Three liquids that don't mix include honey, corn syrup and liquid dishwashing soap. These liquids, when poured in one container, form layers. Other liquids that do not mix include water, vegetable oil and rubbing alcohol.
Liquids that don’t mix are called immiscible liquids. Oil and water are immiscible liquids. The reason why some liquids do not mix is a difference in density, in which liquids of heavier density settle at the bottom and those with lighter density at the top. The density of a substance is the ratio of mass (weight) to volume.
Immiscibility of liquids is tested using a density-column experiment. For example, pouring honey, corn syrup and liquid dishwashing soap into a container results in a column with honey (heaviest density) at the bottom, and liquid soap (lightest density) at the top.
The concept of immiscible liquids has its applications in steam distillation. With immiscible liquids in a closed flask, all conditions held constant, the vapor pressure measured is that of the floating liquid. Immiscible liquids are used in heating liquids with higher boiling points. With a liquid of lower boiling point on top of an immiscible mixture, the boiling point is reached faster than normal. Agitated mixtures of immiscible liquids boil at a temperature below the boiling point of either of the pure liquids.