# Questions tagged [osmosis]

Osmosis the the movement of molecules through a permeable membrane barrier. It occurs as the molecules of a solvent stabilize the concentration of solute particles on either side of the barrier. Osmosis does not require input energy, it does deplete the kinetic energy of the solution.

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### Help understanding osmotic pressure and van't Hoff equation

I am trying to understand how to interpret the van't Hoff equation for movement across a semi permeable membrane. Let's say we have a concentration of 100 mM sucrose on the inside of a semi permeable ...
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### Effects of an electrode in a microchannel flow

I am working on modeling the effects of an electrode in a microchannel flow. I am a little confused about how to model this. I have two electrodes on the boundary. Do I add the electric field effects ...
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### How to find the rate at which water can pass through a semipermeable membrane that allows particles of size lesser than 0.5 mm?

I was wondering what would be the rate at which water would cross a semipermeable membrane that allows particles of size greater than 0.5 mm. Knowing that size of water molecules is 2.75 angstroms ...
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### Osmosis in a U-shaped tube with selective permeable membrane

Starting from the classical osmosis experiment, a U-shaped tube with a semi-permeable membrane, I would like to consider the case when the solute added to one of the compartments (labelled A) is ...
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### The fundamental nature of osmotic pressure: electromagnetic?

As is known to all that osmotic pressure is an entropic force on a larger/statistical scale. But down to a smaller/microscopic sale, what's the origin or nature osmotic pressure? Is it electromagnetic ...
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### Osmotic pressure "equilibrium"?

Consider a box with two compartments separated by a semipermeable membrane. The first compartment is initially at pressure $P_0$ and contains the solvent ; the second compartment is initially at ...
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### Does osmotic pumping continue in plants at sub-zero temperatures?

Every now and again I stop and notice something which I find surprising, and try to work out why or how it happened. In this case I would like some assurance with the explanation. We have a bucket ...
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### Is osmosis stronger or weaker than gravity, and by how much?

Suppose you perpare a jar of salt water and another of sugar water and invert one on top of the other with a divider between them, and then carefully remove that divider so the liquids are in contact. ...
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### What exactly is an entropic force?

In general the entropic force is defined as $\vec{F}(\vec{X}_0) = T \vec{\nabla} S(\vec{X})\vert_{\vec{X}=\vec{X}_0}$, where $T$ is the temperature of the system and $S(\vec{X})$ is the entropy ...
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### Why does osmosis happen they way they do? [duplicate]

We know that in osmosis, the solvent molecules flow from the lower concentration to higher concentration across a semi-permeable membrane. But why do they flow like that? For my explanation, I took ...
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### How does the combination of freeze distillation and osmotic power respect the conservation of energy?

So imagine the following system. You're in a cold part of the world. Sea water isn't isn't frozen over due to the ocean moderating temperature, but smaller lakes definitely have by now. Sea water ...
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### Why reverse osmosis does not cause perpetual motion?

In reverse osmosis, water molecules flow from dense solution to pure water under external pressure. But how does this process prevents perpetual motion? Consider the following example. Salt water ...
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### Where is the energy involved in osmosis?

Osmosis creates pressure on the side of the membrane with higher concentration. But where does the energy for this come from?
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### By what mechanism is a growing tree root able to lift heavy concrete pavement?

A tree root lying under several square meters of 100mm thick concrete pavement can cause the pavement to lift up as it grows. What forces are involved in creating this lift? I vaguely understand that ...
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### Why would tennis balls filled with sulfur hexafluoride explode?

An answer at Chemistry.SE tells the following anecdote: Another fill gas to avoid is sulfur hexafluoride. A tennis ball manufacturer once decided to fill tennis balls with sulfur hexafluoride, ...
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### Why does the ideal gas law exactly match the van't Hoff law for osmotic pressure?

The van't Hoff law for osmotic pressure $\Pi$ is $$\Pi V=nRT$$ which looks similar to the ideal gas law $$PV = nRT.$$ Why is this? Also, in biology textbooks, the van't Hoff law is usually instead ...
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### Is a pressurized tank required for reverse osmosis systems?

It seems that a pressurized tank solves a few problems: a) Where to store the water b) How to get the water from a low point to a high point (floor to sink). c) How to provide water pressure after ...
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### Why is the osmotic force virtual?

In Einstein's treatment of Brownian motion he argues that (in equilibrium) there is an "osmotic pressure force" $\vec O$ that counterbalances the effect of gravity $\vec K$. This allows him to derive ...
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### membrane for a reverse osmosis desalination experiment

We are second year cram schoolers , we are currently working on a project tackling water desalination by reverse osmosis and are required to do an experience. During the process, we encountered some ...
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### Details of forces involved in osmosis at a microscopic level

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 ...
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### How is it possible for tall trees to pull water to heights more than 10m?

Which force actually drives water so high up, since pure atmospheric pressure will only get you up to about 10 meters if you're using suction and a long straw and yet tallest trees are over 100 meters ...
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### How is reverse osmosis even possible for desalination?

