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Video transcript

let's say that I had two compartments of water and they're separated by a semipermeable membrane now what do I mean by a semipermeable membrane that means they allow some things to go through and not other things and let's say this semipermeable membrane it does allow water molecules to pass and a few seconds we'll talk about what it does not allow to pass which makes it semipermeable but let's just think about what would happen if we just had water molecules on the either side well we've already talked about it in the videos on diffusion the water molecule since we have an equal concentration on either side the probability that one of these water molecules goes this way in a certain amount of time is equal to the probability that a water molecule goes from right to left in the same amount of time and that's because if we have equal concentrations and these things are all bouncing around in all different ways they all have they all are they all have different velocities they have different speeds and in different directions and we just have to think about it probabilistically the probability of going from left to right through one of these gaps is going to be equal to the probability of going right to left in any given period of time but now let's make this interesting let's treat our water as a solvent and let's put some solute in it so let's dissolve some solute so let's throw some solute particles here and I'm going to make it make them bigger so you can see they would physically have trouble passing through these gaps there's other ways where you could have semipermeable membranes that use charge to allow certain things to pass through and not others but it's easier to visualize this these the size and thinking about the membrane is only allowing certain things of certain size to pass through so let's let's throw some solid there and actually I'll throw a little bit of solid here to I'll do I'll do one I'll do one or two particles right over here but I'm going to do many more I'm going to do many more over there on the right hand side so we have a higher concentration of solute on the right hand side and this is a semipermeable membrane and you can see even from the size where I drew these gaps these big particles aren't going to be able to go through the membrane they aren't going to be able to diffuse if they were allowed to diffuse and they would just go down their concentration gradient and in any given moment of time you would have a higher chance of one of these big particles moving from the right to the left then from the left to the right because you just have more on the right-hand side but this is a semipermeable this is a semipermeable membrane these things aren't just going to be allowed to naturally diffuse now all of these big particles they all have they all have their own their own unique velocities so they all have their unique velocities what do we think is going to happen well let's just think about the problem we know that the big particles can't diffuse from one side to another but what's going to happen to the water molecules well the water molecules on the left-hand side they're not going to be stopped they if they're bouncing in the right way they can bounce from the left to the right or they could move from the left to the right through one of these gaps but what about the ones on the right side well if things are if they're the just right conditions if there's the just right conditions maybe this character could move through this so you're definitely going to have water molecules going back and forth but I'd argue that the ones on the right hand side there's a lower probability of water molecules from the right hand side moving to the left as from the left hand side moving to the right and why is that well there's all this interference at play from these big molecules that aren't able to diffuse these are going to be bouncing around sometimes they're going to be even sometimes you could imagine them even blocking they're going to be they're going to be blocking the approach to to these to these openings if this membrane wasn't here they wouldn't block the approach they would just keep on going but since that membrane is there they might block it or they might ricochet off and while they ricochet off they might push on some water molecules they might push on some water molecules going in this direction right over there and so an argument can be made that these water molecules some of them will still make it from right to left but you have a lower probability of going from right to left as you have from going to left right and so because of this you would have a net inflow of water from this area where you have a low solute concentration and remember the solute is the thing that's dissolved in the water and in general we always consider the solvent to be whatever there's more of in this case its water and water is probably the most typical solvent and the solute is whatever there's less of so solute is dissolved in the solvent and so we have a net migration of the water molecules from this solution that has a low solute concentration to one that has a higher solute concentration and this phenomenon we call osmosis we call this osmosis and there's other arguments for osmosis and it's something that we've observed many many many times if you think put something that's used to fresh water and it has a if it has skin that is or it has membranes that is that allows water to pass through it put it in salt water you know they're kind of the famous things like slugs do not will not do well in the presence of salt because the water inside the slug will do exactly what is happening in this diagram now this mechanism that I just talked about that the the molecules that cannot pass through the membrane blocking the water molecules from going right to left ricocheting off and maybe causing the ones that are on the right side to to maybe move in this direction when they bounce into them that's one explanation another possibility is many times the solute that's being dissolved in water has some charge associated with it so when we think of say regular table salt you have sodium you have sodium ions that get so do regular table salt is sodium chloride but when you put it in the water you have sodium ions and you have chloride ions and you have chloride ions these are negatives so the chlorides are negative the sodium ions are positive and above and beyond doing some of the mechanical blockage that I just talked about there's also the idea that possibly that because they are because they are ionic they have charge and water has partial charges they also might stick to more of the water so the one the waters that stick to them aren't going to be available to move through the membrane and so what I mean by there the water is going to stick to them well when we think about a water molecule it's an oxygen and on the oxygen end you have a partially negative charge and then you have two hydrogen's to let me write it this way you have two hydrogen's right over here there's a partially positive charge and so they're going to be this this oxygen end away from the hydrogen's it's going to be attracted to the sodium molecule and so it's it's going to be less of the sodium molecule can't make it through this guy's going want to stick to the sodium molecule ins you can kind of imagine all of these water molecules sticking sticking to the sodium molecule which would make it less likely that these would pass from right to left than the ones that are passing from left to right similarly if you have a negatively charged ion like this well then you can orient the water the other way where the partially positive charged hydrogen ends are going to be at at attracted to the chloride ion right over here and since the chloride ion might not be able to get through well then these molecules that are stuck to it are going to be less likely to flow through these molecules are going to be more more attracted to the chloride or more attacked it to the sodium ions then they would be two other water molecules and only have partial charges see if these have full charges so if these can't get through well then maybe it's a lower probability that these are going to get through as well but the combined effects of all of these and I'd love if any of you all to point me to a nice simulation or maybe we'll create one on the Khan Academy computer science program to show this is that you're going to have a higher probability of the water molecules over here going from left to right then the water molecules over here going right to left from mechanical blockage and/or these big molecules ricocheting off and pushing them in the wrong direction or because they're just stuck to the big molecules because the big molecules are charged and that is osmosis
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