Lesson focus: Six reversible state changes rearrange particles without changing their chemical identity.
Name and explain condensationWhen a gas cools and turns into a liquid, like water droplets forming on a cold window., sublimationWhen a solid turns straight into a gas without melting first, like dry ice. and depositionWhen a gas turns straight into a solid without becoming a liquid first, like frost forming on a cold night. alongside melting, freezing and vaporisation.
Build on earlier learning
BoilingPlace boiling and evaporation within the complete state-change map.
You will be successful when you can…
- Name all six state changes.
- State direction of energy transfer.
- Explain conserved mass in a closed system.
- Explain apparent loss from an open system.
Words that carry the science
- condensation
- Change from gas to liquid.
- sublimation
- Change directly from solid to gas.
- deposition
- Change directly from gas to solid.
- closed system
- A system that does not exchange matter with surroundings.
- conservation of mass
- Total mass remains constant when matter cannot enter or leave.
Track energy, arrangement and system boundaries
01One map, six directions
Solid ↔ liquid uses melting/freezing; liquid ↔ gas uses vaporisation/condensationWhen a gas cools and turns into a liquid, like water droplets forming on a cold window.; solid ↔ gas uses sublimationWhen a solid turns straight into a gas without melting first, like dry ice./depositionWhen a gas turns straight into a solid without becoming a liquid first, like frost forming on a cold night..
02Energy changes movement and arrangement
Heating generally increases particle energy and can overcome attractions. Cooling transfers energy to surroundings and allows more ordered or closer arrangements. condensationWhen a gas cools and turns into a liquid, like water droplets forming on a cold window. is when a gas turns into a liquid. The gas particles lose energy to the cold surroundings, so they slow down. Slower particles cannot escape the pull between them, so they come close together and form a liquid. They are still the same particles.
03Mass depends on the boundary
In a sealed container, state changes conserve mass. In an open container, vapour can leave the measured system, creating apparent mass loss.
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 frost formation
Water vapour forms ice directly on a very cold surface.
- Initial state is gas.
- Final state is solid.
- The direct gas-to-solid change is deposition.
- Energy transfers from water particles to surroundings; particle identity and total closed-system mass remain unchanged.
How to make the reasoning visible: Name both states before naming the process.
State-change evidence
Identify direction and energy transfer.
| Example | Start → finish | Process | Energy direction |
|---|---|---|---|
| Dew | Gas → liquid | Condensation | To surroundings |
| Dry ice fog source | Solid → gas | Sublimation | Into substance |
| Frost | Gas → solid | Deposition | To surroundings |
| Ice cube melts | Solid → liquid | Melting | Into substance |
Why can an open beaker lose measured mass during evaporation?
Would a sealed flask lose total mass?
Use the table. Frost forms on a cold car window. Name the change of state and say whether energy goes in or out of the water.
Check your understanding
Answer from memory, then check against the suggested answer.
What is condensation?
What is sublimation?
What is deposition?
What is the change from gas to liquid called?
Name a substance that sublimes.
A freezer wall gains frost without liquid droplets first appearing. Name and explain the change.
Why can dry ice appear to “disappear”?
Water droplets appear on the outside of a cold can of drink. Where did the water come from?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Steam (a gas) touches a cold window and turns into water droplets. This is condensation. Answer the parts to explain it using the particle model.
What do the gas particles lose to the cold window?
(1)So what happens to how fast the particles move?
(1)What happens to the gaps between the particles?
(1)What state forms? Are they still the same particles?
(1)Total for this question: 4 marks
Command word: Put the steps in order: energy → speed → gaps → new state.