaction, and the rate goes up.There are a few catalysts that can be used for hydrogen peroxide decomposition. Two of them are potassium iodide (KI) and sodium iodide (NaI). In both cases, the iodide ion is the part that acts as the catalyst by participating in the below pair of reaction steps.H<sub>2</sub>O<sub>2</sub> (aq) + I<sup>–</sup> (aq) ⟶ H<sub>2</sub>O (l) + IO<sup>–</sup> (aq)
H<sub>2</sub>O<sub>2</sub> (aq) + IO<sup>–</sup> (aq) ⟶ H<sub>2</sub>O (l) + O<sub>2</sub> (g) + I<sup>–</sup> (aq)Because the iodide is a catalyst, it isn’t consumed or used up in the reaction. Instead, the reaction starts with iodide and ends with iodide, and the iodide can be used again.Another possible catalyst to use for the reaction is manganese dioxide (MnO<sub>2</sub>
. The possible reaction mechanisms involving manganese dioxide and hydrogen peroxide can be found at this link.So if you add a catalyst to the hydrogen peroxide in the bottle, the reaction is accelerated, and oxygen and water are quickly created. The “genie” that shoots up from the bottle is the water vapor that is propelled out of the bottle with a little help from the oxygen.Does this demonstration sound familiar? It might be because it’s pretty close to the elephant toothpaste demonstration. It’s the same decomposition reaction sped up with a catalyst, just with some added soap to help make the foam.Like the elephant toothpaste demo, the genie in a bottle demonstration is exothermic, meaning it releases heat. If you were to touch the flask, you would feel just how warm it gets after the reaction has been completed.There are a few little upgrades you can do to make the demonstration more “showy.” For example, a set-up that I used before involved a flask covered in aluminum foil with fake gemstones glued on. I’ve also seen this demonstration where the catalyst was wrapped in a bit of tissue paper and tied to a string. The string was then wedged with a cork so that the packet hovered over the hydrogen peroxide until the cork was removed. The packet of catalyst then dropped in to kick-start the reaction, releasing the genie.Unfortunately, you won’t get three wishes from this demonstration. Sorry.
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