Hydroxide

Quick Facts

Formula: OH⁻

Pronunciation: hy-DROK-side

Etymology: Hydro + oxy + -ide

Meaning: The negative ion made of one oxygen and one hydrogen, responsible for basicity in water.

What It Really Means

Hydroxide is the reason bases are bases. When a base dissolves in water, it produces hydroxide ions. Sodium hydroxide gives Na plus and OH minus. Potassium hydroxide gives K plus and OH minus. Calcium hydroxide gives Ca 2 plus and two OH minus. The OH minus is what makes the solution basic. Now here is a common confusion. Hydroxide and hydroxyl are not the same. Hydroxide is the free ion, OH minus, floating in solution. Hydroxyl is the group, OH, bonded to a carbon. Sodium hydroxide contains hydroxide ions. Ethanol contains a hydroxyl group. Same letters, different state, different chemistry. One is basic, the other is part of a neutral alcohol. Also, hydroxide is the opposite of hydronium. Hydronium is H3O plus, the acidic ion. Hydroxide is OH minus, the basic ion. In pure water, they exist in equal amounts, at a concentration of 10 to the power minus 7 molar each. That is why pure water is neutral. And here is a practical point. Strong bases like sodium hydroxide are corrosive. They react with fats and proteins in your skin to cause deep burns. Weak bases like ammonia are less dangerous but still need care. Hydroxide is not a harmless ion, even though it is small and simple. Respect it in the lab.

Common Examples

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)
• Ammonium hydroxide (NH₄OH)

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