When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.

Formation of Calcium carbonate:

1. So basically, Calcium hydroxide or lime water is formed when Lime or Calcium oxide is treated with Water
2. Reaction: $\underset{\mathrm{Calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}$
3. So now when Calcium hydroxide or lime water is treated with Carbon dioxide gas , it gets converted to Calcium carbonate which is actually a milky solution that is formed due to the precipitation of the insoluble suspension of Calcium carbonate:
4. The precipitate of Calcium carbonate will settle down at the bottom.
5. Reaction for the formation of Calcium carbonate is:

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

Therefore, When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate.