How does hydraulic equilibrium work on the particulate level? I recently learned that osmosis occurs when there is a semipermeable membrane between pure water and a solution of water. The pure water then fills up the solution's side until, as my textbook put it,

&quot;The pressure exerted by this column of solution counterbalances the pressure exerted by the water moving through the membrane from the pure water side.&quot;

My question is how does water just naturally want to move from one gradient to the other? Doesn't water move through the membrane both ways? Also, why does water &quot;want&quot; to go to the more concentrated side when the particles move at random?
In other words, why doesn't the flow of water molecules depend on the number of water molecules in the membrane instead of the concentration gradient?
The membrane has plenty of holes that are a little bit bigger than small molecules like water. Bigger molecules or bigger ions like usual solutes cannot go through these holes. So water has the tendency to cross the membrane both ways. When a water molecule hits a hole in the membrane from the pure water side, it can cross it. Nothing prevents it. When a solute molecule hits a hole of the same membrane, it blocks the passage for a couple of microseconds. No water can go through this hole from the solution side of the membrane. So on average, more water can cross the holes of the membrane in the direction water to solution, then backwards.
Is it what you wanted to know ?