K323 / 2027
Thermal properties of matter overview

Lesson 4 of 6

Boiling and evaporation

  • I can explain the difference between boiling and evaporation.Syllabus

    Syllabus K323, 9(e). Explain the difference between boiling and evaporation.

Make a guess. You are not marked.

Wet washing dries faster on a windy day than on a still day at the same temperature. Why?

  1. Wind pushes the water out of the cloth, like squeezing it.
  2. Wind carries vapour away, so more particles escape than return.
  3. Wind warms the clothes, so the water reaches its boiling point.
Show the answer

Wind carries vapour away, so more particles escape than return.

Above still washing, the air fills with water vapour and some particles return to the cloth. Wind removes that vapour, so the net rate of evaporation rises.

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.

Two ways for liquid to become vapour
BoilingEvaporation
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.

Evaporation occurs at the surface of a moving liquidClose, irregularly arranged circles represent liquid particles. Selected short blue arrows point in different directions to indicate random motion. Two longer blue arrows begin at particles at the upper surface and point out of the liquid, showing particles with sufficient energy escaping into the vapour. There are no vapour bubbles in the liquid. The circles keep the same size and do not represent solid particles. Arrows are qualitative motion indicators, not measured trajectories or numerical speed ratios. Preferential escape of higher-energy particles can reduce the average kinetic energy of the remaining liquid; the remaining particles do not stop.Escape at the surfaceVapourLiquid surfaceLiquid particles keep moving

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.

Some higher-energy surface particles escape while the others continue moving and interacting. Evaporation takes place at the surface; this picture does not show vapour bubbles throughout the liquid.
  1. A range of energies: particles do not all have the same kinetic energy, even at one temperature.
  2. Escape: a surface particle needs sufficient energy to overcome the attractions holding it in the liquid.
  3. What remains: preferential removal of higher-energy particles can reduce the average random kinetic energy of the remaining liquid.
  4. 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.

Check your understanding

A liquid evaporates at its surface below its boiling temperature. Why can the liquid cool if energy from the surroundings does not replace the energy removed quickly enough?

  1. Bubbles of liquid form throughout the gas and release cold.
  2. Escaping particles preferentially remove the higher energies, which reduces the average kinetic energy of those remaining.
  3. The lowest-energy particles escape first, because they are held least strongly, leaving faster particles behind.
Show the answer

Escaping particles preferentially remove the higher energies, which reduces the average kinetic energy of those remaining.

The remaining liquid can have a lower average random kinetic energy and therefore a lower temperature. Energy supplied by the surroundings can offset this cooling, so a temperature fall is not inevitable in every evaporating sample.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Explain, in terms of molecules, why evaporation cools the liquid that remains. [3]

Saved on this browser only. Anyone using this browser can see it.