If a liquid is heated and begins to boil, what has happened?

• A. Melting
• B. Condensation
• C. Evaporation or boiling (liquid → gas)
• D. Freezing

Answer: (C)

When a liquid is heated to its boiling point, it undergoes a phase change from liquid to gas. The heat energy supplied allows molecules to overcome intermolecular attractions, and once boiling begins, vapor forms throughout the liquid (not just at the surface) as bubbles rise and release as gas. The temperature stays around the boiling point until all the liquid has become gas. This is different from evaporation, which can happen below the boiling point and only at the surface. The other options describe melting (solid to liquid) and freezing (liquid to solid), or condensation (gas to liquid), which don’t match the situation of a liquid visibly turning into a gas through boiling.