Effusion is the process in which a gas escapes through a small hole. This occurs if the diameter of the hole is considerably smaller than the mean free path of the molecules. According to Graham's law, the rate at which gases effuse (i.e., how many molecules pass through the hole per second) is dependent on their molecular weight. Gases with a higher molecular weight effuse slower than gases with a lower molecular weight; the number of smaller molecules escaping will be greater but the mass of the larger molecules will be greater. For two gases at the same temperature , and thus having the same kinetic energy, the root mean square molecular speed, , of each gas can be found using the equation
where is the Boltzmann constant. Thus, lighter molecules have a higher speed. This results in more molecules passing through the hole per unit time. This is why a balloon filled with low molecular weight hydrogen deflates faster than an equivalent balloon full of higher molecular weight oxygen.
Application to thermodynamics
 Thomas Graham (1805–1869), a Scottish chemist, found experimentally that the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles. In other words, the relative rates of effusion of two gases at the same temperature and pressure are given by the inverse ratio of the square roots of the masses of the gas particles. The equation is given by
where and represent the molar masses of the gases. This equation is known as Graham's law of effusion.
The effusion rate for a gas depends directly on the average velocity of its particles. Thus, the faster the gas particles are moving, the more likely they are to pass through the effusion orifice. A figure of Graham's law of effusion linked below shows the rate of effusion (the rate at which the gas is transferred across the barrier through the pin hole) is inversely proportional to the square root of the mass of the gas molecules.
- K.J. Laidler and J.H. Meiser, Physical Chemistry, Benjamin/Cummings 1982, p.18
- Zumdahl, Steven S. (2008). Chemical Principles. Boston: Houghton Mifflin Harcourt Publishing Company. p. 164. ISBN 978-0-547-19626-8.
- Effusion of a gas into an evacuated chamber