Explain why evaporation cools a liquid
Connect selective escape to reduced average energy in the remaining liquid and recognise replacement energy.
Core and Extended; Physics 0625, 2026–2028, section 2.2.3. Explain energy input during melting/boiling, water's standard-pressure temperatures, condensation/solidification, surface evaporation and liquid cooling. Detailed boiling/evaporation comparisons, rate factors and cooling a contacted object belong to the labelled Supplement lessons. Written practice is self-checked
Before you begin
- Describe evaporation as surface escape of more energetic particles.
- Connect temperature with average particle kinetic energies.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which particles preferentially escape in the evaporation cooling account?
What you will learn
- Know that evaporation causes cooling of a liquid.
- Explain the cooling tendency when more energetic particles escape and energy is not fully replaced.
Follow the particles left behind
More energetic particles escape preferentially during evaporation. In the cooling account, the particles remaining in the liquid have lower average kinetic energy, so the liquid's temperature falls as it re-equilibrates when energy is not fully replaced. Focus on the remaining population rather than claiming every remaining particle individually loses the same energy at the instant of escape.
Evaporation also needs energy to overcome liquid attractions. The liquid can supply energy and cool, and energy can be transferred in from its surroundings. The mechanism is an energy and particle account, not cold flowing into the liquid. Lower liquid mass alone is not the complete explanation of temperature change.
Distinguish cooling tendency from an imposed reading
An evaporating liquid can be held at steady temperature if energy is supplied to replace what the process removes. Thus evaporation causes a cooling tendency, but the complete experiment may not show a decreasing temperature if other transfers compensate. The same process can be present under different overall thermal conditions.
To interpret observations, state whether the liquid receives additional energy and whether the thermometer represents the sample. Do not infer an exact drop from an uncalibrated particle diagram, or assume a measured constant temperature proves evaporation is absent. Later Extended work explains energy drawn from an object touching an evaporating liquid.
Pause and explain
An evaporating liquid is kept at steady temperature by an energy supply. What follows?
Worked example
A liquid evaporates without full replacement of the outgoing energy. Explain why the remaining liquid can cool.
Show the worked solution
- More energetic surface particles escape.
- The average kinetic energy of the remaining liquid decreases in the stated account.
- Its temperature falls as the remaining particles redistribute energy.
Answer Preferential escape lowers the remaining liquid's average energy and temperature without full replacement.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why an evaporating liquid cools when energy is not fully replaced.
Give me a hint
Track more energetic escape and the remaining average.
Compare my reasoning
- More energetic surface particles escape.
- The remaining population has lower average kinetic energy in the stated account.
- Lower average kinetic energy corresponds to a lower temperature.
The remaining liquid cools through selective energetic escape without full replacement.
Look for these in your work
- Use the remaining average.
- Include the replacement-energy condition.
An evaporating liquid stays at 25°C while receiving energy from warmer surroundings. Explain why there is no contradiction.
Give me a hint
Consider the complete energy account, not evaporation alone.
Compare my reasoning
- Evaporation removes material and energy from the liquid.
- Surroundings supply energy into it.
- Compensation can keep average kinetic energy and temperature steady.
Energy replenishment can maintain 25°C despite continuing evaporation.
Look for these in your work
- Retain evaporation.
- Account for the other transfer.
A diagram shows a few arrows leaving the surface. Can you calculate an exact liquid temperature drop from these arrows? Explain what is missing.
Give me a hint
Schematic arrows do not supply an energy account or calibrated population.
Compare my reasoning
- The arrows identify escape qualitatively.
- They do not provide escaping energy, sample heat properties or replacement transfers.
- An exact temperature change is not determined from that drawing.
No exact drop is determined by the uncalibrated schematic.
Look for these in your work
- Use the diagram at its stated scope.
- Identify missing quantitative information.
Common mistakes
- Explain the remaining particles' average, not an identical loss for each particle.
- Cold is not a substance transferred into the liquid.
- Replacement energy can maintain temperature while evaporation continues.
What can you explain now?
Does steady temperature prove an evaporating liquid has stopped evaporating?
Compare with the explanation
No.
Energy transfer into the liquid can compensate for the cooling effect.
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Make it stick
Explain evaporation cooling with the remaining average and show how additional energy can compensate.
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