Few pieces of marble are dropped in dilute $\mathit{HCl}$ in a test tube. The evolved gas then passed through lime water. The lime water turns

When a few pieces of marble are added to dil. $\mathit{HCl}$, $C{O}_2$ gas evolved, which is then passed through lime water.

The lime water turns milky.
$\mathit{CaC}{O}_3+2\mathit{HCl}\to \mathit{CaC}{l}_2+H_{2}O+C{O}_2$
$\mathit{Ca}{\left(\mathit{OH}\right)}_2+C{O}_2\to \mathit{CaC}{O}_3\left(s\right)\downarrow +H_{2}O$

NOTE: If excess of $C{O}_2$ gas is passed through lime water, milkiness will disappear due to the formation of $\mathit{Ca}{\left(\mathit{HC}{O}_3\right)}_2\left(\mathit{aq}\right)$ which is soluble in water.