In a zeotropic refrigerant blend, the refrigerant with the lowest boiling point boils first.

In a zeotropic refrigerant blend, the characteristic behavior is that the refrigerant component with the lowest boiling point will evaporate first when the blend is heated. This is due to the individual components in a zeotropic mixture not having a consistent boiling point, which is different from an azeotropic mixture where the components share a specific boiling point and behave as a single substance.

As the temperature of the zeotropic blend rises, the component with the lowest boiling point will start to vaporize first. This sequence of boiling leads to changes in the composition of the liquid phase as the lower-boiling refrigerant evaporates, which can affect the performance and efficiency of the refrigeration cycle. Understanding this behavior is crucial for system design and operation, as it influences both the potential for fractionation and the strategies used for charging systems with blends.

Other options such as stating it is only true under pressure or that it depends on the mixture introduce unnecessary conditions or ambiguities. The statement's essence remains true regardless of the operating pressure or specific mixture, reaffirming the fundamental principle of zeotropic blends in refrigeration.