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Water as a solvent | Water, acids, and bases | Biology | Khan Academy

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- [Voiceover] We've already talked about the notion that a water molecule has polarity to it. One end has a partially negative charge, and the other end has partially positive charges. And we've talked about how this leads to hydrogen bonds, and we alluded to the fact that maybe these hydrogen bonds give us all sorts of neat properties of water. And what I want to talk about in this video is one of those very important properties, and that's water's ability to be a solvent Water's ability to be a solvent. And this means that it's easy for certain things to be dissolved inside of water. And that's super important, because that's how a lot of the chemistry occurs, by things getting dissolved in water and then interacting and bumping with other things. And this is actually what's happening inside of cells, in the cytoplasm. The cytoplasm, which is mostly water, is a solvent which allows a bunch of interactions to happen between different types of molecules. But let's think about why water is a good solvent, and what types of things it can dissolve easily and what types of things it might not be able to dissolve so easily. So the key feature that makes water a good solvent, or at least a good solvent for a large class of molecules, is its polarity. If I were to take some sodium chloride, often known as table salt, so let me... So if I were to take sodium NaCl, sodium chloride. Sodium chloride, the sodium and chloride are attracted by ionic bonds. The sodium right over here has a positive charge, it has an electron stripped from it, and the chloride has a-- let me write that in a different color. So the sodium has a positive charge, because it has an electron stripped from it, and the chloride, it is an anion, it has a negative, it is a negatively charged ion, because it gains an extra electron. But they are attracted to each other. This has a positive charge, this has a negative charge. This is called an ionic bond. But if you put sodium chloride in water, something very interesting happens. This is something that we've all experienced. Take some table salt and put it in water and see what happens. It will dissolve. And why does it dissolve? Well, let's draw it out. So this is the sodium right over here. So that's the sodium. It has a positive charge. It has a positive charge. And then this is the chloride right over here. It has a negative charge. What's gonna happen if you put it inside of the water? Let me do that negative charge in the green. What's gonna happen when you put it inside of water. Well, you can imagine. The negative ends of the water molecules are going to be attracted to the sodium ion. So it's going to look something like this. So you have the oxygen, oxygen, oxygen, oxygen, oxygen, I'm clearly not drawing things to scale, but this'll just give you the idea. This end of the water molecule all has a partially negative charge. Partially negative charge. So it's going to be attracted to the positive sodium ion. And then the hydrogen ends, the hydrogen ends, are going to have a partial positive charge, and then they're going to be repelled. They are going to be repelled from the positive sodium ion. So it's gonna look something like this. And it's gonna look something like this. And you're gonna have partial positive charge on the outside. Partial positive charge on the outside. And now these hydrogens over here, this will just interact with water the way that it would typically, with the hydrogen bonding, the molecules just flowing past each other. So the fact that the sodium ion here, it's an ion, it has charge. It is able to dissolve in the water very easily, because it is attracted to the partially negative ends of the water molecule. Now, a similar thing is going to happen with the chloride ion. And we call a negative ion an anion. So over here, over here, and actually let me get some, let me move it over a little bit so that I have some space. So the chloride anion, let me move it over a little bit. So right, or maybe I'll move it over here. So the chloride, the chloride anion, let me see, I'm having trouble with my selection tool. Alright. So there we go. The chloride anion has a negative charge that's going to be attracted to the positive end of the water molecules. So you could imagine something like this. So the hydrogen ends, the hydrogen ends, are going to be attracted to it, they have a partially positive charge. They have a partially positive charge. And of course you have the oxygen end that has a partially negative charge. It has a partially negative charge. And I could draw more of these. Hydrogen, hydrogen, attracted there, you have the oxygen over, let me do that, I wanna keep my colors consistent. The oxygen right over there, we have the hydrogen, once again this isn't drawn to scale. Hydrogen, it is bonded to the oxygen. And so once again, you can form this, so you could almost imagine this shell of water molecules is going to be attracted to it, it's going to be attracted, or I guess you could say the partially positive end, which is where the hydrogen atoms are, is what's going to be attracted to this negative ion. So this is partial positive over here. And then on the partially negative side, outside of this shell, you can imagine it's just gonna interact with the water just the way any water molecule would, and so it's gonna be able to flow very easily. So you probably see something interesting here. If something has charge, if it's an ion, or if something has some polarity, it's very easy to dissolve it inside of water. And in this case, and just to have some terminology here, in this case, water is the, water is the solvent, so water is the solvent. So the solvent is water. And the thing that's being dissolved in the water, we call that a solute. So we call this the solute. So the sodium chloride. That is, you could use sodium chloride as a solute, or you could say that the sodium ions are a solute and so are the chloride ions. That is also considered to be the solute. And so you say, well what kind of things dissolve well? Well, things that that have charge or that are polar. And things that are charged and polar and tend to dissolve well in water, there's another word we use for them. We say that they are hydrophilic. So we could say that this right over here is hydrophilic. And if you look at the word root, hydro is referring to water. So hydro is referring to water. And philic means loving. So this literally means water loving. Water loving. Hydrophilic. And so you might be asking, okay, everything we've talked about, you know, we've seen water molecules, that's polar, we're looking at charged ions, okay we could get that, we can get why they'd be hydrophilic, they can incorporate themselves well into the water, but what are examples of things that would not incorporate themselves well in water? Well, in general, things that don't have charge, or that aren't polar, aren't going to be able to dissolve in water all that well. And a good example is hydrocarbons. So if you took some hexane. And hexane is a major constituent of car gasoline. So hexane, hex the prefix means six carbons. So let's see, one, let me do this in another color. So we have one, two, three, four, five, six carbons. So one two three four five six carbons. And then all the other bonds are with hydrogens. So let me draw this as well as I can. Carbon, at least typically, forms four bonds. So hydrogen, hydrogen, hydrogen, hydrogen, hydrogen, hydrogen, hydrogen. This right over here is hexane. This thing has no polarity to it. It doesn't form hydrogen bonds, it doesn't have any polarity. And so if you were to take hexane and throw it into water, it's not going to dissolve that well. It's actually going to kind bead up. And you would see that if you actually threw some gasoline inside of water. So things like hexane we would call hydrophobic. Hydrophobic. So this right over here is hydrophobic. Hydrophobic. It'll literally ball up to avoid getting in touch, to minimize its contact with water. Because the water is attracted to itself, and it is not so attracted to this stuff right over here. And hydrophobic? You still have hydro, meaning water, and then phobic, it means fearing. So this right over here is water fearing. That's why it's hard to mix things like water and oil or gasoline and water.

Video Details

Duration: 9 minutes and 12 seconds
Language: English
Genre: None
Views: 5
Posted by: jake.goldwasser on Nov 19, 2015

Water as a solvent. Polar solutes. Hydrophilic and hydrophobic substances.
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