Lesson focus: diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. is net movement caused by random motion and a concentrationHow much of a substance is in a certain volume, such as how much sugar is dissolved in a cup of water; a high concentration means lots of particles packed in. difference.
Explain diffusion, dynamic equilibriumWhen particles keep moving both ways at the same rate, so overall nothing seems to change. and rate factors in gasA state of matter that spreads out to fill any container, because its particles are far apart and move quickly. and liquidA state of matter that can flow and takes the shape of its container, but keeps the same volume..
Build on earlier learning
More changes of stateReuse particle motion and state differences to explain spreading.
Practical
Watching diffusion
- Drop a single crystal of a coloured substance into still water in a beaker.
- Do not stir; observe at 0, 5 and 10 minutes.
- Repeat with warm water and compare.
Record: Drawings at each time and an explanation using random particle movement.
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
You will be successful when you can…
- Define diffusion precisely.
- Explain dynamic equilibrium.
- Predict effects of temperature and gradient.
- Distinguish diffusion from bulk flow.
Words that carry the science
- diffusion
- Net movement from higher to lower concentration due to random particle motion.
- concentration
- Amount of a substance in a stated volume.
- concentration gradient
- Difference in concentration across a distance.
- dynamic equilibrium
- Opposing movements continue at equal rates, so no net change occurs.
- bulk flow
- Movement of the material as a whole in one direction.
Track net movement without stopping random motion
01Random motion produces a net direction
Particles move in every direction. When concentrationHow much of a substance is in a certain volume, such as how much sugar is dissolved in a cup of water; a high concentration means lots of particles packed in. differs, more particles cross from high to low than low to high, producing net movementThe overall movement once you take away the movement going the other way, like the final score in a tug of war..
02Equilibrium is dynamic
When distribution becomes even, particles do not stop. Equal random movement in opposite directions means no net concentration change.
03Rate depends on conditions
Higher temperature usually increases speed; a steeper gradient increases imbalance; shorter distance speeds mixing. Gases generally diffuse faster because particles move more freely. diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. is particles spreading out from where there are lots of them to where there are fewer. When it is hotter, particles have more energy and move faster, so they spread out more quickly.
Compare two diffusion tests
Dye spreads 4 cm in warm water and 2 cm in cold water in equal time.
- Independent variable is temperature.
- Distance, dye amount and container must be controlled.
- Greater distance in warm water indicates faster spreading.
- Particle explanation: higher temperature means greater average speed.
How to make the reasoning visible: A fair comparison changes one factor and measures the same outcome.
Diffusion-rate evidence
Predict the faster condition and justify.
| Comparison | Faster condition | Reason |
|---|---|---|
| 20 °C vs 50 °C | 50 °C | Greater average particle speed |
| Small vs steep gradient | Steep gradient | Larger imbalance in crossings |
| 1 cm vs 5 cm distance | 1 cm | Shorter route |
| Gas vs viscous liquid | Gas | Freer, faster motion |
What happens at equilibrium?
Why is a fan-driven smell not pure diffusion?
Use the table. Why does a tea bag diffuse faster in hot water than in cold water?
Check your understanding
Answer from memory, then check against the suggested answer.
What is diffusion?
What is concentration?
What is concentration gradient?
In which direction do particles spread in diffusion?
Name one state of matter in which diffusion happens.
Why does food colouring eventually spread through still water?
Why is comparing gas and liquid diffusion difficult experimentally?
Why does diffusion not happen in a solid in the same way as in a gas?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. Answer the parts to explain why diffusion is faster when it is hotter.
When it is hotter, what do the particles have more of?
(1)So how do the particles move?
(1)What does this mean for how quickly they spread out?
(1)What is the end result?
(1)Total for this question: 4 marks
Command word: Link the temperature to the particles, then the particles to how fast they spread.