What is the boiling point?

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

Answer: (B)

Boiling point is the temperature at which a liquid changes to a gas. This happens when the liquid’s vapor pressure reaches the surrounding pressure, allowing bubbles of vapor to form inside the liquid and rise to the surface. At this temperature, heat goes into changing the phase rather than raising the temperature, so the liquid boils at a constant temperature. This value depends on pressure: lowering the pressure lowers the boiling point, while increasing pressure raises it. The other ideas describe freezing (liquid to solid), condensation (gas to liquid), or general expansion with heating, none of which define the boiling point.