Question

Name two gases which turn lime water milky?

Answer 1

Two gases which turn lime water milky are Carbon dioxide (CO₂) and Sulphur dioxide (SO₂).

• Limewater is an aqueous solution of Calcium hydroxide [Ca(OH)₂].

Sulphur dioxide reacts with limewater to produce a white precipitate of Calcium sulphite (CaSO₃) which makes the solution to look cloudy or milky in appearance.

$$\begin{gathered} {\mathrm{SO}}_2\left(\mathrm{g}\right)+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\to {\mathrm{CaSO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Sulphur}\mathrm{dioxide})(\mathrm{Calcium}\mathrm{hydroxide})(\mathrm{Calcium}\mathrm{sulphite})\left(\mathrm{Water}\right) \end{gathered}$$

Carbon dioxide reacts with limewater to produce a white precipitate of Calcium carbonate (CaCO₃) which makes the solution to look cloudy or milky in appearance.

$$\begin{gathered} {\mathrm{CO}}_2\left(\mathrm{g}\right)+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Carbon}\mathrm{dioxide})(\mathrm{Calcium}\mathrm{hydroxide})(\mathrm{Calcium}\mathrm{carbonate})\left(\mathrm{Water}\right) \end{gathered}$$

• This test is often used to confirm the presence of these gases.