Cambridge Science – States Of Matter Practice Test

How is boiling different from evaporation?

Boiling occurs at the surface below boiling point; evaporation throughout the liquid. Evaporation is slower but occurs through the liquid; boiling is rapid throughout at the boiling point. Evaporation requires heat; boiling does not. Boiling is rapid vaporization throughout the liquid at its boiling point; evaporation is slower at the surface.

Correct Answer:

Boiling is rapid vaporization throughout the liquid at its boiling point; evaporation is slower at the surface.

Explanation:

Boiling is rapid vaporization that happens throughout the liquid once it reaches its boiling point. At that temperature, the vapor pressure inside the liquid equals the surrounding pressure, so bubbles form inside the liquid and rise to the surface, turning the liquid into gas quickly. Evaporation is a slower process that only occurs at the surface and can happen at temperatures below the boiling point. Individual surface molecules gain enough energy to overcome the liquid’s cohesive forces and escape as gas, but the bulk of the liquid doesn’t need to be at the boiling point. So the best description is that boiling is rapid vaporization throughout the liquid at its boiling point, while evaporation is slower vaporization at the surface.

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