Why do hexanes form the top layer in a separatory funnel when mixed with water?

Explanation:

When water and hexanes (C6H14) are mixed together in a separatory funnel, the two liquids will form distinct layers based on differences in their densities. This concept is fundamental in preparative chemistry where recovery of specific substances from a mixture is required. Water is a polar solvent, while hexane is a nonpolar solvent, and due to their differing polarities, they are immiscible—meaning they do not mix.

Given that hexane (and by extension, similar hydrocarbons like heptane C7H16) is less dense than water, when placed in the separatory funnel, it will form the top layer while water remains at the bottom. This is an application of the principle that less dense substances float on more dense substances. The separation of immiscible liquids by density makes the separatory funnel an essential tool in chemistry labs for tasks such as extractions or washing compounds.

The concept mentioned here extends to similar solvent systems as well. When considering the miscibility of other substances in solvents, it's crucial to keep in mind the solubility rules—like dissolves like—meaning polar substances preferentially dissolve in polar solvents and nonpolar substances in nonpolar solvents.