AZEOTROPE: a mixture of two or more refrigerants in which the liquid and vapor have same composition at equilibrium. In addition, the resulting pressure of the mixture is either higher or lower than the pressure of any of the components. Azeotropes behave like pure component refrigerants because there is no change in boiling temperature or shift in composition during phase change, equipment operation, or leakage.

R-502: Before its phaseout in 1995, it was used in low temperature refrigeration (the evaporator pressure stays out of vacuum down to -40°F). R-502 offered lower discharge temperature and improved capacity compared to R-22. Commercial refrigeration systems could perform low temperature refrigeration in a single stage with relatively inexpensive compressors.
An EPA certification card is required to purchase CFC and HCFC refrigerants.