Upper critical solution temperature

The upper critical solution temperature (UCST) or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word upper indicates that the UCST is an upper bound to a temperature range of partial miscibility, or miscibility for certain compositions only. For example, hexane-nitrobenzene mixtures have a UCST of 19 °C, so that these two substances are miscible in all proportions above 19 °C but not at lower temperatures. Examples at higher temperatures are the aniline-water system at 168 °C (at pressures high enough for liquid water to exist at that temperature), and the lead-zinc system at 798 °C (a temperature where both metals are liquid).