For my latest celebrity scientist speaking gig [at a small church youth group] my glamorous assistant [my wife] found a new demonstration to add to my repertoire, Elephants toothpaste. It is described in this Journal of Chemical Education paper.
Hydrogen peroxide is thermodynamically unstable. However, you can buy bottles of it and they will remain useful for months. It will slowly decompose into water and oxygen.
2 → 2 H
2O + O
However, if you add some iron chloride it acts as a catalyst and increases the decomposition rate by a factor of a thousand. You will see some amount of "bubbling" due to the oxygen gas produced. If blood [which contains haemoglobin] is added the rate increases by a factor of a million. Even better, if you add the enzyme catalase, the rate increases by a factor of a billion. In the demonstration the catalase is present in the yeast that is added. Catalase is one of the fastest catalysts known. It performs an incredibly important biochemical function, that is essential to life existing. Hydrogen peroxide is a strong oxidant that could destroy many biomolecules. It is also an unwanted byproduct of many biochemical reactions. Biological systems use catalase to rapidly destroy the hydrogen peroxide before it can do harm.
The demonstration I did (and described in the JCE article) makes use of a dilute aqueous solution [a few per cent] of hydrogen peroxide. The spectacular video below makes use of a highly concentrated solution that is quite dangerous because it can cause chemical burns of the skin.
The above discussion follows the beautiful introduction to enzymes in chapter 11 of my favourite biochemistry text by Matthews, van Holde, Appling, and Anthony-Cahill
It contains the figure below, illustrating the key idea of how catalysts work: by lowering the energy barrier [the transition state] for a chemical reaction.