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Hydrogen bonding in water | Water, acids, and bases | Biology | Khan Academy

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- [Voiceover] I don't think it's any secret to anyone that water is essential to life. Most of the biological, or actually in fact all of the significant biological processes in your body are dependent on water and are probably occurring inside of water. When you think of cells in your body, the cytoplasm inside of your cells, that is mainly water. In fact, me, who is talking to you right now, I am 60% to 70% water. You could think of me as kind of this big bag of water making a video right now. And it's not just human beings that need water. Life as we know it is dependent on water. That why when we have the search for signs of life on other planets we're always looking for signs of water. Maybe life can occur in other types of substances, but water is essential to life as we know it. And to understand why water is so special let's start to understand the structure of water and how it interacts with itself. And so water, as you probably already know, is made up of one oxygen atom and two hydrogen atoms. That's why we call it H2O. And they are bonded with covalent bonds. And covalent bonds, each of these bonds is this pair of electrons that both of these atoms get to pretend like they have. And so you have these two pairs. And you might be saying, "Well, why did I draw "the two hydrogens on this end? "Why didn't I draw them on opposite sides of the oxygen?" Well that's because oxygen also has two lone electron pairs. Two lone electron pairs. And these things are always repelling each other. The electrons are repelling from each other, and so, in reality if we were looking at it in three dimensions, the oxygen molecule is kind of a tetrahedral shape. I could try to, let me try to draw it a little bit. So if this is the oxygen right over here then you would have, you could have maybe one lone pair of electrons. I'll draw it as a little green circle there. Another lone pair of electrons back here. Then you have the covalent bond. You have the covalent bond to one hydrogen atom right over there. And then you have the covalent bond to the other hydrogen atom. And so you see it forms this tetrahedral shape, It's pretty close to a tetrahedron. Just like this, but the key is that the hydrogens are on one end of the molecule. And this is, we're going to see, very very important to the unique properties, or to the, what gives water its special properties. Now, one thing to realize is, it's very, in chemistry we draw these electrons very neatly, these dots up here. We draw these covalent bonds very neatly. But that's not the way that it actually works. Electrons are jumping around constantly. They're buzzing around, it's actually much more of a, even when you think about electrons, it's more of a probability of where you might find them. And so instead of thinking of these electrons as definitely here or definitely in these bonds, They're actually more of in this cloud around the different atoms. They're in this cloud that kind of describes a probability of where you might find them as they buzz and they jump around. And what's interesting about water is oxygen is extremely electronegative. So oxygen, that's oxygen and that's oxygen, it is extremely electronegative, it's one of the more electronegative elements we know of. It's definitely way more electronegative than hydrogen. And you might be saying, "Well, Sal, "what does it mean to be electronegative?" Well, electronegative is just a fancy way of saying that it hogs electrons. It likes to keep electrons for itself. Hogs electrons, so that's what's going on. Oxygen like to keep the electrons more around itself than the partners that it's bonding with. So even in these covalent bonds, you say, "Hey, we're supposed to be sharing these electrons." Oxygen says, "Well I still want them to "spend a little bit more time with me." And so they actually do spend more time on the side without the hydrogens than they do around the hydrogens. And you can imagine what this is going to do. This is going to form a partial negative charge at the, I guess you could say, the non-hydrogen end that is the end that has, that's well I guess this top end, the way I've drawn it right over here. And this Greek letter delta, this is to signify a partial charge, and it's a partial negative charge. Because electrons are negative. And then over here, since you have a slight deficiency of electrons, because they're spending so much time around the oxygen, it forms a partial positive charge right over there. So right when you just look at one water molecule, that doesn't seem so interesting. But it becomes really interesting when you look at many water molecules interacting together. So let me draw another water molecule right over here. So it's oxygen, you have two hydrogens, and then you have the bonds between them. You have a partially negative charge there. Partially positive charge on that end. And so you can imagine the partial, the side that has a partially negative charge is going to be attracted to the side that has a partially positive charge. And that attraction between these two, this is called a hydrogen bond. So that right over there is called a hydrogen bond. And this is key to the behavior of water. And we're going to see that in future videos. All the different ways that hydrogen bonds give water its unique characteristics. Hydrogen bonds are weaker than covalent bonds, but they're strong enough to give water that kind of nice fluid nature when we're thinking about kind of normal, or you could say, normal temperatures and pressures. This nice fluid nature, it allows these things to be attracted to each other, to have some cohesion, but also to break and reform and flow past each other. So you can imagine another hydrogen bond with another water molecule right over here. So put my hydrogens over there. Put my hydrogens, your bonds, partial negative, partial positive right over there. And so we'll see in future videos, hydrogen bonds, key for water flowing past itself. Key for its properties to its ability to take in heat. Key for its ability to regulate temperature. The key for its ability is why lakes don't freeze over. It's key for some of it's properties around evaporative cooling and surface tension and adhesion and cohesion, and we'll see that. And probably most important, and it's hard to rank of these things, if we're thinking about biological systems, this polarity that we have in water molecules and these hydrogen bonding, it's key for its ability to be a solvent, for it to be able to have polar molecules be dissolved inside of water. And we'll see that in future videos.

Video Details

Duration: 6 minutes and 47 seconds
Language: English
Genre: None
Views: 7
Posted by: jake.goldwasser on Nov 19, 2015

Reactants and products in reversible and irreversible chemical reactions.
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