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Main content
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AP.BIO:
SYI‑1.A (LO)
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Video transcript

I don't think it's any secret to anyone that water is essential to life most of the biological or actually frankly all of the significant biological processes in your body are dependent on water and are probably occurring inside of water when you think of the cells in your body in the cytoplasm inside of your cells that is mainly water in fact me who's talking to you right now I am 60 to 70% water you can 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's why when we have the search for for signs of life on other planets we're always looking for signs of water maybe life can occur and other types of and other types of substances but water is what 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 water is you probably already know is made up of one oxygen atom and two hydrogen atoms and two hydrogen atoms that's why we call it h2o h2o and they are bonded with covalent bonds and covalent bonds each of these bonds this is 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 don't I draw the two hydrogen's 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 three dimensions the oxygen molecule is kind of a tetrahedral 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 to this 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 one hydrogen atom right over there and then you have the covalent bond then you have the covalent bond to the other hydrogen atom and so you see it forms this this 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 this is we're going to see very very important to the unique properties or to this the the what gives water its special properties now one thing to realize is you know 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 is you know 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 and they jump around and what's interesting about water is oxygen is extremely electronegative so oxygen that's oxygen 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 what does it mean to be electronegative well electronegative is just a fancy way of saying is just a fancy way of saying that it hogs electrons hogs electrons it likes to keep electrons for itself hogs hogs electrons so that's what it's that's what's going on oxygen likes to keep the electrons more around itself than the partners that it's bonding with so even in these covalent bonds you could 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 hydrogen's than they do around the hydrogen's and you could imagine what this is going to do this is going to form a partial negative charge at the I guess you say the non hydrogen end is the end that has that that's why I guess this this top end the way I've drawn it right over here and this Greek letter Delta this is to signify a partial charge 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 partial positive charge right over there so right when you just look at one 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 hydrogen's 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 that that 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 the fluid nature when we're thinking about kind of normal where you take normal temperatures and and pressures this nice fluid nature it allows these things to 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 another water molecule right over here so put my hydrogen's over there put my hydrogen's here bonds partial negative partial partial positive right over there and so we'll see in future videos hydrogen bonds key for water flowing past itself key for its its properties to its ability to take in Heat key for its ability to regulate temperature the key for its abilities why why lakes don't freeze over its key for some of its pop properties around around evaporative cooling and surface tension and adhesion and cohesion and we'll see that and probably most important is hard to rank of these things if we're thinking about biological systems this this polarity that we have in water molecules in 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
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