What is bubble point?

• A. The temperature at which the first refrigerant in a blend turns to vapor
• B. The temperature at which the last component turns to liquid
• C. The boiling point of the pure component at standard pressure
• D. The dew point of the vapor above the mixture

Answer: (A)

Bubble point is the temperature at which a liquid mixture begins to boil at a given pressure—the moment when vapor bubbles first form in the liquid. In a refrigerant blend, the component with the lowest boiling point tends to vaporize first, so the initial vapor consists of that most volatile refrigerant. That’s why describing the bubble point as the temperature at which the first refrigerant in a blend turns to vapor best fits. The other ideas describe different phenomena: the last component turning to liquid relates to condensation behavior, the pure-component boiling point at standard pressure doesn’t reflect a mixture’s boiling at a specific pressure, and the dew point refers to condensation of vapor, not boiling of liquid.