A refrigerant should have a low freezing point, below the lowest operating temperature, to avoid solidification and blockage. It normally changes between liquid and vapour during operation. High latent heat and low toxicity, corrosiveness and flammability are desirable; high dielectric strength is especially useful in hermetic compressors. These are selection goals, and actual refrigerants involve trade-offs. Thus a high freezing point is the undesirable property.