The initial temperature at which propane boils is -44 degrees Fahrenheit. Which of the following temperatures would indicate propane is at its boiling point?

• A. -44 degrees F
• B. -100 degrees F
• C. 0 degrees F
• D. 32 degrees F

Answer: (A)

Boiling point is the temperature at which a liquid’s vapor pressure equals the surrounding pressure, so liquid and vapor exist in balance. For propane at standard atmospheric pressure, that point is -44 degrees Fahrenheit. At this temperature, propane is at equilibrium between liquid and vapor, meaning it’s just beginning to boil. Any temperature much lower than that, like -100 degrees Fahrenheit, would keep propane in the liquid phase, not at the boiling point. Temperatures higher than -44 degrees Fahrenheit, such as 0 or 32 degrees Fahrenheit, would push propane into the gaseous phase rather than at the state where liquid and vapor coexist. So the temperature indicating propane is at its boiling point is -44 degrees Fahrenheit.