What is a base?
A base is any metal oxide (contains oxide ions, O2-) or metal hydroxide (contains hydroxide ions, OH–).
What is an alkali?
An alkali is a base that is soluble in water. Alkalis produce hydroxide ions when dissolved in water.
Note: It is a common misconception that all hydroxides are alkalis.
Most of the metal hydroxides are, in fact, insoluble in water. Hence, they are bases, not alkalis.
Examples of alkalis
Group (I) hydroxides are all alkalis. Example: Sodium hydroxide (NaOH), Lithium hydroxide (LiOH), Potassium hydroxide (KOH)
Other common alkalis include Calcium hydroxide (Ca(OH)2, Barium hydroxide Ba(OH)2 and aqueous ammonia (NH3 (aq)/ NH4OH (aq))
Some Metal oxides that are soluble in water to form alkalis
Sodium oxide: Na2O(s) + H2O(l) → 2NaOH(aq)
Potassium oxide: K2O(s) + H2O(l) → 2KOH(aq)
Bases that are insoluble in water
Copper(II) oxide: CuO
Magnesium oxide: MgO
Iron(III) oxide: Fe2O3
Aluminium hydroxide: Al(OH)3
Zinc hydroxide: Zn(OH)2
Iron(II) hydroxide: Fe(OH)2