Hydroxide Ion: Definition & Formula

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  • 0:01 Hydroxide Ion:…
  • 1:27 Formula for Hydroxide
  • 2:07 Hydroxide Ion Facts
  • 3:12 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

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.

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.

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