I've been pondering osmosis for a while now. I get most of the theory behind it but theres a few things they say in the tutorials that are confusing me. Main one is the fact they say that a substance (that can permeate the membrane) will diffuse until its concentration on one side of the membrane is equal to its concentration on the other but it does not matter what other substances may be on either side of the membrane.

I'll use sugar and water as an example. Lets say I have a half liter of pure water (say 100 moles) on one side of the membrane (side A) and a half liter of water (say 90 moles water + 10 moles sugar) with sugar dissolved in it on side b.

The membrane is permeable to water but not sugar. From what I've read the water on side A will diffuse and pass through the membrane until there is an equal amount of moles on each side and there is no longer a concentration gradient. Now both sides have equal concentrations of water but now side b has over a half liter volume and side a has under a half liter because side b has sugar on top of water.

Is this what really happens? The water molecules ignore the concentration of any other compound and diffuse until there are equal amounts of water everywhere regardless of the concentration gradients of the other compounds in the environment?

yes, i have seen it happen in real life. it looks kind of strange

however, you have to consider one thing:
when one side is pure water, theoretically, all the water should move to the other side since the pure water side will never increase in concentration (zero times anything is still zero). however, it doesnt work exactly like that. not exactly sure, but i think the high amount of water on one side creates a downward pressure on the water, pushing the water back against the concentration gradient, until it reaches equilibrium. though the salty side will have more water.

Yeah thats what I read just after I asked this question. I read that if the osmosis took place in an environment with no gravity then one side of the membrane really would rise alot higher than the other but since theres gravity this won't happen. The gravity will force more water back into the hypotonic side and will reach an equilibrium that favors more water flowing into the hypotonic side of the membrane.

One other thing I was wondering is what osmotic pressure is exactly. I read the wiki definition but its not very clear or concise. Is osmotic pressure just the pressure caused by an imbalance of impermeable solutes? For example if I had pure water on both sides of the membrane and I decided to add a bit of sugar to side B then would side by have higher osmotic pressure :confused:

take hypertension (high blood pressure) for example. one of the leading causes of it is due to a high salt diet. salt makes your blood salty. the walls of your blood vessels are semi permeable. if you blood is very salty, water will osmose through it. this increases the amount of water inside your circulatory system. however, your blood vessels is a closed system, and it can only stretch a little bit. as more water goes in there, because volume stays similar, pressure increases. this i think is an example of osmotic pressure

its also the technique used to kill people in lethal injections. a high concentration potassium chloride solution is injected intravenously, which spikes the blood pressure.

I have never understood osmosis, but, from this thread, I am to assume that a force acts on the water in our salt/water setup, that causes it to move. Like mantikore said, you could use this to raise the depth of the water on one side slightly with respect to the other.

This means work is being done. I could, for example, use a tiny turbine to turn the height difference into electricity when I remove the membrane, or even as the water flows through it.

Where is this energy coming from? How is this not the start of a perpetual motion machine?

Is it something to do with the salt dissolving in the water?

Nereth all the sources say that natural osmosis is caused by Brownian motion. That means the energy comes from normal air temperature. The random movement of particles in a fluid. All molecules in a liquid or gas have some degree of kinetic energy induced by the temperature so they are constantly moving around and banging into each other. Its this chaotic flow of molecules that causes gases and liquids to expand and encompass every part of a container.

When salt is dissolved in water it will spread until theres an equal concentration of salt everywhere in the water medium. Its a confusing concept to me because I would assume a chaotic force like this to give chaotic results and not perfect balanced results like a homogenous solution.

Hears what I've gathered about using osmosis to clean water.

Reverse osmosis: This is basically just forcing water through a very fine filter to filter out any molecules larger than water. The driving force in this is not osmotic pressure its hydraulic pressure. They literally pressurize the water on one side of the membrane which forces it through the membrane so it comes out a fair bit cleaner on the other side of the membrane. I don't know why they even call this osmosis though this is just filtration.

Forward osmosis: This uses the real principles of osmosis. I don't fully understand this but I know it uses osmotic pressure as its driving force. On one side of the membrane you have your dirty water called the feed solution and on the other side you have the draw solution which pulls water out of the feed solution. This draw solution has high osmotic pressure. This is what I think it is but I'm not fully sure. I think the draw solution is water with a non toxic solute like sugar dissolved in it. This sugar cannot pass through the membrane so it causes osmotic pressure. The water from the feed solution will diffuse to try and get equal concentrations on both sides of the solution so it will pass through the membrane into the draw solution.

I still don't really understand that because why would the draw solution having high osmotic pressure help at all. If you didn't have any sugar in the draw solution just a small amount of water wouldn't the same thing happen? Water from the feed solution seep into the draw solution?

If the energy comes from the kinetic energy of the water molecules, does that not imply that the temperature of the water will decrease?

And does that not then imply that you could create even more energy by exploiting the flow of heat from ambient temperature sources to the water?

The suns constantly replacing the thermal energy on earth. Osmosis is just exploiting the energy thats already there.I'd say there are loads of methods of exploiting natural phenomena that we don't even know about yet. For example this Norweigen scientist discovered a method of harvesting extraterrestrial gamma rays using satellites in space.

I'd say theres some potential in the earths magnetic field and the kinetic energy of the earth itself. Maybe a gigantic outer space electrical conductor. I don't know shit about space and the way planets revolve but don't masses outside the earths atmosphere revolve around earth?