Which of the following is NOT considered a thermodynamic property of refrigerants?

The critical point is indeed a thermodynamic property of refrigerants, referring to the temperature and pressure at which a substance can no longer exist as a distinct liquid and gas phase. It is crucial for understanding the behavior of refrigerants in various applications since it defines the limits of their liquid and vapor phases.

In contrast, viscosity, while related to fluid dynamics, is not classified as a thermodynamic property; it describes a fluid's resistance to flow and shear rather than its thermal or phase behavior. Therefore, the correct answer to the question is viscosity, as it does not fit the definition of a thermodynamic property like boiling point, critical point, and density do. These properties are integral to understanding how refrigerants behave under different pressures and temperatures in thermodynamic cycles used for refrigeration and air conditioning.