Topic 4 of 6
Boiling and evaporation
Boiling forms vapour throughout a liquid. Evaporation occurs at its surface and can happen below the boiling temperature.
Particles in a liquid move randomly with a range of kinetic energies. Attractions keep them close together, but some particles at the surface have enough energy to escape into the gas above.
| Boiling | Evaporation |
|---|---|
| Vapour bubbles form throughout the liquid and rise. | Particles escape at the liquid surface; bubbles throughout the liquid are not required. |
| A pure liquid boils at its boiling temperature for the stated pressure. | It can occur below the boiling temperature, over a range of temperatures at which the liquid exists. |
| During boiling at fixed pressure, continued energy input changes the state without raising the temperature. | It can reduce the liquid's temperature if incoming energy does not replace the energy removed quickly enough. |
Boiling bubbles contain the substance's vapour. They are not simply bubbles of liquid water or evidence that water has split into hydrogen and oxygen. The boiling temperature depends on pressure; 100°C is the familiar value for water at approximately normal atmospheric pressure, not a rule for every liquid under all conditions.
Why evaporation can cool a liquid
Some particles escape from the liquid surface
This particle model shows evaporation below the boiling temperature. All the liquid particles keep moving; only selected motion arrows are drawn.
Particles that escape take energy with them. The average kinetic energy of the particles remaining can fall, so the liquid cools.
Cooling depends on the energy account: incoming energy may replace the energy removed quickly enough to maintain the temperature. The arrows show particle motion, not a separate energy-transfer pathway.
- A range of energies: particles do not all have the same kinetic energy, even at one temperature.
- Escape: a surface particle needs sufficient energy to overcome the attractions holding it in the liquid.
- What remains: preferential removal of higher-energy particles can reduce the average random kinetic energy of the remaining liquid.
- Temperature: the lower average kinetic energy corresponds to a lower temperature, unless incoming energy offsets the loss.
The whole liquid does not need to reach its boiling temperature for some surface particles to escape. A wet surface can therefore dry in an ordinary room.
Worked explanation
Why evaporating sweat cools skin
Sweat on the skin changes from liquid to vapour. Energy needed for this change can transfer from the skin into the evaporating liquid, so the skin loses energy.
The cooling effect depends on evaporation, not merely on the presence of liquid. If surrounding air already contains much water vapour and little evaporates, this cooling route is reduced. Incoming energy can also partly or fully replace the energy lost, so an evaporating surface need not always fall in temperature.
Interpret a temperature comparison
Compare two similar temperature probes initially at the same room temperature. Cover one sensing region with a small wet covering and leave the other dry. Under the same airflow, evaporation from the wet covering can make that probe read a lower temperature.
The lower reading concerns the evaporating wet region. It does not show that the room air as a whole has reached that temperature. Keep probe type, initial conditions and airflow comparable, and allow for response time before interpreting the difference.
Cooling needs an energy explanation. Evaporation does not release a substance called cold. It removes energy, and the remaining liquid cools when other transfers do not replace that energy fast enough.