Evaporation: Quick Revision

A brief note on Evaporation. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Evaporation is a liquid turning into a gas at its surface, at any temperature. Boiling happens all through the liquid, at the boiling point.
• It is faster with a larger surface, a higher temperature, a lower humidity and more wind.
• Evaporation cools: the fastest particles leave, and the liquid takes latent heat from its surroundings.
• Humidity is the water vapour in the air. Humid air slows evaporation, so we feel hotter.
• Warm air holds more vapour than cool air. Cooling air makes dew, and evaporation and condensation drive the water cycle.

Evaporation and boiling at a glance

A worked example

A tray holds $220\ \text{g}$ of water, which all evaporates in $5$ hours. Find the rate, the heat taken from the surroundings, and the average power. Use $2.26 \times 10^{6}\ \text{J/kg}$.

1. The rate. $\dfrac{220\ \text{g}}{5\ \text{h}} = 44\ \text{g/h}$.
2. The heat. $Q = 0.22\ \text{kg} \times 2.26 \times 10^{6}\ \text{J/kg} = 4.97 \times 10^{5}\ \text{J}$.
3. The power. $\dfrac{4.97 \times 10^{5}\ \text{J}}{5 \times 3600\ \text{s}} = 27.6\ \text{W}$.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Saying water evaporates only when it boils. It evaporates at any temperature.
• Saying evaporation warms a liquid. It cools it.
• Forgetting humidity. A hot, humid day dries clothes slowly.
• Calling a cloud water vapour. Vapour is invisible; a cloud is tiny drops.

Check yourself

1. Why does a wet floor dry faster when the fan is on?

2. A dish loses $36\ \text{g}$ of water in $4$ hours. What is the rate of evaporation?

3. Why does an earthen pot keep water cool?

4. Why do drops form on the outside of a cold bottle on a warm day?