If the name ‘hydroxide’ sounds toxic to you, that’s good. Hydroxide is a powerful chemical with potentially dangerous properties. Learn the definition and formula of hydroxide, as well as some interesting facts about it, in this lesson.

Hydroxide Ion: Definition and Properties

It may just be the similar ‘oxic’ sound, but to many, ‘hydroxide’ sounds like a toxic compound. It you believe this, you’re not exactly wrong. Hydroxide is made of hydrogen and oxygen. Chemicals containing hydroxide can be very corrosive, and sometimes very dangerous.

The hydroxide ion is a negatively charged molecule made up of one oxygen bonded to one hydrogen. When dissolved in water, the hydroxide ion is an incredibly strong base. In fact, according to the Arrhenius definition of a base, the presence of a hydroxide ion is what makes a chemical a base. A base is a chemical that has a high pH, tastes soapy, feels slippery and reacts well with acid.

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Because hydroxide has a negative charge, it is often found bonded to positively charged ions in what are known as ionic compounds. An ionic compound is a chemical composed of a positively charged ion bonded to a negatively charged ion. Some ionic compounds containing hydroxide dissolve well in water, like the corrosive bases sodium hydroxide or potassium hydroxide.

Other hydroxide containing ionic compounds are fairly insoluble in water, like bright blue copper (II) hydroxide or the brown iron (II) hydroxide. While they may be toxic, these compounds don’t pose as much immediate danger as sodium hydroxide or potassium hydroxide.

Formula for Hydroxide

The formula for hydroxide is OH-. In this compound, oxygen bonds with hydrogen by sharing two electrons. Hydroxide carries a negative charge because it has gained an electron. Oxygen, depicted as an O, is bonded to hydrogen, depicted as an H, and we can see where the most negative part of the compound is with the negative sign.

If you were to zoom in to the atomic level, hydroxide would not look like a couple of letters, however. Instead, you might see a particle that looks like this model, where the large red sphere is oxygen and the smaller gray hemisphere attached to the side is hydrogen.

Hydroxide Ion Facts

Chemicals containing hydroxide that easily dissolve in water are often some of the most potent, corrosive chemicals. Sodium hydroxide, often known as caustic soda or lye, is the active ingredient in powerful drain cleaners. Human tissues can be severely damaged by this compound if inhaled, ingested or even touched. High concentrations of sodium hydroxide can dissolve aluminum or even glass!

Dissolved hydroxide ions are aggressive chemicals. Because of their negative charge, they are very attracted to anything with even a slight positive charge. This attraction causes hydroxides to attack positively charged things vigorously, breaking chemical bonds and damaging structures.

On the other side of the spectrum, insoluble compounds that contain hydroxides are quite useful to humans. Antacid tablets often rely on aluminum hydroxide or magnesium hydroxide to quell heartburn. Other hydroxide containing compounds may just be beautiful, like the rare Henmilite crystal.

Lesson Summary

The hydroxide ion is a negatively charged molecule made of one hydrogen atom bonded to one oxygen atom. It is a base that often forms ionic compounds when bonded to a positively charged ion. Dissolved hydroxide ions are aggressive chemicals that can be very dangerous and corrosive. Often times they are useful as cleaning agents or antacids.