I read about osmosis and most of the chemistry books did not delve into intricate details so I consulted many sites to get the "physics approach" to osmosis and they mostly talked about the Gibb's energy and chemical potential differences which I found hard to understand without having much fundamental knowledge about them.

But I want to know isn't there anything to say about the forces involved and pressure exerted by the particles on the SPM? I read about Van't Hoff's theory of dilute solutions and got an interesting analogy with gaseous molecules and solute particles in a very dilute solution. Can't we use it to comment intuitively on the real physical processes involved in osmosis without getting in much deep into the mathematics?(though I understand that entire statistical physics and physical chemistry stands on math). I have a theory that more no. of solute particles on hypertonic side exerts more pressure on the SPM wall and hence to achieve equilibrium the solvent flows from hypo to hypertonic side and hence exerts a counter-pressure. Is this correct?

It would be very helpful if someone could provide a clear and deeper understanding about this process without involving the energy concept(if possible) for a high school student.


1 Answer 1


It is my understanding that the primary factor in buildup of osmotic pressure is probability.

For a particularly simple case let's take water as the solvent. Pure water at one side of the selectively permeable membrane, and a salt solution on the other side.

Water molecules enter the membrane, and any water molecule has a statistical probability of migrating all the way to the other side of the membrane.

However, the level of saturation will not be the same at both sides of the membrane.

The water molecules at the not-pure-water side have a smaller probability of entering the membrane. Each of the ions of the salt in the solution has an electric effect on the polar water molecules. The ions tend to attract a "coating" of water molecules that interferes with water molecules entering the membrane (when that "coated" ion happens to be snug against the membrane.)

Conversely, the ions (when snug against the membrane) will tend to draw water molecules out of the membrane.

So the side of the membrane adjoining the salt solution will be less saturated with water than the pure water side.

This difference in water saturation gives that on average water molecules will migrate towards the salt solution side.

Over time a pressure difference builds up. The larger pressure at that salt solution side increases the probability of water molecules entering the membrane.

When the probabilities on either side of the membrane attain equilibrium the buildup of osmotic pressure comes to a halt.

So: it's about probability, and about reaching a state of equilibrium of probability.

Additional remark:
The solution of salts in water is a matter of what is energetically favorable. The attraction between the salt's ions and the polar water molecules is what makes it energetically favorable for many salts to dissolve in water. As discussed above, this attraction is instrumental in buildup of osmotic pressure.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.