eli5 how molecules move during diffusion



No, this is not for homework, I’m genuinely interested in stuff like this and I couldn’t find s lot online about this that explained it easy enough for me.

In: Physics

Any atom or molecule at a temperature above absolute zero is moving (that’s basically the physical manifestation of temperature). Since we never get to absolute zero, all atoms/molecules are moving all the time. They constantly are bouncing off each other but, at the molecular level, all collisions are perfect and nothing is ever lost so they “never” stop, they just change direction.

Diffusion is just what happens if you let molecules run around randomly for long enough.

Despite it seeming like there is something actively moving them, the molecules pretty much move randomly. Here’s an explanation I thought was quite good (though it’s probably better with the drawing):

Assume you would have a cube divided in two equally large parts by a wall. Inside the cube there is a solvent with the same temperature (so it would be homogeneous if the wall wasn’t there). On either side of the wall the same substance is dissolved, but on one side the concentration is much larger. On both sides the random motion of the dissolved substance means that molecules will bounce off of the wall. Because of the larger concentration on one side, more molecules will bounce off on the side with the larger concentration because there are more of them there. Now assume you would remove this wall. The same number of molecules would still hit the wall, they only don’t bounce off anymore and move into the other part of the cube. Because there would be more molecules hitting the wall from the side with the larger concentration, more molecules would move into the part of the cube with a lower concentration. Over time this means that the concentration will rise on one side and lower on the other. This will continue until the concentration is the same anywhere, at which point the substance has diffused. This is how diffusion occurs even though the molecules just move in random directions.

Classically, molecules generally moves in straight lines until they collide with other ones. When they collide, they change directions (and speed).

So you can pretty much imagine a billiards table without the walls. If the walls of a billiards table didn’t keep the balls within the table, where would they tend to go? Outwards from the center of the table. It helps if you think of a lot of balls within a billiards table rather than just 15.