Which statement best defines the boiling point?

• A. The temperature at which a liquid boils
• B. The temperature at which a liquid freezes
• C. The temperature at which a liquid condenses
• D. The temperature at which a liquid changes to a gas

Answer: (D)

Boiling point is the temperature at which a liquid changes into a gas at a given pressure. This choice is the most precise because it describes the actual phase transition—the liquid becoming vapor. The other ideas describe aspects of the process or related changes: stating the temperature at which a liquid boils is a circular way of defining the moment the liquid boils, rather than naming the transition itself; freezing and condensation are different phase changes (liquid to solid, and gas to liquid, respectively) and do not define the point at which a liquid turns into a gas. Remember, the boiling point can vary with pressure (water boils at 100°C at 1 atm, but changes if the pressure changes).