Why does a high osmotic pressure pull water in, yet a high hydrostatic pressure pushes water out?

Question

When I normally think of high pressure, I think of it as pushing something away from the zone of high pressure.

Yet for a high osmotic pressure, water is pulled IN to this high osmotic pressure region.

Why is this the case?

Answer 1

Great question, the answer has to do with the definition of osmotic pressure and the difference between this pressure and "hydrostatic pressure" which is the water pressure you are thinking of.

Here is the definition of "osmotic pressure" given by Google:

the pressure that would have to be applied to a pure solvent to prevent it from passing into a given solution by osmosis, often used to express the concentration of the solution.

Osmotic pressure in this definition is best illustrated by the commonly used tool to show osmotic pressure, often seen in a textbook as something like this:

In this picture, osmotic pressure is roughly how hard do you have to push down on the water on the right to make the levels the same on the left and right, despite the difference in concentrations. Osmotic pressure is not a pressure of water, as your question seems to imply, it is a pressure of the solutes in the water.

If it helps, I think it would be appropriate to think of this as sort of a "suction" on the water. Net flow of water stops when:

osmotic pressure - hydrostatic pressure = 0

...on the right side of the diagram.

Answer 2

• What is "Osmotic Pressure"?

Consider the following system -1 :

Fig.1: Left container: Pure water (Solvent). Right container: Solution of glucose (solute) in water (solvent); separated with a semi-permeable membrane.

In this system; as we know, water (solute) will flow from left container (Lower concentration of solute) to the right (higher concentration of solute); and such phenomenon is called Osmosis. If we use more-concentrated sugar solution to the right-container; the "tendency" to take water from the left container. We need to "define" this tendency.

Why the flow is taking place? Whatever going on molecular scale; on gross-scale we can imagine a 'pressure' that is driving the solvent only. (As we know, to pump a fluid we need to generate a difference in pressure). But it is not actually a real pressure. It is just used to measure/ describe the "Tendency to suck solvent". Now we have to "define" this pressure.

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Consider the following system - 2

Fig-2: A piston has been added on the second container of system-1, with which a variable (as we wish) amount of pressure could be applied on the content of right container.

We can see the following 3 conditions:

A. Below a certain applied pressure, osmosis continues, but the rate is lesser than no-piston (system-1)

B. At certain applied pressure osmosis stops.

C. Above certain applied pressure; the solvent follow a reverse path. i.e. get squeezed out back-to left container. This is called Reverse osmosis.

Condition- B; i.e. the applied pressure at which the flow stops, is considered as the magnitude of osmotic pressure.

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• Why osmotic pressure pulls the water in?

Better to ask,

• Why a solution try to suck solvent through a semi-permeable membrane? Or why a solution do have an osmotic pressure?

According to this article ( * ) by Frank G. Borg ;

The standard treatment of osmosis in thermodynamics employs the concept of the chemical potential and does not give any clues how the process "really" works.

The answer is controversial; and there are 3 hypotheses.

• 1. The theory you mentioned: the concentration of water explanation-

Fig: 3, Diagram taken from (**) experiment- article by Robert J. Kosinski and C. Kaighn Morlok

This theory tells; if we increase concentration of solute in a solution, the concentration of solvent would decrease.

But they also told that this theory doesn't work. See appendix section of the paper.

The Handbook of Chemistry and Physics has a large section on solutions of common solutes, and it discloses that a 0.2 M solution of NaCl has a markedly higher water concentration (995 g/L) than a 0.2 M solution of sucrose (958 g/L) (Wolf et al., 1982, pp. D261 and D270). Yet our experiments will disclose that a potato cores loses water to a 0.2 M solution of NaCl but it gains water from a 0.2 M solution of sucrose.

• 2. The “bound water” explanation for osmosis

Fig 4: The “bound water” explanation for osmosis. Diagram taken from following (**) research article by Robert J. Kosinski and C. Kaighn Morlok.

According to this hypothesis; due to the attraction (or hydrophilicity) the solute molecules bind the water; thus decrease water's concentration.

The paper told this theory is proven wrong because:

If the bound water explanation were true, we would expect that a greater mass of hydrophilic solute would bind more water. Whether a certain mass of solute is present in a few large molecules or in many small ones shouldn’t matter.

And in wikipedia's language (latest edition),

" The "bound water" model is refuted by the fact that osmosis is independent of the size of the solute molecules—a colligative property".

But this argument is not clear to me, I confess, because if solute particles break into fragments; the exposed area should be increased, so, more surface area would become available to get bind with solvent; and vice versa if more than 1 solute particles bound with each other the surface area should get decreased. So I did not understand why they thought the attraction model conflicts with number-of-particle theory.

3. the number of particles” explanation (According to the article by Kosinski and Kaighn, it is as same as Van't Hoff 's theory). According to this theory; only the number of particles per certain amount of solution; (and not its chemical nature) determines the osmotic pressure. i.e. osmotic pressure acts as Colligative property of a solution.

Though its derivation is thermodynamic (using concept of energy loss-gain); (And I confess, I've not understood whole mathematical derivation), I could not found any portion of it directly claiming a physical mechanism. However according to wikipedia;

It is hard to describe osmosis without a mechanical or thermodynamic explanation, but basically, there is an interaction between the solute and water that counteracts the pressure that otherwise free solute molecules would exert. One fact to take note of is that heat from the surroundings is able to be converted into mechanical energy (water rising).

Reference:

1. Plant Physiology by Taiz and Zeiger, Ed3, Sinauer.

2. * What is Osmosis? Explanation and Understanding of a Physical Phenomenon / Frank Borg , Jyväskylä University, Chydenius Institute

3. ** Challenging Misconceptions about Osmosis / Robert J. Kosinski and C. Kaighn Morlok

4. My high-school chemistry textbook, দ্বাদশে রসায়ন (Chemistry for class 12), 3rd edn 2008/ Mitra, Paul, Choudhury.

5. Wikipedia

Answer 3

Osmotic pressure is simply... Negative pressure exerted by solutes in the solvent.. That will draw the solvent into the solution. External pressure,(+VE)exerted on the solution side to prevent the flow of solvent into solution across semipermeable membrane measures... Total magnitude of OSMOTIC PRESSURE (-VE) created by solutes.