Key Stage 4
Meaning
A Hydroxide ion (OH-), is a negative ion of Hydrogen bonded to Oxygen found in acid solutions.
About Hydroxide Ions
- When a basic compound dissolves in water it forms Hydroxide ions.
- Hydroxides in solution are alkalis.
- Hydroxide ions are responsible for the corrosive properties of alkalis.
- Hydroxide has gained an electrons to have a relative charge of -1.
- Hydroxide forms ionic bonds with positive ions.
- Hydroxides are ionic compounds.
Examples
Metal Hydroxides form Hydroxide ions in water.
- NaOH(aq) → Na+(aq)+OH-(aq)
- KOH(aq) → K+(aq)+OH-(aq)
- Mg(OH)2(aq) → Mg+2(aq)+2OH-(aq)
- Ca(OH)2(aq) → Ca+2(aq)+2OH-(aq)
- Al(OH)3(aq) → Al+3(aq)+3OH-(aq)
Metal Oxides react with water to produce Metal Hydroxides which also form Hydroxide ions.
- Na2O(s) + H2O(l) → 2Na+(aq)+2OH-(aq)
- K2O(s) + H2O(l) → 2K+(aq)+2OH-(aq)
- MgO(s) + 2H2O(l) → Mg+2(aq)+2OH-(aq)
- CaO(s) + 2H2O(l) → Ca+2(aq)+2OH-(aq)
- Al2O3(s) + 3H2O(l) → 2Al+3(aq)+6OH-(aq)
Ammonia also reacts with water to produce Hydroxide ions:
- NH3O(aq) + H2O(l) ⇌ NH4+(aq)+OH-(aq)
Edexcel
- Hydroxide ions, page 43, GCSE Chemistry; The Revision Guide, CGP, Edexcel
- Hydroxide ions, pages 118, 120, GCSE Chemistry, CGP, Edexcel