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  • Hi. It's Mr. Andersen and in this video I'm going to talk about water, a polar molecule.

  • What does it mean to be polar? It means that you have a charge. The electrons are shared

  • unequally. And as a result of that water has these amazing behaviors that allow us to survive

  • on our planet. And so let's start by looking at one water molecule. It's made up of 1 oxygen

  • atom and 2 hydrogen atoms. And what you should know is that oxygen in highly electronegative.

  • What does that mean? It's going to pull electrons towards it. And if we were to look at those

  • levels here on the periodic table, aside from fluorine, it's going to be the highest one

  • there. And so why is that? Well if we go across on the periodic table we're seeing an increase

  • in electronegativity. Why is that? We're increasing the number of protons in the nucleus. Therefore

  • there's a more positive charge on the inside, so it's pulling those electrons towards it.

  • And you might think, well then as we go down on the periodic table, why doesn't it increase

  • there? Well as we go down we're filling in more levels of electrons. And so these electron

  • levels that are filled are shielding that pull of the protons on the inside. And therefore

  • we have to go up on a periodic table to get electronegativity increases. And so oxygen

  • is really high. It's pulling the electrons of hydrogen closer to it. And so what you

  • get is a partial positive and a partial negative charge. In other words the oxygen is always

  • going to be negative. And then the hydrogen is always going to be positive. And so you

  • can think of it almost like a magnet. There are going to poles, or two sides to it. And

  • just like magnetic fields move out and around the magnet, you can imagine these imaginary

  • fields around the water molecule as well. And so on the top where the hydrogen is we're

  • going to have positive charges. And then on the bottom we're going to have negative charges.

  • And so imagine trying to put a bunch of magnets together like this and just holding them there.

  • Now you know that those similar poles are going to repel each other. And so if you were

  • eventually to let got it would quickly shift to a shape like this, where south meets north

  • and north meets south. The same thing would occur with water. If we put water molecules

  • like this, all of these negative charges of the oxygen molecules are going to repel each

  • other. So it will quickly orient itself like this. And so water will always be oriented

  • so the positive of the hydrogen and the negative of the oxygen are attracted to each other.

  • And so what do we call that attraction between these? We call those simple hydrogen bonds.

  • And so the hydrogen bond would be represented with these dotted lines. It's an attraction

  • between the positive hydrogen and the negative oxygen. Sometimes we'll see hydrogen bonding

  • with nitrogen as well. Now what are some of the properties that we get as a result of

  • this polarity? The biggest one that you can see right away is cohesion. What does that

  • mean? If you think of this as one water molecule and this as another water molecule then the

  • positive here is attracted to the negative here. So if I pull this is this direction,

  • this is going to go with it. And so it's holding all of those water molecules together. And

  • so the surface tension in this water right here is caused by all of the hydrogen bonds

  • in the water molecules. Where this doesn't occur on our planet, but in space or in orbit

  • around our planet, this is a big globule of water. And it's just floating there. What's

  • holding it together is going to be the cohesion or the hydrogen bonds of those water molecules.

  • What's another property? We have capillary action. What does capillary action mean? Well

  • if we were to look in this tube, it's just sitting in water, there is starting to be

  • adhesion to the sides of the tube. Now how is adhesion different from cohesion? Adhesion

  • is when water is sticking to another surface. Anything that has a charge. And so you can

  • see it's kind of creeping up on the side. What's pulling the other water molecules along

  • with it? That's going to be cohesion. But you can see as we decrease the size of that

  • tube it goes higher. And as we make it smaller it goes higher yet. And in fact if we make

  • it microscopically small, so like the xylem which are these tiny tubes in a tree, that's

  • how water is moving all the way up a tree. There's a evaporation of water up at the top.

  • And then there's this connection of water all the way up these tubes to the surface.

  • And so it's moving all that water through a tree. What's pulling it up? It's going to

  • be the driving of that energy from the sun. But it's the connection of all those water

  • molecules. It also accounts for the high specific heat of water. What does that mean? It's really

  • hard to change the temperature of water. And the reason why is to change the temperature

  • you have to pull those molecules apart. And if you want to evaporate it you really have

  • to free up one of those water molecules. And so I live in Bozeman. And so Bozeman looks

  • just like this, surrounded by beautiful mountains. But we are about at the same latitude as Seattle.

