The correct answer is: D. Water and diethyl aniline.
A lower critical solution temperature (LCST) is the temperature below which two liquids that are immiscible at higher temperatures become miscible. An upper critical solution temperature (UCST) is the temperature above which two liquids that are miscible at lower temperatures become immiscible.
Water and diethyl aniline are a partially miscible liquid pair that has both an LCST and an UCST. This means that at temperatures below the LCST, water and diethyl aniline are immiscible, but at temperatures above the LCST, they are miscible. The UCST for water and diethyl aniline is approximately 293 K.
The reason why water and diethyl aniline have both an LCST and an UCST is because of the difference in their polarity. Water is a polar molecule, while diethyl aniline is a nonpolar molecule. When water and diethyl aniline are mixed at low temperatures, the water molecules form hydrogen bonds with each other, while the diethyl aniline molecules do not interact with each other. This results in the two liquids being immiscible.
However, as the temperature increases, the water molecules begin
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