Upper critical solution temperature

Have you ever made a cake and had to mix all the ingredients together? Imagine that two of the ingredients you used were oil and water. Normally, oil and water do not mix together because they consist of different substances that do not dissolve in one another.

Similarly, some chemicals when mixed together do not mix or dissolve completely. These chemicals have different properties and behavior that prevents them from becoming a solution. However, when we heat up the mixture, some chemicals start to react differently and can be dissolved, forming a solution.

Upper critical solution temperature (UCST) is the highest temperature at which two chemicals can still exist as separate liquids. Above this temperature, the two chemicals become more miscible, meaning they can mix together and form a solution. This temperature is called the "upper" critical solution temperature because it is the highest temperature that still allows the two substances to exist separately.

For example, think about mixing salt and water. The solubility of the salt in water changes as the temperature of the water increases. The upper critical solution temperature in this case refers to the temperature above which salt and water cannot be fully separated anymore, and the salt completely dissolves in the water forming a clear solution.

Understanding the upper critical solution temperature of different chemical mixtures can help us better control reactions in the lab, as well as understand natural processes such as the behavior of different substances in the ocean.