Name the gas which turns lime water milky.

Carbon dioxide when passed through lime water $\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$ turns it milky.

This is due to the formation of insoluble calcium carbonate.

$$\begin{gathered} \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{lime}\mathrm{water})(\mathrm{calcium}\mathrm{carbonate} \\ \mathrm{insoluble}) \end{gathered}$$