Melting, Boiling & Evaporation

Objectives

Lesson outcomes

Explain why melting and boiling need energy even though the temperature stays constant during the change of state.
State the melting and boiling temperatures of water at standard atmospheric pressure.
Describe evaporation as the escape of the more energetic particles from the surface of a liquid.
Explain the differences between boiling and evaporation and why evaporation causes cooling.

Syllabus

CIE 0625 syllabus points

8 linked

2.2.3.1 Describe melting and boiling in terms of energy input without a change in temperature
2.2.3.2 Know the melting and boiling temperatures for water at standard atmospheric pressure
2.2.3.3 Describe condensation and solidification in terms of particles
2.2.3.4 Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid
2.2.3.5 Know that evaporation causes cooling of a liquid
2.2.3.6 Describe the differences between boiling and evaporation
2.2.3.7 Describe how temperature, surface area and air movement over a surface affect evaporation
2.2.3.8 Explain the cooling of an object in contact with an evaporating liquid

Definitions

Required definitions

Evaporation
the escape of more energetic molecules from the surface of a liquid.
Specific latent heat
the energy needed to change the state of 1 kg of a material without changing its temperature.
Latent heat of vaporisation
the energy needed to boil 1 kg of a liquid without changing its temperature.
Latent heat of fusion
the energy needed to melt 1 kg of a solid without changing its temperature.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson is about keeping several similar-looking processes separate. You need to understand not just what melting, boiling, condensation, and evaporation are, but why some of them happen at fixed temperatures while evaporation can happen from the surface at many temperatures.

What You Need to Know

Melting and boiling require energy even though the temperature stays constant during the change of state.
For water at standard atmospheric pressure, melting happens at 0 °C and boiling at 100 °C.
Condensation is gas changing to liquid, and solidification is liquid changing to solid.
Evaporation happens when the more energetic particles escape from the surface of a liquid.
Evaporation causes cooling because the particles left behind have a lower average energy.
Boiling happens throughout a liquid at a fixed temperature, while evaporation happens only at the surface and can happen below the boiling point.
A higher temperature, larger surface area, and more moving air all increase the rate of evaporation.

How to Work Through It

Start with a heating curve or familiar water example so the main state changes are visible.
Compare melting and boiling with evaporation using particle and energy ideas.
Practise explaining why temperature does not rise during melting or boiling.
Finish with examples where you predict how quickly evaporation will happen and whether cooling will be stronger or weaker.

Check Your Understanding

Why can energy be added during boiling without a temperature rise?
What is the difference between boiling and evaporation?
Why does evaporation make the remaining liquid cooler?
Which conditions make evaporation faster?

Common Mistakes

Saying evaporation only happens at the boiling point. It can happen at many temperatures.
Forgetting that boiling happens throughout the liquid, not just at the surface.
Thinking temperature always rises when energy is supplied. During a change of state, the energy is used differently.

Next Steps

Rehearse clear particle explanations for each change of state.
Keep the differences between conduction, convection, and radiation separate for the next two lessons on thermal transfer.