Hydrogen Peroxide (H2O2) is a colourless liquid that resembles water in many respects. Its physical properties are very similar to those of water, except that it is 40% denser. The main difference between hydrogen peroxide and water, however, is in its chemical behaviour. The single bond between the two oxygen atoms is weak, so that H2O2 readily fragments into either H and HO2 or two OHs. Either way, the resulting species are free radicals, which means they are very reactive, and this makes H2O2 a very powerful oxidising agent. For this reason it has been utilised in rocket propulsion, when it is used to oxidise the hydrazine fuel, liberating hot gases (steam and oxygen) which propel the rocket forward.
The oxidising power of H2O2 has also been used for decades in homes as a treatment for minor cuts and scratches. Its oxidising properties are effective in inhibiting growth and killing microbes. What makes hydrogen peroxide unique is the foaming action that takes place whenever it is placed on a cut. Some people used to believe that this foaming action indicated the presence of infection, and that the H2O2 was foaming as it got to work destroying the disease. In reality hydrogen peroxide foams any time it comes into contact with blood, because an enzyme in blood catalyses the decomposition of H2O2 to water and oxygen gas. But a benefit of the foaming action is that hydrogen peroxide acts as a cleaning agent, bringing any embedded dirt up to the surface. H2O2 is present in trace amounts in honey, which is why before the advent of modern chemical preparation methods, honey used to be used for dressing wounds.
H2O2 is such a strong oxidising agent that it is sold in a concentration of only 3% in water. This makes it much safer to handle, but has the disadvantage of slowly reacting with the water and decomposing. This decomposition is increased when the molecules are in the presence of light, so H2O2 is normally sold and stored in brown bottles.
Another role for H2O2 is as a bleaching agent. The oxidising ability of H2O2 allows it to react directly with double bonds in large organic molecules, forming organic peroxides. Since the double bonds are often what causes the molecules to absorb light, and therefore give the molecule its colour, removal of them destroys the pigments and so removes the colour. H2O2 is used in this way to bleach wood pulp to make white paper, and the melanin in hair. Some of the most famous Hollywood movie stars, such as Marilyn Monroe and Jean Harlow, were 'peroxide blondes' and perhaps owe some of their fame to the H2O2 molecule. One advantage of hydrogen peroxide over some other bleaching agents, such as chlorine gas, is that the decomposition products, water and oxygen, are not harmful.
Hydrogen peroxide also plays a less beneficial role in modern life, as one of the causes of photochemical smog. The action of sunlight decomposes it and forms free radicals, which then attack unburned molecules of hydrocarbon fuel and turn them into molecules which irritate the nose and eyes.