Beginner Topic 239 of 1102 States of Matter & Solutions
Diffusion
Spontaneous mixing of particles due to random motion from high to low concentration.
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In-depth explanation of Diffusion
Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration, driven by random thermal motion. Gas molecules move very fast, but they collide with other molecules billions of times per second, so diffusion in air is much slower than their speeds suggest. The rate of diffusion increases with temperature and decreases with molar mass, because heavier molecules move more slowly. Diffusion occurs in gases, liquids and solids, and it is slowest in solids. It is the reason gases mix uniformly over time and why sugar spreads through a cup of tea.
Going deeper: Diffusion
Key ideas: Diffusion time grows with distance squared, t ~ x^2 / D, so doubling the distance quadruples the time. Typical gas diffusion coefficients are around 0.1 to 0.2 cm2/s, versus around 10^-5 cm2/s in liquids. Graham's law gives only relative rates and is not exact for diffusion because collisions matter.
Common mistakes and connections: Many students think diffusion is fast because molecular speeds are hundreds of metres per second, ignoring frequent collisions. Another error is treating diffusion as the same as effusion; effusion passes through a small hole. Diffusion needs no stirring but is greatly sped up by convection in practice. Connects to osmosis, Fick's law and mixing of gases.
Visualization of Diffusion
Diffusion spreads particles from high to low concentration through random thermal motion until the mixture is uniform. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The smell of perfume spreads gradually across a room as its molecules diffuse through the air.
Frequently asked questions about Diffusion
What is Diffusion?
Spontaneous mixing of particles due to random motion from high to low concentration.
Is Diffusion a beginner, intermediate or advanced topic?
Diffusion is a beginner topic in States of Matter & Solutions. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Diffusion?
Study these earlier topics in States of Matter & Solutions first: Mole Fraction, Gas Collection Over Water, Graham's Law of Effusion.
What comes after Diffusion?
The next topics in States of Matter & Solutions are Effusion and Root Mean Square Speed.
Can I practise Diffusion with quizzes?
Yes. The interactive lesson includes a self-check question on Diffusion, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Diffusion fits in States of Matter & Solutions
Diffusion is topic 20 of 109 in States of Matter & Solutions.