Graham's law, known as Graham's law of effusion, was formulated by Scottish physical chemist Thomas Graham in 1846. Graham found experimentally that the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles. This formula can be written as:
where:
- Rate1 is the rate of effusion of the first gas (volume or number of moles per unit time).
- Rate2 is the rate of effusion for the second gas.
- M1 is the molar mass of gas 1
- M2 is the molar mass of gas 2.
Graham's law states that the rate of effusion of a gas is inversely proportional to the square root of its molecular weight. Thus, if the molecular weight of one gas is four times that of another, it would diffuse through a porous plug or escape through a small pinhole in a vessel at half the rate of the other. A complete theoretical explanation of Graham's law was provided years later by the kinetic theory of gases. Graham's law provides a basis for separating isotopes by diffusion — a method that came to play a crucial role in the development of the atomic bomb.
Graham's law is most accurate for molecular effusion which involves the movement of one gas at a time through a hole. It is only approximate for diffusion of one gas in another or in air, as these processes involve the movement of more than one gas.
Famous quotes containing the words graham and/or law:
“You had disadvantages. You’re insane.”
—Michael Mann, U.S. screenwriter. Will Graham (William Petersen)
“Our law very often reminds one of those outskirts of cities where you cannot for a long time tell how the streets come to wind about in so capricious and serpent-like a manner. At last it strikes you that they grew up, house by house, on the devious tracks of the old green lanes; and if you follow on to the existing fields, you may often find the change half complete.”
—Walter Bagehot (1826–1877)