  • Now Seattle, instead of being surrounded by mountains is surrounded by water. And since

  • that water has this high specific heat, what it does is it moderates their temperature.

  • And so this is the average high temperature in Bozeman and in Seattle. And you can see

  • Bozeman is this yellow line. In January it's going to be in the 30s or high 30s. And then

  • in the summer it's going to be in the upper 80s, pushing 90. But if we were to look at

  • Seattle its temperature is really not going to vary that much. And it's rare that they

  • have really icy roads and a lot of snow. And it's also rare that they're going to have

  • 100 degree temperatures in the summer. Why is that? It's the high specific heat of the

  • water that's surrounding Seattle. So for example in the summer when it gets really hot outside

  • a lot of that energy is going into the water itself. It's absorbing a lot of that energy.

  • And as it does that it's kind of moderating the temperature. It's a good reason why you're

  • filled with water as well. If you weren't our temperature would change radically over

  • the course of even a day. Another property is that ice floats. And you might think, well

  • that's obvious. But with most matter, with most molecules, when you cool them down they

  • become more dense and they would therefore sink. But water doesn't do that. And that

  • has to do with these hydrogen bonds in here as well. So as it cools down it's forming

  • this beautiful three-dimensional matrix which allows it to actually decrease density. Now

  • what is that important? Well, if it didn't do that then ice would settle at the bottom

  • of the oceans and we would quickly be a frozen planet. Another important thing about water

  • being polar is that it's a good solvent. And so this is a phet simulation. What I'm doing

  • is adding a little bit of sodium chloride or salt to water. And you'll notice what happens,

  • that when we add that to water it breaks apart into its ions. And so the sodium and chloride

  • ions are actually being ushered away by the water molecules. So if we get rid of the water

  • it goes right back to that ionic salt. So if we zoom in a little bit closer, what's

  • going on? Well let me kind of pause it right here. So what do we have? We have this chloride

  • ion which is going to have a negative charge and it's going to be surrounded by the positive

  • parts of the hydrogen. Likewise we have this sodium ion. And so it's a positive charge.

  • You can see that it's being surrounded by the negative charges of the oxygen molecules.

  • And so as a result, that allows us to dissolve material really, really quickly. And as we

  • move materials around in our body, it's important that we're made of water, a very good solvent.

  • And there's this saying that like dissolves like. And so let's say we have water and then

  • we throw a little sugar, a disaccharide next to that. Well what do you see? A bunch of

  • oxygen. A bunch of hydrogen. And so that's going to be polar as well. And so we're going

  • to have areas that are negative. And so the positive hydrogen is going to connect to that.

  • And areas that are positive and then the negative oxygen is going to bond to that. And so what

  • happens when we add sugar to water? It's going to dissolve quickly. And the reason why is

  • that like is dissolving like. Now if we were to look at another molecule, this is a triglyceride,

  • or a fat. It doesn't have a lot of oxygen and hydrogen together. It's mostly made up

  • of carbon and hydrogen. And those are sharing electrons equally. And so it's hard to dissolve

  • something like that. And so what happens when you pour, for example, oil into water? It

  • will settle out. And so anything that has a charge, water is able to breakdown. The

  • only exception would be things that are non-polar. So water is an amazing molecule. It allows

  • life to exist on our planet. And if it wasn't polar in nature, life probably wouldn't exist.

  • And as we start to look for life out in the universe, what are we generally looking for?

  • Water. And so where is a great place to look? Europa is one of the moons of Jupiter and

  • we think that there is a liquid water ocean underneath an ice surface. And so if we could

  • eventually get there, we might find life there. And so that's water. It's polar. It's amazing.

  • And I hope that was helpful.

Hi. It's Mr. Andersen and in this video I'm going to talk about water, a polar molecule.

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水。極性分子 (Water: A Polar Molecule)

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    李掌櫃 に公開 2021 年 01 月 14 日
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