The temperature at which a liquid is ready to begin changing phase at a given pressure is called what?

• A. Boiling point
• B. Freezing point
• C. Melting point
• D. Saturation temperature

Answer: (D)

The main idea is the point where a liquid is in equilibrium with its vapor at a given pressure. At that temperature, the liquid is just ready to change phase into gas, and any added heat will cause vaporization while the temperature stays the same. That specific temperature is called the saturation temperature. It’s the thermodynamic description of the same boundary you’d think of as the boiling point at that pressure—the liquid-vapor equilibrium temperature for that pressure. The other terms describe different phase changes (freezing/melting) or are less precise for this boundary, so saturation temperature is the best fit for “the temperature at which a liquid is ready to begin changing phase at a given pressure.”