A single component refrigerant always had a "boiling point." Zeotropic blends change composition when they boil or condense, and therefore have a continuously changing boiling point. The most useful temperatures to know are where the boiling starts and ends. Bubble point and dew point are terms used in the chemical industry to define these two temperatures. Bubble point is the temperature where the saturated liquid starts to boil off its first "bubble" of vapor. (Picture a pot of liquid with the first bubbles starting to appear.) It is also called the "liquid side temperature/ pressure relationship." Dew point is the temperature where saturated vapor first starts to condense, or the last drop of liquid evaporates. (Picture a room full of vapor with a few drops forming on the ceiling.) This is also called the "vapor side temperature/ pressure relationship."