What is a Base?
A base is a substance that contains oxide ions (O²⁻) or hydroxide ions (OH⁻). Most bases are metal oxides or metal hydroxides.
What is an Alkali?
An alkali is a base that is soluble in water.
When dissolved in water, alkalis produce hydroxide ions (OH⁻) in solution.
Important Note: Not all bases are alkalis. Most metal hydroxides are insoluble in water, so they are bases but not alkalis.
Examples of alkalis
Group I hydroxides (all are alkalis):
- Sodium hydroxide, NaOH
- Potassium hydroxide, KOH
- Lithium hydroxide, LiOH
Other common alkalis:
- Calcium hydroxide, Ca(OH)₂ (slaked lime)
- Barium hydroxide, Ba(OH)₂
- Aqueous ammonia, NH₃(aq) or NH₄OH(aq)
Soluble Metal Oxides that Form Alkalis
Sodium oxide: Na2O(s) + H2O(l) → 2NaOH(aq)
Potassium oxide: K2O(s) + H2O(l) → 2KOH(aq)
Bases that are Insoluble in Water (Bases but not Alkalis)
Metal Oxides:
Copper(II) oxide: CuO
Magnesium oxide: MgO
Iron(III) oxide: Fe2O3
Metal Hydroxides:
Aluminium hydroxide: Al(OH)3
Zinc hydroxide: Zn(OH)2
Iron(II) hydroxide: Fe(OH)2
Key Summary
- All alkalis are bases.
- Not all bases are alkalis (because most are insoluble in water).
- Alkalis produce OH⁻ ions in aqueous solution.
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