If a liquid reaches its boiling point and begins to boil, what happens inside the liquid?

• A. Only the surface boils, rest remains liquid
• B. It becomes a solid
• C. It changes to gas throughout the liquid
• D. No change occurs

Answer: (C)

When a liquid reaches its boiling point, energy allows molecules throughout the liquid to become vapor. Vapor bubbles form inside the liquid, not just at the surface, and these bubbles rise and release gas as steam. This happens because the liquid’s vapor pressure equals the surrounding pressure, so the phase change occurs inside the liquid as well as at the top. So the liquid changes to gas throughout the liquid. The alternatives describe situations that don’t match boiling, such as boiling only at the surface or turning into a solid.