What is mutual solubility temperature?
The mutual solubility temperature is the highest temperature you can reach before two partially miscible liquids become miscible.
Oil and water don’t mix. Ethanol and water mix in all proportions.
Many liquid fall between these two extremes. If you shake equal volumes of two of these liquids together, you often get two layers with unequal volumes. These liquids are “partially miscible”.
As the temperature rises, both liquids become more soluble in each other. They reach a mutual solubility temperature or critical solution temperature. Above that point, the mixture becomes homogeneous. Below that point, the mixture separates into two layers.
The diagram below is a solubility curve for mixtures of phenol and water. The critical solution temperature for phenol-water mixtures is about 67 °C.
Above 67 °C, phenol and water are miscible. Below 67 °C, the mixture separates into two phases.