Final answer:
Option 3. is correct. Zeotropic refrigerant blends condense and evaporate at multiple temperatures because they consist of various components with different boiling points, and during phase transition, the vapor and liquid phases have different compositions.
Step-by-step explanation:
Zeotropic refrigerant blends condense and evaporate at multiple temperatures. This occurs because the blend is made up of different substances, each having a different boiling point. When a zeotropic blend is undergoing a phase change, it doesn't happen at a single temperature, but over a range of temperatures, where each component of the blend transitions at different points along this range. This characteristic also means that during condensation and evaporation, the composition of the vapor phase and liquid phase differ.
Notably, two phases can coexist at a set of pressures and temperatures, which is described by a line on a phase diagram. However, all three phases (solid, liquid, and vapor) coexist only at one specific triple point, which is a single point on a phase diagram. The understanding of phase changes and thermal equilibrium plays a vital role in refrigeration systems, particularly when working with zeotropic blends.