On a warm, dry day a shallow puddle of water on the ground slowly disappears without ever bubbling. (a) Explain what is meant by the term evaporation. [2] (...

This question tests evaporation, how it differs from boiling, and the particle explanation of cooling by evaporation. Evaporation is a surface process that can occur at any temperature below the boiling point.

(a) [2] Evaporation is the change of a liquid into a gas (vapour) [1] that happens only at the surface of the liquid and at temperatures below its boiling point [1].

(b) [2] Two differences from boiling (any two, [1] each):

• evaporation happens only at the surface, while boiling happens throughout the liquid;
• evaporation happens at any temperature, while boiling happens at a fixed boiling point;
• evaporation is slow and gives no bubbles, while boiling is fast and produces bubbles.

(c) [3] The water particles are moving at a range of speeds [1]; the fastest, most energetic particles at the surface have enough energy to escape into the air as vapour [1]; over time more and more particles escape until the puddle has gone [1].

(d) [3] Three changes that speed up evaporation (any three, [1] each): raise the temperature (warmer day); increase the surface area (spread it out); blow air across it (windier); drier air (lower humidity).

(e) [1] The water left behind decreases in temperature (cools), because the most energetic particles leave and the remaining ones have a lower average energy.

Exam tip: evaporation cools the liquid left behind. This is why sweating and a wet towel both feel cold as they dry.