Lesson focus: Changes of state alter particle energy, movement and arrangement without changing chemical identity; diffusionThe net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, without needing energy from respiration. produces net spreading from random motion.
Explain state interconversions and diffusion using energy transfers and random particle motion.
Build on earlier learning
DiffusionYear 7 introduced diffusion. This lesson connects it to temperature, random motion and state changes.
You will be successful when you can…
- Name six state changes.
- Explain energy changes.
- Explain diffusion as net movement.
- Distinguish physical from chemical change.
Words that carry the science
- melting
- Change from solid to liquid.
- boiling
- Rapid vaporisation throughout a liquid at its boiling point.
- evaporation
- Vaporisation from a liquid surface below or at its boiling point.
- condensation
- Change from gas to liquid.
- sublimation
- Direct change from solid to gas.
- deposition
- Direct change from gas to solid.
- diffusion
- Net movement of particles from higher to lower concentration due to random motion.
State changes transfer energy without making a new substance
01State changes transfer energy without making a new substance
Heating can cause meltingWhen a solid warms up and turns into a liquid, like ice turning to water., evaporationWhen a liquid slowly turns into a gas from its surface, like a puddle drying up., boilingWhen a liquid turns to gas all the way through, with bubbles forming inside it. or sublimationWhen a solid turns straight into a gas without melting first, like dry ice.. Cooling can cause condensationWhen a gas cools and turns into a liquid, like water droplets forming on a cold window., freezing or depositionWhen a gas turns straight into a solid without becoming a liquid first, like frost forming on a cold night.. During these physical changeA change, such as melting, where no new substance is made and which can usually be reversed., chemical identity is retained.
Energy supplied increases internal energy. At a change-of-state temperature, transferred energy is used to overcome particle attractions rather than produce a temperature rise.
02Boiling and evaporation are not synonyms
Evaporation occurs at a liquid surface and can happen below the boiling point when higher-energy particles escape. Boiling occurs throughout a liquid when vapour bubbles form at a characteristic temperature for the pressure.
Evaporation cools the remaining liquid because escaping particles carry away more energy than average.
03Random motion creates net diffusion
Particles move randomly in all directions. Where concentration differs, more particles cross from the crowded region per second, creating net movementThe overall movement once you take away the movement going the other way, like the final score in a tug of war. down the concentration gradient.
Higher temperature increases particle speeds and usually increases diffusionThe net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, without needing energy from respiration. rate. Lower particle mass can also increase gas diffusion rate under comparable conditions.
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
Diffusion in a liquid
A drop of dye spreads out because its particles are knocked about by water particles. Warmer water particles move faster, so the dye spreads faster.
Controls
Explain a cooling curve plateau
A liquid cools and its temperature remains constant while it freezes.
- Particles lose internal energy to the surroundings.
- At the freezing point, particles become more ordered.
- Energy is released as attractions form or strengthen.
- Temperature stays constant until the state change is complete.
How to make the reasoning visible: Constant temperature during freezing does not mean no energy transfer occurs.
Connect observations to mechanisms
Interpret each physical process without inventing new substances.
| Observation | Process | Energy transfer | Particle account |
|---|---|---|---|
| puddle disappears below 100 °C | evaporation | from surroundings | surface particles escape |
| steam forms droplets | condensation | to surroundings | particles slow and come closer |
| iodine solid forms purple vapour | sublimation | into sample | particles separate into gas |
| gas colour spreads | diffusion | not a state change | random motion gives net spreading |
Why does warming speed diffusion?
Why is condensation physical?
Check your understanding
Answer from memory, then check against the suggested answer.
What is gas to liquid?
Where does evaporation occur?
Define diffusion.
Why does wet skin feel colder in moving air?
At diffusion equilibrium, have particles stopped?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A smell spreads through a room faster from a warm liquid than from the same liquid when cold.
Explain why more vapour forms from the warm liquid.
(2)Explain why the smell spreads faster.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., connect temperature to both production of vapour and its subsequent diffusion.