Khan Academy - Chemistry of Life NOTES

Current transcript segment:0:00- [Voiceover] I don't think it's any secret to anyone

0:02that water is essential to life.

0:05Most of the biological, or actually in fact

0:07all of the significant biological processes

0:09in your body are dependent on water

0:11and are probably occurring inside of water.

0:13When you think of cells in your body, the cytoplasm

0:15inside of your cells, that is mainly water.

0:19In fact, me, who is talking to you right now,

0:21I am 60% to 70% water.

0:24You could think of me as kind of this big

0:26bag of water making a video right now.

0:29And it's not just human beings that need water.

0:31Life as we know it is dependent on water.

0:33That why when we have the search for

0:36signs of life on other planets

0:37we're always looking for signs of water.

0:40Maybe life can occur in other types of substances,

0:44but water is essential to life as we know it.

0:47And to understand why water is so special

0:50let's start to understand the structure of water

0:52and how it interacts with itself.

0:54And so water, as you probably already know,

0:58is made up of one oxygen atom and two hydrogen atoms.

1:05That's why we call it H2O.

1:09And they are bonded with covalent bonds.

1:10And covalent bonds, each of these bonds

1:12is this pair of electrons that both of these

1:14atoms get to pretend like they have.

1:17And so you have these two pairs.

1:19And you might be saying, "Well, why did I draw

1:21"the two hydrogens on this end?

1:22"Why didn't I draw them on opposite sides of the oxygen?"

1:25Well that's because oxygen also has two lone electron pairs.

1:30Two lone electron pairs.

1:32And these things are always repelling each other.

1:34The electrons are repelling from each other, and so,

1:36in reality if we were looking at it in three dimensions,

1:38the oxygen molecule is kind of a tetrahedral shape.

1:42I could try to, let me try to draw it a little bit.

1:45So if this is the oxygen right over here

1:48then you would have, you could have

1:50maybe one lone pair of electrons.

1:52I'll draw it as a little green circle there.

1:54Another lone pair of electrons back here.

1:57Then you have the covalent bond.

2:00You have the covalent bond to

2:03one hydrogen atom right over there.

2:07And then you have the covalent bond

2:11to the other hydrogen atom.

2:12And so you see it forms this tetrahedral shape,

2:16It's pretty close to a tetrahedron.

2:18Just like this, but the key is that the hydrogens

2:21are on one end of the molecule.

2:23And this is, we're going to see, very very important

2:25to the unique properties, or to the,

2:29what gives water its special properties.

2:32Now, one thing to realize is, it's very, in chemistry

2:35we draw these electrons very neatly, these dots up here.

2:37We draw these covalent bonds very neatly.

2:40But that's not the way that it actually works.

2:41Electrons are jumping around constantly.

2:44They're buzzing around, it's actually

2:45much more of a, even when you think about electrons,

2:48it's more of a probability of where you might find them.

2:50And so instead of thinking of these electrons as

2:54definitely here or definitely in these bonds,

2:56They're actually more of in this cloud

2:58around the different atoms.

3:00They're in this cloud that kind of describes a probability

3:03of where you might find them as they buzz

3:04and they jump around.

3:07And what's interesting about water

3:08is oxygen is extremely electronegative.

3:13So oxygen, that's oxygen and that's oxygen,

3:16it is extremely electronegative, it's one of the more

3:19electronegative elements we know of.

3:22It's definitely way more electronegative than hydrogen.

3:25And you might be saying, "Well, Sal,

3:27"what does it mean to be electronegative?"

3:29Well, electronegative is just a fancy way of

3:31saying that it hogs electrons.

3:35It likes to keep electrons for itself.

3:39Hogs electrons, so that's what's going on.

3:43Oxygen like to keep the electrons more around itself

3:46than the partners that it's bonding with.

3:48So even in these covalent bonds, you say,

3:51"Hey, we're supposed to be sharing these electrons."

3:52Oxygen says, "Well I still want them to

3:54"spend a little bit more time with me."

3:56And so they actually do spend more time

3:58on the side without the hydrogens

4:00than they do around the hydrogens.

4:03And you can imagine what this is going to do.

4:05This is going to form a partial negative charge at the,

4:09I guess you could say, the non-hydrogen end

4:11that is the end that has, that's well I guess this top end,

4:14the way I've drawn it right over here.

4:16And this Greek letter delta, this is to signify

4:17a partial charge, and it's a partial negative charge.

4:20Because electrons are negative.

4:21And then over here, since you have a slight

4:23deficiency of electrons, because they're

4:24spending so much time around the oxygen,

4:26it forms a partial positive charge right over there.

4:31So right when you just look at one water molecule,

4:34that doesn't seem so interesting.

4:36But it becomes really interesting when you look at

4:38many water molecules interacting together.

4:41So let me draw another water molecule right over here.

4:45So it's oxygen, you have two hydrogens,

4:49and then you have the bonds between them.

4:52You have a partially negative charge there.

4:54Partially positive charge on that end.

4:58And so you can imagine the partial,

5:00the side that has a partially negative charge is going to be

5:02attracted to the side that has a partially positive charge.

5:05And that attraction between these two,

5:08this is called a hydrogen bond.

5:11So that right over there is called a hydrogen bond.

5:15And this is key to the behavior of water.

5:16And we're going to see that in future videos.

5:19All the different ways that hydrogen bonds

5:20give water its unique characteristics.

5:23Hydrogen bonds are weaker than covalent bonds,

5:25but they're strong enough to give water that kind of nice

5:29fluid nature when we're thinking about kind of normal,

5:32or you could say, normal temperatures and pressures.

5:35This nice fluid nature, it allows these things to be

5:38attracted to each other, to have some cohesion,

5:40but also to break and reform and flow past each other.

5:43So you can imagine another hydrogen bond with another

5:46water molecule right over here.

5:49So put my hydrogens over there.

5:52Put my hydrogens, your bonds, partial negative,

5:57partial positive right over there.

6:00And so we'll see in future videos, hydrogen bonds,

6:03key for water flowing past itself.

6:05Key for its properties to

6:08its ability to take in heat.

6:10Key for its ability to regulate temperature.

6:13The key for its ability is why lakes don't freeze over.

6:16It's key for some of its properties around

6:20evaporative cooling and surface tension

6:22and adhesion and cohesion, and we'll see that.

6:25And probably most important,

6:27and it's hard to rank of these things,

6:29if we're thinking about biological systems,

6:31this polarity that we have in water molecules

6:33and these hydrogen bonding,

6:35it's key for its ability to be a solvent,

6:37for it to be able to have

6:39polar molecules be dissolved inside of water.

6:43And we'll see that in future videos.