Why does diffusion occur, even when the pressure at both regions are equal
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The answer is that all systems will change until they reach equilibrium. So if part of the system isn’t in equilibrium, it will diffuse.
If it’s too concentrated, it’ll disperse. If it’s more favourable for it to concentrate (say, solidify), it’ll diffuse to do that too.
It works for all states of matter, just the details change.
Pressure is related to the kinetic energy of the molecules. It means the molecules have velocity and bounce around in the system frequently colliding. The bet effect is that they spread around over macroscopic distances if you wait long enough.
Let’s start with a few details.
1- there is some of empty space between liquid and gas atoms. This allows them to move past each other.
2- materials with a temperature have vibrating or moving atoms. Hotter means more of then are moving faster.
3 pressure arises when there is a collision between two atoms. And this doesn’t cause then to stop moving, just bounce off and continue on a new direction.
So a group of atoms free to move (liquid it gas) had them in constant motion bouncing off each of their. They mill around, go past each other and such.
If they are all concentrated in one spot, but surrounded by a vacuum (no other atoms) this random motion means the atoms at the edge will face less obstacle in their outward direction, and will spread out.
If they are surrounded by a heavier gas there is resistance to this outward expansion. Even if there is pressure, and equal on both sides, that means the two sets are colliding. Our lighter gas is also smaller, and will bounce off the heavier gas… Into the Harris between them. The light gas will slip between the heavy atoms like cats in a crowd of people.
If surrounded by a lighter gas, the same commission that creates the pressure we measure well simply push the lighter atom away. The heavy atoms will just Wade through the light atoms, like people on a crowd of cats.
And it will spread out, on average, as collisions with the other gas impede the motion less than with the same gas. At least as long as the gasses have differing concentrations.
Edit: autocorrect weirdness
Concentration differences of the components. This drives diffusion when pressures are equal.
Pressure is just the average force on the boundary of a fluid from the internal motion inside it. Unlike a solid where there is a hard boundary, fluids are lots of small things moving around at random.
If you have 2 volumes of fluid right next to each other at equal pressure then there’s no difference in resistance for the small parts to move from one fluid to the other compared to moving around inside the one they started in. This means that the random motion can cross the boundary and, being random, sooner or later it will
If you have a higher pressure on one side then the tiny parts are moving more from the higher pressure fluid to the lower pressure fluid. If the difference in pressure is high enough and the space is open enough (think a pipe discharging out the side of a building into the atmosphere) then you’ll get effectively no back flow until it starts to equalise
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