Evaporation and what changes its speed

Drying without boiling

Wet clothes dry in the sun, and a puddle of water disappears after the rain. The water is far below $100\ ^\circ\text{C}$, so it is not boiling. How does the liquid go into the air?

Particles with a lot of energy

The particles of a liquid are always moving, but they do not all have the same energy. Some have more energy, and some have less. At the surface of the liquid, a particle that has enough energy can overcome the force of attraction of the other particles and go into the air as a gas.

This change of a liquid into a gas at the surface, at a temperature below the boiling point, is called evaporation.

A helpful picture is a crowd of people in a stadium, all moving about. A few of the fastest people get close to the exit and slip out. The ones who stay behind are the slower ones. In the same way, the particles that leave the liquid are the fastest, and the particles that stay are the slower ones. So the liquid that remains becomes a little cooler.

Evaporation and boiling

What changes the speed of evaporation?

1. The surface area. Evaporation happens at the surface, so a larger surface gives faster evaporation. This is why clothes spread out dry faster than clothes kept in a heap. A wide dish of water dries faster than the same water in a narrow glass.

2. The temperature. At a higher temperature, more particles have enough energy to escape. So evaporation is faster. Clothes dry faster on a warm day.

3. The humidity. Humidity is the amount of water vapour in the air. The air can hold only a certain amount. If the air already has a lot of water vapour, then less water evaporates. So evaporation is slower when the humidity is high. In the rainy season, clothes dry slowly.

4. The wind speed. Wind carries the water vapour away from the surface, so the air near the liquid does not become full of vapour. Evaporation is faster on a windy day.

There is also a fifth factor, which depends on the liquid: some liquids evaporate faster than others. A drop of acetone (nail polish remover) or petrol on your hand is gone in seconds. A drop of water takes much longer, and a drop of oil hardly goes at all. Liquids with weaker forces between their particles evaporate more easily.

You should find that halving the humidity, from $80\ \%$ to $40\ \%$, roughly triples the rate, and that the wind matters a lot on a still day.

Evaporation causes cooling

When the fastest particles escape, the rest have less energy on average, so the temperature of the liquid falls. The energy taken away is the heat of vaporisation. You can feel it in daily life.

• Sweat. When we sweat, the water on the skin evaporates and takes heat from the body. Evaporating one gram of sweat takes about $2400\ \text{J}$ of heat from the body. This is how the body keeps cool.
• An earthen pot (matka). Water slowly passes through the tiny holes of the clay, and evaporates from the outside. This takes heat from the pot, and the water inside stays cool.
• Acetone on the skin. A drop of acetone feels cold, because it evaporates fast.
• A cloth on the forehead. On a hot day, a wet cloth on the forehead takes heat away as the water evaporates. Doctors use this to bring down a high fever.
• A dog pants with its tongue out, so that the water on the tongue evaporates and cools the animal.

On a hot summer day, we wear cotton clothes because cotton takes up sweat and lets it evaporate. A synthetic cloth that does not absorb sweat feels hotter.

Practise with new numbers

Try it in Python

The program uses the same model to compare four dishes of water.

Common mistakes

• Thinking that evaporation needs boiling. It happens at any temperature.
• Thinking that the whole liquid is involved. Evaporation happens at the surface.
• Thinking that evaporation warms the liquid. It cools the liquid that is left.
• Forgetting the humidity. Wet clothes dry slowly in damp air even if it is hot.

Remember these

• Evaporation is the change of a liquid to vapour at the surface, below the boiling point.
• It is faster with a larger surface, a higher temperature, a lower humidity and a higher wind speed.
• Evaporation happens at any temperature, and only at the surface. Boiling happens in the whole liquid at the boiling point.
• Evaporation causes cooling.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name four things that make a wet towel dry faster.

2. A dish loses $15\ \text{g}$ of water in $3$ hours. At the same rate, how long does it take to lose $40\ \text{g}$?

3. Give two differences between evaporation and boiling.

4. Why does petrol spilled on a road disappear faster than water spilled beside it?