So we need some liquid.
I, I think it's fair to say that that is a necessity.
I can't imagine any way to make general life that
doesn't involve some sort of liquid for storing, for transporting chemicals.
But is it water?
Do we need water?
Is water something special?
Well, there are couple of reasons why water is special.
First, astrophysically.
Water is an abundant molecule in the solar system.
In addition to being common, it's one of a very small number of molecules that
we know to be a liquid in a liquid phase anywhere in the solar system.
And it's by far the most common one and
by far the most common liquid in our solar system.
So, those are good reasons to be intimately concerned with water.
Of course photosynthesis the, the version we showed involves water.
You don't have to have water for photosynthesis.
That's, that's a, we showed that because that's the one is most common.
So, that's not a good reason think there has to be water.
There's one other good reason that water is, is a critical thing here.
And that is the water is a polar molecule.
you, you might have heard water being described as the, the universal solvent.
Things dissolve in water better than in almost anything else, and that
the inherent reason for that is because water is a polar molecule.
Let me show you what I mean by polar molecule.
Here's a sketch of what water molecule looks like.
Here's, here's the oxygen atom here in the middle,
hydrogen on the end, hydrogen on the other end.
And it's bent, which is just a function of
the physical chemistry of these particular atoms coming together.
And what I'm showing you in the, in the shaded
part here, is the residual electrostatic charge that this molecule has.
Okay, let's think of it this way.
The hydrogen atom is really just a proton which is a positive charge.
And another proton with its positive charge.
No neutrons involved.
And, of course the, the oxygen nucleus has
a bunch of positive charges in through here.
But the oxygen also has a lot of electrons around.
And, and this covalent bond that water has
they are hydrogen and the oxygen are sharing
electrons back and forth between them in orbitals
that look, you know, sort of like this.
The fact that they're sharing the electrons
is what causes this bond between them.
But it also means that in, in very simplest
terms, the electron that's associated with this hydrogen atom tends
to spend more time between the two of them
than it does on [SOUND] this side of the hydrogen.
The electron tends to spend over here, more time than it does over here.
As a net result, you could almost think of it as this bare
proton over here sticking out on this end, sticking out on this end.
And because it's overall neutrally charged, you can tell from
that that there is a net negative charge on this end.
This is what's meant by a polar molecule.
It has a polarity just like a battery has a polarity.
And this shading that you see in the
background shows you the overall regions of the charge.
Over here it's generally positively charged.
Over here it's generally negatively charged.
Now what happens?
Well, if we have this thing that's generally
positively charged over here and generally negatively charged
over here, and we have water, which is
a fluid which can move around, what's going to happen?
Well, you're going to have things sort of forming like this.
Positive and negative charges like to move together.
You're going to have hydrogen bonding, it's called.
Hydrogen bonding is not the same as the bonds
that, that form the water molecule, the, the, oxygen
and the hydrogen that the, that, are very tightly
bounded in this covalent bond, where they're sharing electrons.
It's a weak bond, it's just that there's a slight force between the two of
them and so they tend to preferentially line up like this in the hydrogen bond.
Why does this matter?
Well, because, not just do they do it in the water molecules themselves, but
if I put some other substance in there, let's say NaCl, also known as salt.
What happens to NaCl?
Well, NaCl, when you put it in water, it it dissolves, and when it
dissolves, it turns into a positively charged
sodium atom, and a negatively charged chlorine atom.
It turns into that because this polar molecule wants to grab onto
those things it'll attach hydrogen bond to this water molecule this way.
There's the positive, positive-negative.
The chlorine will be over here.
There'll be the oxygen would grab onto the sodium and
that, that attraction will pull those, will break this ionic bond.
Pull those things apart, the object will dissolve.
Other liquids that don't have this polar nature, they're not
very good solvents, they will not dissolve these ionic compounds.
Why do you need things to be good solvents?
Again if your going to transport molecules, if your going to transport
energy, if your going to transport things that you're going to build things,
and you're going to transport them in a liquid, you need to
be able to dissolve in that liquid, if i wanted to transport.
Sodium chloride, and, I, put it
into something where sodium chloride didn't dissolve.
Well, you can imagine trying to push those salt grains along in some sort of river.
