Drop a little ink into a glass of water and leave it alone, and without any stirring the ink slowly spreads until the whole glass is evenly colored.
This happens because the particles of water and ink keep moving about randomly and mix with each other; particles spreading out on their own like this is called diffusion.
Particles are far too small to see, but you can see traces of their motion in Brownian motion, the jittery, random movement of tiny grains from pollen floating in water.
Raise the temperature and the particles move faster, so in the pictures the red ink particles spread faster and the yellow grain jiggles more.
Why Ink Spreads by Itself
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Diffusion: ink particles (red) spread on their own among water particles (blue)
25℃
💡 Diffusion
①The particles that make up matter never sit still; they keep moving about randomly.
②So they spread from where there are many of them (high concentration) to where there are few (low concentration), until the concentration is even.
③The higher the temperature, the faster the particles move and the faster diffusion happens: ink spreads faster in hot water than in cold.
④Perfume sprayed in one corner filling the whole room is diffusion in air.
Traces of Particle Motion: Brownian Motion
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Brownian motion: a yellow grain is knocked about by invisible water particles
🔍 Brownian Motion
①In 1827 the botanist Robert Brown watched under a microscope as tiny grains released from pollen in water kept moving this way and that.
②The grains were not alive, so at first no one knew why they moved.
③Today we explain it by invisible water particles striking the grain from all sides, slightly harder from some directions than others, so the grain jiggles.
④It is evidence that particles are moving, even though we cannot see them directly.
Particle Motion and Temperature
Particle motion and temperature
temperature ↑ → particles move faster → diffusion speeds up
Temperature can be thought of as a measure of how lively the particles' motion is.
Direction of diffusion
high concentration → low concentration
It goes on until the concentration is even; after that the particles still move, but the overall picture no longer changes.
🌡️ Diffusion in Everyday Life
①A tea bag in hot water brews faster than in cold.
②The smell of cooking reaches other rooms (diffusion in a gas).
③Ink dropped into water spreads without stirring (diffusion in a liquid).
④Diffusion is fastest in gases, where particles are far apart and fast, and slower in liquids and slowest in solids.
Exam Key Points
Summary
diffusion: particles spreading out on their own (high → low concentration)
Faster at higher temperature, and fastest in gases, then liquids, then solids.
🎯 Exam Points
①Diffusion is particles spreading on their own from high to low concentration.
②The higher the temperature, the livelier the particle motion and the faster the diffusion.
③Brownian motion is indirect evidence that particles move.
④Diffusion is fast in gases because the particles are far apart and move quickly.
⑤Diffusion happens in gases, liquids and solids, but very slowly in solids.
Examples and Unit Test
Example 1
Ink dropped into water spreads by itself. What is this called, and what happens to the speed of spreading if the water is warmer?
1
Particles moving on their own and spreading from high to low concentration is called diffusion.
2
The higher the temperature, the faster the water and ink particles move, so diffusion speeds up.
▸ Diffusion / it speeds up as the temperature rises
Switch the picture between 5 ℃ and 80 ℃ and you can see how much faster the ink becomes even.
Example 2
Why does Brownian motion, the random movement of tiny grains from pollen in water, happen?
1
Around each grain, invisible water particles are constantly moving.
2
They hit the grain from all sides, but not equally hard from every direction, so the grain is pushed this way and that; this is evidence that particles really move.
▸ Water particles hit it unevenly from all sides (evidence of particle motion)
The grain is not moving by itself; it reveals the motion of particles we cannot see.
Unit-test style
Which statement about molecular motion and diffusion is correct?
①Diffusion goes from low concentration to high concentration
②The higher the temperature, the faster the diffusion
③Diffusion happens fastest in solids
④Brownian motion is evidence that molecules are at rest
⑤Diffusion happens even when molecules do not move
▸ ② The higher the temperature, the faster the diffusion
1
The higher the temperature, the livelier the particle motion and the faster the diffusion, so ② is correct.
2
Diffusion goes from high to low concentration (①), it is fastest in gases (③), Brownian motion is evidence that particles move (④), and diffusion happens because particles move (⑤).