Chloroform-acid wash
Usually, you shake chloroform with concentrated sulfuric acid, wash with water, dry over $\ce{CaCl2}$, and distill.

The density of chloroform is approximately $\rho = 1.48\ \mathrm{g\ cm^{-3}}$, whereas the density of concentrated sulfuric acid is approximately $\rho = 1.84\ \mathrm{g\ cm^{-3}}$.
Therefore, after you allow the layers to separate, the concentrated sulfuric acid is in the lower layer and the chloroform is in the upper layer.

After that, however, when you wash with water ($\rho = 1.00\ \mathrm{g\ cm^{-3}}$), the chloroform is in the lower layer and the water is in the upper layer.

If there is any doubt about the identity of a layer, add a drop of the layer in question to a small amount of water in a test tube. If the drop does not dissolve, the layer in question is the organic layer.

Caution: The mixture may become hot when the remaining sulfuric acid is removed with water, and chloroform boils at $61.2\ \mathrm{^\circ C}$.