Evaporation is a liquid turning into a gas at its surface, at any temperature below its boiling point. Boiling is a liquid turning into a gas throughout the liquid, at its boiling point, with bubbles of vapour forming inside. Questions ask you to tell them apart, or to explain a real situation such as wet washing or a kettle.
This lesson follows on from explaining diffusion and belongs to the module on particle models and changes of state.
What is happening to the particles in each case?
In a liquid the particles are close together and move around each other. They do not all move at the same speed. Some have more kinetic energy than others, and the energies change constantly as they collide.
Evaporation. A particle at the surface that has enough energy can overcome the attractions holding it in the liquid and escape into the air. Only surface particles can do this, and only the higher-energy ones. This is why evaporation can go on at room temperature.
Boiling. When the liquid reaches its boiling point, particles anywhere in the liquid have enough energy to form bubbles of vapour inside it. The bubbles rise and burst at the surface. The temperature of the liquid stays at the boiling point while it boils.
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it happens | Surface only | Throughout the liquid |
| Temperature | Any temperature below the boiling point | One fixed temperature (at a given pressure) |
| Bubbles inside the liquid | No | Yes |
| Speed | Slow unless conditions help | Fast |
| Effect on the liquid | It cools | It stays at the boiling point while heated |
Water boils at 100 °C at standard atmospheric pressure. The boiling point changes if the pressure changes, so check your syllabus for how far you need to go with that idea.
Worked example
A student hangs wet clothes indoors on a day when the temperature is 28 °C. Separately, a kettle of water is heated until it reaches 100 °C.
Question: Explain why the clothes dry but do not boil.
Step 1, name the process. Water in the clothes is evaporating, not boiling.
Step 2, use the particles. Water particles at the surface of the wet cloth have different energies. Those with enough energy escape into the air as water vapour.
Step 3, say why it is not boiling. The temperature is 28 °C, well below the boiling point of 100 °C, so particles throughout the water do not have enough energy to form bubbles.
Step 4, add the extra detail if asked. Drying is faster with a higher temperature, a larger surface area or moving air that carries vapour away from the surface.
Notice that the answer uses the two words in the correct places and ties each to a condition: surface at any temperature, or throughout at one temperature.
The mistake to watch for
A common error is to describe boiling as “fast evaporation”.
Mistaken answer: “Boiling is when a liquid evaporates very quickly at a higher temperature.”
The two processes differ in kind, not only in speed. Evaporation happens at the surface and boiling happens throughout. A correct reply says: “Evaporation occurs only at the surface at any temperature below the boiling point. Boiling occurs throughout the liquid at the boiling point, with bubbles of vapour forming.”
Another slip is to say the bubbles are “air” or “hydrogen and oxygen”. They are vapour of the same substance. That point returns in separating a substance change from a change in spacing.
Check yourself
1. Give two differences between boiling and evaporation.
Show answer
Any two of: evaporation occurs only at the surface while boiling occurs throughout; evaporation occurs at any temperature below the boiling point while boiling occurs at one temperature; boiling has bubbles forming in the liquid while evaporation does not.
2. Explain why a cup of hot drink cools a little as steam rises from it.
Show answer
The particles with the most kinetic energy are the ones that escape from the surface by evaporation. The particles left behind have a lower average kinetic energy, so the drink gets cooler.
3. A puddle of water disappears on a warm afternoon. Is this boiling or evaporation? Give one reason.
Show answer
Evaporation. The puddle is far below 100 °C and no bubbles form throughout the water. Particles escape only from the surface.
Where this leads next
Next, follow a substance through all the states in interpret a heating curve. Then try the mixed particle practice.
If you keep losing marks because a question uses one word and your answer uses the other, a teacher in online one-to-one Chemistry tuition can go through your wording with you.