Intermediate Topic 240 of 1102 States of Matter & Solutions
Effusion
Escape of gas molecules through a tiny hole into a vacuum.
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In-depth explanation of Effusion
Effusion is the escape of gas molecules through a tiny hole into a vacuum or a region of lower pressure, where the hole is smaller than the mean free path of the molecules. Because molecules pass through the hole without colliding with each other, the effusion rate depends on their speed and therefore on their molar mass and temperature. The rate is proportional to the pressure difference across the hole and to the average molecular speed. Effusion is the basis of Graham's law, and it is used in gas separation and in measuring vapour pressures by the Knudsen method.
Going deeper: Effusion
Key ideas: The effusion rate per unit area is p / sqrt(2 pi m kT), so it rises with pressure and falls with mass and falls slightly as temperature increases at fixed density. The hole diameter must be much smaller than the mean free path, which is about 70 nm in air at 1 atm. Mixture components effuse independently, so the escaping gas is enriched in the lighter component.
Common mistakes and connections: A typical mistake is calling any leak effusion; a large hole produces bulk flow instead. Another is expecting effusion rates to be equal for equal masses of different gases. Effusion rates compare as sqrt of inverse masses, not as masses. Connects to Graham's law, isotope separation and rms speed.
Visualization of Effusion
In effusion, molecules pass one by one through a tiny hole into a vacuum; lighter, faster molecules escape more frequently. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A small pinhole in a helium balloon lets helium escape faster than air would through the same hole, because helium atoms are lighter.
Frequently asked questions about Effusion
What is Effusion?
Escape of gas molecules through a tiny hole into a vacuum.
Is Effusion a beginner, intermediate or advanced topic?
Effusion is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Effusion?
Study these earlier topics in States of Matter & Solutions first: Gas Collection Over Water, Graham's Law of Effusion, Diffusion.
What comes after Effusion?
The next topics in States of Matter & Solutions are Root Mean Square Speed and Maxwell-Boltzmann Distribution.
Can I practise Effusion with quizzes?
Yes. The interactive lesson includes a self-check question on Effusion, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Effusion fits in States of Matter & Solutions
Effusion is topic 21 of 109 in States of Matter & Solutions.