Chemicals float over one another because of differences between their densities. Density is the property of a substance relating to its mass and volume. It is the volume of space a specified mass occupies.
Density = mass / volume
Most oils are composed of triglycerides as their predominant components. Glycerides are organic compounds made by the condensation reaction between carboxylic acids and alcohols. The carboxylic acids used are usually high molecular weight and the alcohols are glycerol molecules for triglycerides to form. The condensation reaction taking place between glycerols and fatty acids leads to the esterification process because glycerides are in fact esters in composition.
The glyceride molecules are larger than molecules like water which is the reason, they take more space than water molecules, making them less dense. As less dense substances tend to float over high dense, glycerides (oils) float over water.
This floating nature of oils on water can be seen clearly in the solvent separatory funnel.
This hydrophobic nature of oils is useful in many instances:
- Spilled oils can be cleaned off the surfaces of seawater in oil rigs regions especially.
- Toxic organic liquids remain on the surfaces so the aquatic and marine life is undisturbed.
- Various solutes can be separated from water (solvent) if they have a higher Ksp value in other solvents e.g. oils, through separatory funnels.
- Splitting patterns of light are seen on water surfaces when oils float over.
The reason why oils float on water surfaces is also related to the immiscibility of water and oils. Immiscibility is the property of two liquids that makes them incapable of mixing and attaining homogeneity. This immiscibility can be explained by the general principle of solubility, like dissolves like. Oils are non-polar whereas water is a highly polar compound.