Water: A Polar Molecule

Uploaded by bozemanbiology on 27.11.2010


Hi. It's Mr. Andersen. Today I am going to be talking about water. I love
this picture of a water droplet. The forces that hold this water droplet together are
the same forces that allow us to survive. And a neat thing about water is that wherever
we find water on our planet we also find life. And so it's very important. And so this podcast
is dedicated to water. Okay. First of all what do we mean by a polar molecule? Well
oxygen is unique. And oxygen is unique in that it is almost the most electronegative
atom on the whole periodic table. So in other words it gets second place. The most electronegative
is actually fluorine. Fluorine can even form compounds with some of the noble gases which
is pretty amazing. First of all we should probably talk about what electronegative actually
means. Electronegative means how much you want electrons. And so if we think about the
valence electrons of oxygen, oxygen remember has six valence electrons, it would love to
have eight and so if it can just gain two more electrons then it can be happy. And so
if we look at this down here, so right here we've got an oxygen attached to two hydrogen
atoms. Oxygen's highly electronegative and so what it's going to do is it's actually
going to share electrons with hydrogen. And so it's not going to form an ionic bond, but
it's going to share those so unequally that it's really, really, really a polar molecule.
What does that mean? It means that on one side of the molecule, so on this, on this
oxygen side it's actually going to have a negative charge over here. And then on the
other side it's going to have a positive charge up here. So we get a positive charge up here.
And as a result of that the whole molecule has a charge. And a good way to think about
how that work is it's almost like a magnet. And so if a magnet looks like this and it's
got this north and south, water's going to behave kind of the same way. And so let's
talk about how that actually works. The most important bond that we need to talk about
when we're talking about water is what's called a hydrogen bond. And so I have a little model
for a water molecule right here. And so the red represents the oxygen and the white represents
the hydrogen. And so if we were to look at this bond right here between the hydrogen
and the oxygen we'd call that a polar covalent bond. But these ones are special in that they
have little magnets inside of them. And so if I try to hold it so the two hydrogens are
next to each other you'll notice that they'll repel each other. In other words they'll push
each other apart, so it's hard for me to get them to stick together. But if I hold the
oxygen next to the hydrogen like that then there'll be an attraction. And so that attraction
between the hydrogen on one side and the oxygen on one side is called a hydrogen bond. It's
much weaker than the actual covalent bond between the hydrogen and the oxygen, but what
it does is it holds that water molecule together. And so if it's water vapor and it's shooting
around there's not going to be much attraction, but when we get liquid water and the water
is flowing around itself, we actually get these hydrogen bonds that hold it together
and give it that property of cohesion. And so if you think about hydrogen bonds, and
we'll see hydrogen bonds in like ammonia as well. Sometimes with nitrogen. But usually
what a hydrogen bond is is an attraction between hydrogen on one side, which has a positive
charge and then oxygen on the other side which has a negative charge. And so in this diagram
right here this would be a hydrogen bond right here. Hydrogen bonds are really important
in holding water molecules together but they're also important in biological life. In other
words, the DNA, the two helixes or the two sides of the double helix is actually held
together with a hydrogen bond. Okay. Polar vs Nonpolar. I have two things on this slide
that are polar. So this would be water on this side. These are water molecules. And
you can see that the water molecules are kind of lining up so we get that hydrogen bond
between the hydrogen on one side and the oxygen on the other. This would be a fat. And so
this would be like an oil, like an olive oil would be mostly made of this triglyceride
that looks like that. So what is this made up of? Well you can see we've got a little
bit of oxygen up here, but most of it is actually carbon. And so carbon is making up most all
of this fat. Now carbon is not highly electronegative, it kind of fits somewhere between the two.
