The hydroxyl radical is the dominant oxidizing chemical in Earth's atmosphere. It is also the most common and most active free radical present in human and other animal tissue, yet it is contained in chemically pure water.

A radical is a molecular ion, a group of two or more atoms bonded together into a single unit that possesses an electric charge. A free radical is usually negatively charged and looking for a “;free”; positively charged ion to bond with by stealing or borrowing a proton in a process known as oxidation.

“;Hydroxyl”; refers to a chemical grouping in which oxygen and hydrogen are covalently bonded into a single negatively charged entity (OH). Chemically speaking, the hydroxyl radical is a hydroxide ion, and it frequently bonds with positively charged metal ions, for example sodium hydroxide (NaOH), commonly known as lye.

The most common place to find a hydroxyl radical is in a water molecule. Water might also be thought of as “;hydrogen hydroxide”; because the hydroxyl radical preferentially bonds with a positive hydrogen ion to form a water molecule (H0H). This reaction occurs when an acid and an alkali react chemically, forming water and a salt.

The reaction between acid and alkali is a unique type of oxidation-reduction reaction that is continually occurring in liquid water at the molecular level.

Although water molecules are exceptionally stable, in the liquid a small fraction spontaneously dissociates into positive hydrogen ions and negative hydroxide ions, maintaining a dynamic equilibrium as equal numbers of individual ions recombine to form water molecules.

This dissociation happens as water molecules bounce around with thermal energy at a thousand miles per hour, forming, breaking and re-forming incomplete nanoscale ice crystals in the liquid state. Both of these processes occur billions of time every second. The high rate at which hydroxide ions combine makes them a particularly aggressive oxidant.

In neutral (neither acidic nor alkaline) water at pH 7, the fraction of dissociated molecules is 10 to the minus-seventh, meaning that one out of every 10 million molecules is split into their component hydrogen and hydroxide ions at any instant of time.

The hydroxyl radical exists as a free radical in the atmosphere, where it is the primary removal mechanism for methane and volatile organic carbon compounds (VOC). It behaves like a typical gas molecule until it comes into contact with an organic molecule, virtually all of which contain hydrogen atoms bonded to carbon atoms. Because the hydroxyl radical's activity is so vigorous, for hydrogen it plucks hydrogen atoms from their bonds with carbon atoms, causing their ultimate destruction.

This is yet another example of the intricacies and complexities of water and the multiple roles it plays in supporting life and the global ecosystem necessary to maintain it.