Diffusion of Gases
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-187
2025-11-12
7
Diffusion of Gases.
If a closed vessel containing a gas is connected with another containing any different gas, no matter what its density, the gases will diffuse completely.
Demonstration. Fit a large rubber stopper with one hole into the open end of a porous cup, such as is used in small battery jars. Put one end of a glass tube about 2 ft. long through the stopper, and hold the tube inverted with the open end below the surface of water in a dish. Bring over the porous cup a jar filled with hydrogen, or with common illuminating gas, and bubbles will be seen to rush from the end of the tube and rise through the water, showing that gas has passed into the cup. After the bubbles stop rising, remove the jar and notice what follows.
It has been proved that a given volume of a gas contains the same number of molecules as the same volume of any other gas at the same pressure and temperature, and hence that the molecules of a light gas have a greater velocity than those of a heavy gas. Since the velocity of the molecules of the gas in the outer jar is much higher than that of the air molecules in the inner cup, the number that strike the outside of the porous cup is correspondingly greater than the number that strike the inside. This means that a greater number will pass through the pores of the cup from the outside to the inside than in the opposite direction, hence the excess has to pass out of the lower end of the tube. As the number of gas molecules increases within the cup, the number of impacts on the inside finally becomes equal to the number on the outside, and no further bubbles will escape. When the outer jar is removed, the conditions and results are reversed.
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