Two partially miscible liquids are completely miscible and yield one-phase systems in all proportions above their upper critical solution temperature
The miscibility of two liquids refers to their ability to mix and form a homogeneous solution. When two liquids are completely miscible, they can mix in all proportions to form one-phase systems.
However, there are cases when two liquids are only partially miscible, meaning they can mix in limited proportions before reaching a point where two separate phases are formed. In such cases, the two liquids form a two-phase system.
The upper critical solution temperature (UCST) is the temperature above which two partially miscible liquids become completely miscible in all proportions, forming a one-phase system. Below the UCST, the liquids remain partially miscible and form a two-phase system.
When the UCST is reached, the attractive forces between the molecules of the two liquids overcome the intermolecular forces between like molecules, allowing for complete mixing and the formation of a homogeneous solution. This typically occurs when the molecular structures and intermolecular forces of the two liquids match in a way that promotes mixing.
So, when two partially miscible liquids are completely miscible and yield one-phase systems in all proportions above their UCST, it means that they can mix fully to form a homogeneous solution at temperatures above the UCST, regardless of the proportion of each liquid in the mixture.