A liquid transforms into a gas at the

• A. melting point.
• B. boiling point.
• C. freezing point.
• D. sublimation point.

Answer: (B)

This question is testing understanding of when a liquid becomes a gas. That transition happens at the boiling point. At this temperature, the liquid’s vapor pressure equals the surrounding pressure, so bubbles form within the liquid and it rapidly turns into vapor. Evaporation can occur at any temperature on the surface, but boiling is a specific temperature-driven process that converts most of the liquid to gas. The other terms refer to different phase changes: melting point is solid to liquid, freezing point is liquid to solid, and sublimation point is solid to gas. For water at 1 atm, the boiling point is 100°C.