Brian Bi

## What do we see the phenomenon of osmosis occuring?

I know osmisis occurs when there is a movement of a solvent, like water, through a semipermeable membrane, with a tendence to equalize these different concentrations. But,why this equilibrium must be achieved? I'm not asking the explanation of the osmosis phenomenon, but why it happens. Why there is a tendence to balance the concentration?

Note: Sorry for the bad english, it is not my first language. Thank you all.

The spontaneity of a process depends only on the free energy change, and free energy is a state function. Consequently, you can predict whether a process is spontaneous just by comparing the initial and final states; it doesn't matter how it got from the initial state to the final state.

So let's say we have two solutions of glucose in water: one dilute and one concentrated. Let's put them in two separate compartments of a beaker separated only by a semipermeable membrane. In the instant before any water starts to cross the membrane, let's call that initial state 1. Now, remove the membrane, let the two solutions freely mix, and then add the membrane back. After doing this, we get final state 1. Call this entire process (1). We know this process is spontaneous, since adding or removing the membrane doesn't change the free energy much, and obviously the mixing step is spontaneous.

In process (2), we don't mix the two solutions directly. We start with the same initial state as in (1), with the two solutions separated by a semipermeable membrane. This time, we allow water to flow from the dilute solution to the concentrated solution until both sides have equal concentration. Now we have final state 2, which is the same as final state 1. Denote this by process (2).

Since process (1) is spontaneous, and process (2) has the same initial and final state as process (1), we conclude that process (2) must be spontaneous as well. It makes no difference that process (1) occurs by the diffusion of both water and glucose, whereas in process (2) only water crosses from one side to the other.

tl;dr: It's more energetically favourable to have equal concentrations, which means the system will try to equalize the concentrations through any possible process.

I added in the comments: It's also worth pointing out that, by this logic, the process would still be spontaneous even if the membrane were impermeable to both water and glucose. It would just have an extremely high activation energy.