Many processes in chemical engineering do not only involve a single phase but a combination of two immiscible liquids, or a stream containing both light and heavy liquid. It is very important to recognize and be able to calculate adjustable variable when these phases are in equilibrium with each other, and how much is in each phase. This knowledge will be especially useful upon studying separation processes, in which the solute is distributed between the two phases according to its solubility in each. The precise liquid-liquid equilibrium data are necessary for the design of extraction processes.