Lesson focus: evaporationWhen a liquid slowly turns into a gas from its surface, like a puddle drying up. occurs at a surface; boilingWhen a liquid turns to gas all the way through, with bubbles forming inside it. occurs throughout a liquidA state of matter that can flow and takes the shape of its container, but keeps the same volume. at a pressure-dependent temperature.
Distinguish evaporation from boiling and explain both using particles.
Build on earlier learning
Melting and freezingExtend energy and particle-arrangement reasoning from melting to liquid–gas changes.
You will be successful when you can…
- Define evaporation and boiling.
- Explain evaporative cooling.
- State what bubbles in boiling water contain.
- Relate external pressure to boiling point.
Words that carry the science
- evaporation
- Escape of particles from a liquid surface into the gas state.
- boiling
- Vaporisation throughout a liquid when vapour bubbles can grow.
- vapour
- Gas-phase particles of a substance normally considered liquid or solid.
- boiling point
- Temperature at which boiling occurs at a stated pressure.
- vapour pressure
- Pressure produced by vapour in equilibrium with its liquid.
Separate surface escape from vapour-bubble formation
01Evaporation selects faster surface particles
At any temperature some surface particles have enough energy to escape. Removing above-average-energy particles can lower the remaining liquid’s average energy, causing cooling.
02Boiling forms vapour bubbles throughout
At the boiling pointThe temperature at which a liquid boils, such as 100 °C for pure water., vapour pressureThe push from gas particles that escape from a liquid; it rises as the liquid gets hotter. matches surrounding pressure so vapour bubblesBubbles of gas that form inside a liquid when it boils. can grow throughout the liquid. In pure boilingWhen a liquid turns to gas all the way through, with bubbles forming inside it. water, bubbles contain water vapourThe gas form of a substance that is usually a liquid, such as water vapour..
03Pressure changes boiling point
At lower external pressure, vapour bubbles can grow at a lower temperature. Therefore boiling point must be quoted with pressure; 100 °C applies approximately at normal atmospheric pressure. Two key differences: evaporationWhen a liquid slowly turns into a gas from its surface, like a puddle drying up. happens only at the surface of a liquid and can happen at any temperature (a puddle dries on a cool day). boilingWhen a liquid turns to gas all the way through, with bubbles forming inside it. happens all through the liquid, with bubbles of gas, and only at the boiling pointThe temperature at which a liquid boils, such as 100 °C for pure water..
Heating curve of water
Ice is heated steadily. While it melts and while it boils, the temperature stays the same: the energy is breaking the forces between particles.
Controls
Explain cooling by evaporation
A wet surface becomes cooler as water evaporates.
- Particles have a range of kinetic energies.
- Faster surface particles are more likely to escape.
- They remove more than the average kinetic energy.
- Average kinetic energy and temperature of remaining liquid fall.
How to make the reasoning visible: Cooling is a change in the remaining particle population.
Boiling or evaporation?
Classify each observation and justify it.
| Observation | Process | Evidence |
|---|---|---|
| Puddle shrinks at 20 °C | Evaporation | Surface loss below boiling point |
| Bubbles form throughout water | Boiling | Vapour bubbles grow in liquid |
| Perfume dries from skin | Evaporation | Surface particles escape |
| Water boils on a mountain below 100 °C | Boiling | Lower external pressure |
What fills bubbles in boiling pure water?
Why can evaporation happen at any temperature?
Use the table. Why does a puddle disappear on a warm day even though it never reaches 100 °C?
Check your understanding
Answer from memory, then check against the suggested answer.
What is evaporation?
What is boiling?
What is vapour?
What is the boiling point of pure water at sea level?
Name one way that evaporation is different from boiling.
Why does blowing across warm tea speed cooling?
Why does a pressure cooker raise water’s boiling point?
Why do you feel cold when you get out of a swimming pool?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Evaporation and boiling both turn a liquid into a gas. Answer the parts to explain two differences.
Where in the liquid does evaporation happen?
(1)Where in the liquid does boiling happen?
(1)At what temperature can evaporation happen?
(1)At what temperature does boiling happen?
(1)Total for this question: 4 marks
Command word: Give each difference as a pair: one fact about evaporation and one about boiling.