I don't understand how reverse osmosis for desalination is possible for existing RO membranes of pore size 10 angstroms (A) if the radius of a sodium atom is 3.14 A, chloride is 3.7 A and maximum ...
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### Does diffusion MRI measure diffusion or osmosis?

I am trying to understand the physical property which is measured in diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI). I read that these methods estimate the apparent diffusion ...
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### Osmotic pressure as a force

Where does osmotic pressure as a force originate? When salt water and fresh water are put together, they would be comfortable to stay as they were if the concentration difference weren't considered, ...
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### Water equilibrium while dying of thirst in a confined space

I'm trying to understand what happens with the water while a person is dying from thirst in a deep, narrow mine shaft. Assuming he has dry food for a long time, and some liters of water. Dry air is ...
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### Where does the energy for osmosis come from? [duplicate]

A thought experiment: A U-shaped tube with semi-permeable membrane at the base. The tube is completely thermally isolated from its surroundings. The liquid (solvent) is at some temperature $T$. ...
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### Dependence of Osmotic Effects on Particle Size

van't Hof's equation for osmotic pressure is derived with the assumption, among others, that the particle size of the solute is comparable to the particles size of the solvent. It holds well so long ...
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### What is osmotic pressure?

Many textbooks refer that osmotic pressure is the pressure we must exert in order for osmosis to stop. But other also refer that osmosis will stop eventually when a specific hydrostatic pressure ...
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### Where does energy in osmosis come from? [duplicate]

So I get that work is done when a net force converts potential to heat or kinetic. But I also understand that osmosis is driven purely by entropic processes. There is no net force, only a statistical ...
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### Is the mechanism behind osmosis attraction of water molecules or pressure differences?

I am trying to get an understanding of the physical mechanics behind osmosis. About half the sources I have found say that it is due to differences in pressure, but some say that it is due to the ...
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### Entropy as a driver behind processes

I sometimes encounter explanations of phenomena where only entropy is quoted as a driver, for example, osmosis, diffusion, hydrophobic effect... But can entropy ever be the sole driver of a process? I ...
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### How can we visualize osmotic pressure of seawater?

Wikipedia says here that seawater has an osmotic pressure of 27 atm, which in my opinion is very high. I guess, an application of this in practice is that any reverse osmosis desalination should ...
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### Osmosis and halocline

Osmosis can release a lot of power - 2.5 kJ per litre. This is equivalent to 250 m fall or rising temperature of the water more than half of degree. But mixing of salted and fresh water is ...
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### Osmotic Pressure: Comparing Solutes and Ideal Gases

The equation describing osmotic pressure is $\Pi=\frac{n}{V} RT$ which is just like the ideal gas equation law $PV=nRT$ So how much of an analogy is there between ideal gases and solutes? Is ...
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### Does there exist a membrane that has unbalanced concentration as equilibrium?

In osmosis, there is flow of solvent particles from a region of lower solute concentration to a region of higher solute concentration, until such time that the concentration on each sides are equal. ...
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### Can osmosis go the other way?

There is an effect called diffusio-osmosis, by which a flow is induced close to walls, and is ultimately caused by the repulsion of solute molecules by the solid wall around the pore. See this older ...
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### Physical reason of adding salt when we're cooking rice (osmosis)

Why do we add salt when we're cooking rice? I know one reason is related to the boiling point of water but someone said it is also related to "Osmosis". What is the relation between adding salt and ...
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### How exactly does osmosis or any other related phenomena work?

How do the molecules of one type 'know' there are less number of the same type on the other side of a semipermeable membrane? Does it have to do with the overall energy state of the system?
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### Reverse osmosis Water desalination

Is it feasible to submerge a reverse osmosis system in the sea in order to use the increasing pressure to push through the membrane and as the salt was removed would the lighter water rise on its own?
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### What happens when a semipermeable membrane allows only one solute to pass through?

I'm essentially a medical student where we deal a lot with osmosis. But when we are taught, it is done generally with only a single solute in consideration. What if two different solutes are used on ...
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### Physical explanation of osmosis

I want to understand the forces involved in osmosis. If I have a molecule of water and one of a salt in the left side of a semi-permeable membrane, and a water molecule at the right side, what forces ...
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### Paper Napkin Folded

Why folded napkins are better absorbing water than ordinary napkin. I think there must be related to cohesion of water element. Water are captivated between layers because they appeals each other or ...
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### Basics of osmosis. What about excluded volume?

I may not understand osmosis very well. Let us suppose two compartments filled with water, separated by a semi-permeable membrane. At equilibrium, both levels are equals. Let us introduce now a given ...
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### Acer saccharum (Sugar Maple) Sap & Freezing

This may or may not be the appropriate scientific discipline to ask this question of, but I'll give it a shot. I'll happily invite chemists to chime in as well. It's time to tap sugar maples in the ...
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### Can a long salt bath increase water retention?

So it's Thanksgiving here in the states and an odd combination of things are on my mind. In the past day, I've Brined a turkey whole, skin on Taken a long epsom salt bath (Same thing, right? What a ...
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