Example 1
Dye takes 80s to spread in sample A and 40s in B over the same distance. Which is faster?
B is faster because the same spreading distance is reached in less time. This does not mean every particle follows a straight path.
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Year 7
Diffusion and particle properties
Apply the particle model to diffusion, compressibility and temperature patterns.
Method and worked examples
Diffusion is the net spreading of particles from higher to lower concentration because particles move continually in many directions. Dye can spread through still water without stirring. Individual particles do not all travel straight down the concentration difference. Higher temperature generally increases particle motion and diffusion rate, although currents can also spread dye.
For a stated spreading distance, a shorter time means faster spreading. Keep temperature and time units distinct: degrees Celsius and seconds measure different things. To find a mean time, add the times and divide by their number. Use particle spacing to explain compressibility: large gas gaps can decrease, whereas liquid particles are already close together.
Example 1
Example 2
Why is a liquid much harder to compress than a gas?
Its particles are already close together. A gas has much larger gaps that can decrease.
Watch out: Individual particles move in many directions. Their collective motion produces net spreading down a concentration difference.