Question

What is the difference between
(a) an alkali and base?
(b) an alkali and a metal hydroxide?

Answer 1

(a)

Alkali	Base
An alkali is a basic hydroxide that is soluble in water. It produces hydroxyl (${\mathrm{OH}}^{-}$) ions as the only negatively charged ions when dissolved in water. Examples include NaOH, Ca(OH)2 and KOH.	A base is a compound that contains displaceable oxide (${\mathrm{O}}^{2-}$) or hydroxide (${\mathrm{OH}}^{-}$) ion or ions. It reacts with ${\mathrm{H}}^{+}\mathrm{or}{\mathrm{H}}_3{\mathrm{O}}^{+}$ ions present in an acid to produce a salt and water. Examples include NaOH, CuO and Fe(OH)3.
Bases that are soluble in water are alkalis. NaOH, KOH and Ca(OH)2 are water-soluble bases; thus, they are alkalis.	A few bases like ferric hydroxide and cupric hydroxides are not soluble in water; thus, they are not alkalis.

(b)

Alkali	Metal Hydroxide
An alkali is a basic hydroxide that is soluble in water. It produces hydroxyl (${\mathrm{OH}}^{-}$) ions as the only negatively charged ions when dissolved in water. Examples include NaOH, ${\mathrm{NH}}_3$$\mathrm{Ca}(\mathrm{OH}{)}_2\mathrm{and}{\mathrm{NH}}_3$.​	A metal hydroxide is a basic salt of a metal. It may be soluble or insoluble in water. Examples include NaOH, $\mathrm{Ca}(\mathrm{OH}{)}_2$, $\mathrm{Fe}(\mathrm{OH}{)}_3$ and $\mathrm{Cu}(\mathrm{OH}{)}_2$.
An alkali can be a water-soluble metal hydroxide. Examples include NaOH, ${\mathrm{NH}}_3$ and $\mathrm{Ca}(\mathrm{OH}{)}_2$.	A metal hydroxide may or may not be an alkali.