Diffusion
Diffusion in Liquids
Lesduur is: 55 minOnderdelen in deze les
Diffusion in Liquids
Open from a familiar observation: a drop of food colouring spreads through water without stirring. Keep the lesson rooted in particle movement and concentration differences.
Learning goal
I can explain diffusion as the movement of particles from a region of higher concentration to a region of lower concentration, and describe factors that affect its speed.
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What words come to mind when you hear diffusion?
Use this to surface prior ideas such as spreading, particles, movement, colour, high concentration, and low concentration. Briefly note misconceptions like 'sinking' or 'stirring' to revisit later.
Movement of particles
Particles in liquids are always moving
Their movement is random in all directions
This motion allows substances to spread out
No stirring is needed for movement to happen
Emphasise particle-level reasoning: students may think the colour 'wants' to spread. Clarify that spreading happens because many tiny particles move randomly.
What diffusion means
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Over time, the particles become more evenly spread through the liquid.
The key word is net. Individual particles still move randomly both ways, but overall there is a spread from high concentration to low concentration.
Before and after diffusion
At first, many coloured particles are in one small area
Later, particles are spread more evenly
The colour looks less concentrated in one place
The particles have not disappeared
Use a simple board diagram if needed: one side crowded with particles, then a later diagram with particles distributed across the whole beaker.
Concentration gradient
A concentration gradient is a difference in concentration between two regions
Diffusion happens when this difference exists
Particles move down the concentration gradient
As the difference becomes smaller, diffusion slows
Keep the phrasing consistent: higher concentration, lower concentration, concentration gradient, down the concentration gradient.
Why the gradient matters
A large concentration gradient means there is a big difference between one region and another, so there is a faster net spread of particles. As the particles become evenly distributed, the gradient decreases.
Students often confuse 'more particles' with 'faster particles'. The gradient changes the net movement, not the basic random motion itself.
What does diffusion describe?
Net movement to lower concentration
Movement only when stirred
Particles stopping in water
Colour disappearing completely
Checks the core definition from slides 2–6. Distractors target common misunderstandings: stirring is not required, particles do not stop, and they do not disappear.
Complete the explanation
Particles in liquids are always moving in random directions. Diffusion is the net movement of particles from high concentration to low concentration. As particles spread more evenly, the concentration gradient becomes smaller.
The blanks focus on the load-bearing terms from the first subtopic: random, net movement, and gradient.
Why does a larger gradient speed up diffusion?
Expected elements: a larger concentration gradient means a bigger difference between regions, so there is faster net movement overall. Individual particles still move randomly in all directions; the gradient affects the net spread, not whether particles move.
Factors affecting diffusion
Temperature
Size of the concentration gradient
Distance particles must travel
Type of substance and medium
This slide introduces the factors briefly; the next slides unpack them. Keep examples in liquids because that matches the practical.
Temperature and diffusion
Higher temperature gives particles more kinetic energy
Particles move faster
Faster movement leads to quicker spreading
Diffusion is usually faster in warm water than in cold water
Link kinetic energy directly to particle speed. Avoid drifting into convection unless it is clearly separated from diffusion.
Other factors
A steeper concentration gradient increases the net movement
A shorter distance means particles spread through the liquid more quickly
Diffusion is slower when particles move through a more crowded medium
For 'more crowded medium', keep it simple and age-appropriate: particles have less space to move through, so spreading is slower.
Why is diffusion usually faster in warm water?
Particles are heavier
Particles move faster
The gradient disappears
Water becomes empty
Checks the temperature explanation from slide 8. Keep the focus on kinetic energy and faster particle motion, not on unrelated ideas.
Complete the factors summary
A higher temperature gives particles more kinetic energy, so they spread faster. A steeper gradient increases net movement. A shorter distance means particles can spread through the liquid more quickly.
This summarises the key factors from slides 7–9 using the exact lesson language.
Demonstration with coloured solution
Equipment: beakers, water, food colouring, timer
Add one drop of colouring to still water
Watch how the colour spreads over time
Record visible changes
If comparing temperatures, use identical beakers and equal volumes of water. Do not stir, as the aim is to observe diffusion rather than mixing by motion.
What to observe
Time when the drop is added
Shape of the coloured region at the start
How far the colour has spread after set times
When the colour appears evenly distributed
This slide connects to the observation table mentioned in the plan. Encourage precise observations rather than vague comments like 'it mixed'.
Explaining the demonstration
Food colouring particles begin in one concentrated region. Water particles and colouring particles move randomly, so the colouring spreads from the high-concentration region into lower-concentration regions until it is more evenly distributed.
Make clear that the water is not empty space; both water particles and dye particles are moving. The visible colour shows where the dye particles are.
Using diagrams to explain diffusion
Draw particles close together in one region at the start
Show arrows in random directions
In a later diagram, spread particles more evenly
Label the high and low concentration regions
Students often draw arrows in one direction only. Remind them that random motion continues in all directions, even though the net movement is down the concentration gradient.
What should you record during the demonstration?
Only the final colour
How much water evaporates
Time and spread of colour
Mass of the beaker
Based on slides 10–11. The correct answer reflects the observation table focus: timing, shape, spread, and when the colour looks evenly distributed.
Complete the demonstration explanation
Food colouring particles begin in one concentrated region. Water particles and colouring particles move randomly, so the colouring spreads into lower-concentration regions until it is more evenly distributed.
This anchors the explanation of the practical and links it directly to the particle diagram language on slides 12–13.
What must a good diffusion diagram show?
Expected elements: arrows should point in random directions, not one way only; the diagram should show particles starting crowded in one region and later more evenly spread; labels for high and low concentration may also appear.
What is one thing you learned about diffusion today?
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What helped you understand diffusion best?
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What part of this lesson felt hardest?
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Glossary
diffusion - the net movement of particles from a region of higher concentration to a region of lower concentration
particle - a tiny part of a substance, such as an atom or molecule
concentration - how much of a substance is present in a certain volume
concentration gradient - a difference in concentration between two regions
net movement - the overall movement in one direction when random movement happens both ways
kinetic energy - the energy that particles have because they are moving
evenly distributed - spread out so that concentration is similar throughout
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