And so this whole molecule, this fat molecule is a nonpolar molecule. And you've probably
noticed other things that are nonpolar, like oil or gasoline. If you pour them into water
they don't mix. And the reason why is that water has a charge and so it's only able to
grab on to parts that are also charged. And so you might be a little confused with polar
and nonpolar but a way that I always remember this is that like dissolves like. And so if
you want to break down something that is charged then you should use a charged particle. If
I want to break something down like fat, what am I going to do? I'm going to use something
that's nonpolar to break that down. So I've come up with five properties of water that
are important just based on it being a polar molecule. First one is called the high specific
heat. High specific heat is a measure of how much energy you have to put into a molecule
to actually change its temperature. And so remember temperature is the speed of those
molecules as they move around each other and it's way easier to heat up something like
alcohol. And the reason why is that alcohol doesn't have much of these hydrogen bonds
and so it's not held together so it's easier to heat it up. A real world example, I'm actually
in West Yellowstone right now at a ski, my kids are at a ski camp, but we're just skiing
down here and this is the temperature in West Yellowstone. This is the temperature last
night. It was negative 22 degrees and so that's pretty darn cold. Now if we were to go straight
across as far as latitude, we'd end up in Seattle. And Seattle weather right now is
going to be much nicer than it is in Yellowstone. And the reason why is that Seattle's sitting
right on the ocean. And as that ocean absorbs energy during the summer, it doesn't change
much because water has a high specific heat. And likewise during the winter when it cools
down, it actually is giving off some of that energy and so it keeps Seattle fairly temperate.
And so high specific heat of water just, it explains why it takes a long time for water
to actually boil when you put it on a stove. Okay, next thing that's important as far as
these properties go is that since it's a polar molecule it's a really good solvent. And a
solvent remember is something that breaks down a solute. And so I love this diagram
right here. What we have is sodium chloride, which is just regular table salt, and that's
an ionic compound. Remember we have the cations - sodium ions, and these anions - chlorine
ions and so what happens with water, if we look right here in the middle is that chlorine
is actually surrounded, since it's a negative charge, it's surrounded by the positive hydrogen
atoms. And so they'll circle around that ion. And likewise the sodium which has a positive
charge is surrounded by the negative oxygen atoms. And so water works as a really really
good solvent. And in fact, that's why we're filled up mostly with water. It keeps our
heat the same but also it moves nutrients around. It's really good at that. Next we
have cohesive properties. Okay, so water remember is held together by this hydrogen bond and
so if we have a group of these molecules they'll all be held together. And so we get these
cohesive forces holding the water together. And so in this picture right here we've got
a paper clip that's actually floating on water. So water has a high surface tension and the
reason why is if we were to look way down here there's these cohesive forces between
the water molecules. And it almost works like a skin giving surface tension to that surface
of the water. Next one would be capillary action. Capillary action works like this.
If you have one water molecule attached to another water molecule you can get this long
string of water molecules like this. And so how does water get to the top of a tree? The
way it works is you'll evaporate one water molecule at the top and as we lose that, the
water will flow up. And so we have this long flow of water molecules moving all the way
up a tree. So that's capillary action. An example of that, let me try to sketch one
out is if we had a tube. Let's say we had a tube that looked like this. Then that tube
was just sitting in water. Water's going to start flowing up the tube. And the reason
it flows up the tube is the water molecules are attached to water molecules farther down.
If I were to make a thinner tube, so let me try to make a thinner tube, let me try. A
thinner tube, the water would actually flow up a larger amount. Or if I were to make a
microscopic tube the water might flow all the way up and even start to flow out the
top. And that's called capillary action. And so the microscopic tubules or tubes inside
a tree are actually called xylem. And then the last thing is the idea that as water becomes
colder it actually becomes less dense. And so the density of water is 1.0, so that's
going to be grams per cubic centimeter. But the density of ice is actually closer to 0.9
grams per cubic centimeter. And that's unlike anything else you can think of. If you've
ever played basketball on a cold day you'd realize the gas inside the basketball is actually
compressing and so it's becoming less dense. But water is unlike that. And the reason why,
again, is these hydrogen bonds between the water molecules. And so what it does is it
creates these beautiful hexagonal shapes. And you'll get these beautiful lattices. And
that's why on this picture right here we've got snowflakes and those snowflakes are held
together by these beautiful, based on hexagon kind of patterns. And the reason why that
we see that is again those hydrogen bonds. And so water is actually going to be, have
a greater density than ice because as those water molecules actually start to slow down
they will form these beautiful kind of three dimensional structures. And so that's it as
far as water goes. In the next few podcasts I'll talk a little bit more about solutions
and how they actually break down material. But for now that's a good introduction to