Immiscible Liquids
Two liquids that do not dissolve in one another and separate into distinct layers.
Unit
Dimensionless
In depth
Immiscibility results when the two liquids differ greatly in polarity, so mixing would require breaking strong interactions and replacing them with much weaker ones. The denser liquid forms the lower layer, and the pair can be separated cleanly in a separating funnel.
Examples in the real world
Oil floating on water; dichloromethane forming the lower layer beneath an aqueous phase during extraction.