Everyday Chemicals – Hydrogen Peroxide

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It’s been a while since the last update to the Everyday Chemicals series – this latest graphic looks at hydrogen peroxide. Everyone’s familiar with the term ‘peroxide blonde’, stemming from the use of hydrogen peroxide in hair dyes, but this accounts for just one of the compound’s many uses; it’s also found in several other products you come across on a regular basis, and even in some rocket fuels.

Since the most well-known use of hydrogen peroxide is in hair dyes, it makes sense to start there. In dye mixtures, hydrogen peroxide acts as an oxidising agent. It oxidises melanins, organic compounds which absorb visible light and give hair its colour; this oxidation removes the parts of the compounds that cause colour, and causes the natural pale yellow of keratin (the protein that makes up hair) to show through. An alkaline compound, for example ammonia, is also present in the dye, to soften the hair cuticles and allow the oxidiser to reach the melanin. Peroxide is also responsible for oxidising other chemicals in the dye mixture, to result in the chemical compounds that bond to the hair as dyes.

However, it’s not just hair that hydrogen peroxide helps bleach, nor is this the most common use for the compound. Approximately half of all the hydrogen peroxide produced per year is used to bleach wood pulp and paper, and it’s also used in household bleaches as an alternative to chlorine-based compounds such as sodium hypochlorite (itself the subject of a previous entry in the everyday compounds series). Use a stain remover on your clothes before throwing them in the washing machine? It’s probable that that, too, contains hydrogen peroxide. It reacts with the colour-causing parts of molecules, which usually absorb in the visible spectrum, to change their structure and hence their absorption, and make stains disappear.

Because hydrogen peroxide is such a strong oxidising agent, it also finds applications as a disinfectant. It’s commonly available from pharmacies as a 3% solution in water, and is often used to clean cuts. If you’ve tried this, you’ll have noticed it bubbles when it comes into contact with the cut – this is due to the action of an enzyme called catalase in the blood. This enzyme catalyses the breakdown of hydrogen peroxide into oxygen and water, and the oxygen that’s produced causes the bubbling. The breakdown of hydrogen peroxide is a reaction that occurs anyway, the enzyme just speeds it up; this is why hydrogen peroxide is stored in bottles with dark glass, as even light can cause it to break down faster. The efficacy of hydrogen peroxide as a disinfectant for cuts is actually somewhat debated, so whether it’s beneficial is a moot point.

More of a novelty use of the compound comes in the shape of glow sticks. Hydrogen peroxide is one of the chemicals present in the stick, separated from the other compounds until the glow stick is snapped. When this happens, it mixes and reacts with the ester compound contained in the other compartment; one of the products this reaction produces is peroxyacid ester, an unstable compound which decomposes. When it does so, it produces energy which excites the dye molecules. When the excited dye relaxes, it releases a photon of visible light, causing the glow stick’s glow.

One final use of hydrogen peroxide is as a rocket fuel. At concentrations of 90% and greater, it can be used either on its own, with its decomposition producing oxygen, or as the oxidiser for other fuels such as hydrazine. Although liquid oxygen is more commonly used in rocket fuels, hydrogen peroxide satellite thrusters are still in use on some satellites. Hydrogen peroxide was also formerly used in submarine propulsion systems. Its use on the Russian submarine, the Kursk, was to blame for its sinking, as a hydrogen peroxide fuel leak from a torpedo is thought to have caused the explosion that ultimately led to the death of all 118 crew.



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References & Further Reading