Fundamentals of Physics 5a)
Falsification of Kinetic Theory
Diffusion
The transport phenomena of the admixture of two different gases does not proceed as the theory predicts. This observation is confirmed in practice by the mixing of gases for commercial use and an example is the mixture of nitrogen and helium, which is used to test high pressure piping and equipment for leaks, as it escapes through the smallest of apertures and simple equipment is available to detect the gas.
However Helium, as a rare gas, is in short supply and is very expensive, and a mixture of 5% helium and 95% nitrogen serves the purpose, and companies producing medical and industrial gases are able to supply this mixture.
But simply introducing both gases into a storage cylinder, in any order, does not achieve a homogeneous mixture suitable for practical use (i.e. with the helium atoms evenly distributed within the more numerous nitrogen atoms) unless it is left for a week or more.
A quicker method of mixing is achieved by placing such a cylinder horizontally on rollers and rotating (or ‘rumbling’) it for some time (a few hours), which process creates a frictional effect between the internal walls of the rotating cylinder and the gases in contact with it.
If the principles of the kinetic atomic theory of gases are applied to the example of static mixing, we see that, according to the theory, the average velocity of nitrogen molecules in air is around 500 metres per second and that of helium atoms is 1300 metres per second. The relative mass of nitrogen is around 14, and that of helium 4, and a typical industrial gas cylinder is around 1.5 metres high and 200 mm in diameter. If introduced after the nitrogen, the lighter helium content would be positioned in the cylinder at the top, and occupy 75mm of the internal height, while the nitrogen the remaining 1425mm.
The diagram above depicts the nitrogen molecules and the helium atoms at the separation point and the numbers conform to Avogadro’s Law.
If uninhibited by collisions, at these velocities it would be possible for the slower nitrogen molecules that are in the vicinity of the helium atoms at the top of the cylinder to travel to the top of the cylinder and back 3,300 times in one second, 200,000 times in one minute. Extending the time to one hour would enable each nitrogen molecule to travel this distance 12 million times, a total distance of 600 kilometres.
With respect to each of the helium atoms, in one second they could each travel in the other direction to the bottom of the cylinder and back around 400 times, 24,000 times in a minute. In one hour 1,500,000 times and traveling a total distance of over 2,800 kilometres.
But of course collisions of any single atom with others are frequent and such an atom would not move linearly, but in a completely random manner.
This is an unusually frank comment from a Russian textbook:- (1)
“Since this transport is ensured by motion of the molecules, and the velocities of the molecules are high, diffusion should seem to occur rapidly with the concentrations leveling out almost instantaneously. Experiments show, however, that at atmospheric pressure diffusion is a very slow process, and mixing in the absence of motion of the gas as a whole may last several days.” (My emphases)
In an attempt to explain this problem the proponents of Kinetic Theory suggest that while the molecules in the above example move chaotically at high velocity, somehow collisions with the molecules of the other gas mean they always end up in the area from which they started in the first place, somehow, in this particular instance, showing both chaotic and ordered characteristic’s at the same time. In other words suggesting that any collisions that they endure with molecules of the other gas must result in their returning to the area in which they originated.
But this is a direct contradiction of the principle that the collisions are completely random or chaotic, and instead is saying that these interactions are, by some inexplicable means, regulated or controlled.
Given the postulated random, kinetic movement, together with the large volumes of empty space separating molecules and atoms and their high velocities, the fact that mixing, in commercial and experimental practice, is very slow, is direct and incontrovertible proof that this theory is invalid.
(1) Molecular Physics, Kirkoin and Kirkoin, Mir Publishers, Moscow