I would like to understand what happens when there are two containers which are connected together and separated by only semipermeable membrane. Then in one container I fill up (all the way to the top) pure water and in the second container I put water with a lot of salt ($\ce{NaCl}$). Now In container 2 it said to be an high osmotic pressure.

Then my question is: Is the water in container 1 will drain out / will be pushed out by the osmotic pressure in container 2? (see sections a,b,c below).

■  My question popped up after reading the following things in a book called "RBS - physiology" (6t edition p. 5):

Osmosis is the flow of water across a semipermeable membrane from a solution with low solute concentration to a solution with high solute concentration.

Example of osmosis:

a. Solutions 1 and 2 are separated by a semipermeable membrane. Solution 1 contains a solute that is too large to cross the membrane. Solution 2 is pure water. The presence of the solute in solution 1 produces an osmotic pressure.

b. The osmotic pressure difference across the membrane causes water to flow from solution 2 (which has no solute and the lower osmotic pressure) to solution 1 (which has the solute and the higher osmotic pressure).

c. With time, the volume of solution 1 increases and the volume of solution 2 decreases.


2 Answers 2


In your described scenario, there will be osmotic flow in an attempt to equalize the chemical potential of the solvent (water in this case, assuming the membrane is only permeable to water). The effect of the osmotic flow will be that pure water from container 1 will flow into container 2. Since this is a closed system, one will observe the liquid level rise in container 2 and fall in container 1, and the amount of displacement will be a function of how different the chemical potential of the water is between container 1 and container 2. There is no classical force that will push or pull the liquid from one container to another, osmotic flow is an instance of diffusion which is governed by the concentration gradient that is present.

  • $\begingroup$ Thank you 1+ What do you mean by "closed system"? My case, is for example two cups (glasses) connected and between them a membrane permeable to water. Now the two cups are open from above. And if I understood you well, you are talking about hermetically closed container (which is an interesting by itself) while I'm talking about open system. Isn't it? $\endgroup$ Jun 5, 2017 at 13:17
  • $\begingroup$ Closed system meaning that there are no energy or mass flows into and out of the system you have described, nothing special like a hermetically sealed container. $\endgroup$
    – J. Ari
    Jun 5, 2017 at 13:23
  • $\begingroup$ Then basically to answer to my question is that the water will not flow out of the container because it needs energy. Isn't it? If it is, I don't really understand why if the containers were filled with half of their space, some of the water had flown from side to side. Does it not need the same energy as it would needed when they were full up? $\endgroup$ Jun 5, 2017 at 13:33
  • $\begingroup$ Water will flow from container 1 to container 2, the driving force (i.e. energy) for this flow is the concentration gradient of the salt between the two containers. $\endgroup$
    – J. Ari
    Jun 5, 2017 at 14:06
  • $\begingroup$ So it cannot be the driving force to push out the water in the case of the full up containers? $\endgroup$ Jun 5, 2017 at 14:23

Indeed when one speaks of a flow from one compartment to the other, this is not true: water exchanges in the two directions, but the equilibrium is when there is a net flow, as water molecules are more attracted by salt than from nothing.


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