Question

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

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Solution

## (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}\left(\mathrm{OH}{\right)}_{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, $\mathrm{Ca}\left(\mathrm{OH}{\right)}_{2}$, $\mathrm{Fe}\left(\mathrm{OH}{\right)}_{3}$ and $\mathrm{Cu}\left(\mathrm{OH}{\right)}_{2}$. An alkali can be a water-soluble metal hydroxide. Examples include NaOH, ${\mathrm{NH}}_{3}$ and $\mathrm{Ca}\left(\mathrm{OH}{\right)}_{2}$. A metal hydroxide may or may not be an alkali.

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