Zeotropes are mixtures of two or more refrigerants that?

Zeotropes are mixtures of two or more refrigerants that separate when changing state. This phenomenon occurs due to the different boiling points of the components in the mixture. As the mixture is heated, the refrigerants will vaporize at different rates, causing a change in composition during the phase change. This characteristic is critical in refrigeration systems as it affects efficiency and performance; the mixture does not maintain a consistent refrigerant composition throughout the phase change process.

When discussing the other options, becoming solid upon cooling is not a characteristic of zeotropes, since they typically remain a liquid mixture until they reach extremely low temperatures where solidification may occur. Remaining in a single phase pertains more to azeotropes, which maintain a consistent ratio and thus a uniform phase during boiling or condensation. Lastly, the statement about always having a constant boiling point is also incorrect for zeotropes, as their boiling point changes with different proportions of the mixture, which distinguishes them from azeotropic mixtures that do boil at a constant temperature under given conditions.