Question

Is calcium hydroxide a weak alkali?

Answer 1

Bases are ionic compounds that produce negative hydroxide (${\mathrm{OH}}^{-}$) ions when dissolved in water.

• A "strong base" is a compound that has the ability to remove a proton from a very weak acid.
• They completely dissociate into it is ions when in water.
• The strong bases are known as Alkali.
• Weak bases are the basic substances that do not completely ionize in water.

Calcium hydroxide $\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$ is an Alkali (strong base) not a weak base because it completely dissolves in water.

• When dissolved in water, it dissociates into two ions (${\mathrm{Ca}}^{2+}$ and 2${\mathrm{OH}}^{-}$).
• It becomes 100% ionized into calcium ions and hydroxide ions.
• The presence of ${\mathrm{OH}}^{-}$ ions in an aqueous solution of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$makes it alkaline in nature.
• Since $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ dissociates into ${\mathrm{Ca}}^{2+}$ and 2${\mathrm{OH}}^{-}$and the ${\mathrm{OH}}^{-}$ io accepts a proton (${\mathrm{H}}^{+}$) to form water.
• $\underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}\to \underset{\left(\mathrm{Calcium}\right)}{{\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Hydroxide}\right)}{2{\mathrm{OH}}^{–}\left(\mathrm{aq}\right)}\stackrel{2{\mathrm{H}}^{+}}{\to }\underset{\left(\mathrm{Water}\right)}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}$
• The $\mathrm{pH}$ value of calcium hydroxide is (11−12.5) and the $\mathrm{pKb}$ value is 1.37 (First ${\mathrm{OH}}^{-}$) and 2.43 (second ${\mathrm{OH}}^{-}$).

Hence, calcium hydroxide $\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$ is not weak alkali