- [Instructor] When we
look out into the cosmos
for alien life, many folks
look for signs of water
on moons or planets.
And that's because life as we know it
is dependent on water.
And to understand that,
we just have to take a closer look
at some of the properties of water.
So what you see here are
some molecules of water.
This might be a review for you.
Every water molecule has one oxygen atom.
And it is bonded to two hydrogens.
So that is a hydrogen, and
that is a hydrogen as well.
And the nature of that bond,
it is a covalent bond,
which means that the
oxygen shares electrons
with each of the hydrogen atoms.
But oxygen is more electronegative,
and that's just a fancy way of saying
that even though those
electrons are shared,
they're going to be spending more time
around the oxygen than
around the hydrogens.
One way to think about it
is oxygen likes to hog electrons
more than hydrogen does.
And since the electrons
will spend more time
around the oxygen than
around the hydrogen,
and because it's a bent molecule
with the hydrogens on
one side of the molecule,
what happens is the side
where the oxygen is,
where the electrons spend more time,
that gets a partially negative charge.
So this is the lowercase
Greek letter delta.
That just means partially negative charge.
And then the sides
where the hydrogens are,
those acquire a partial positive charge.
And so what you see here
is that a water molecule is
not charged in aggregate.
But either side has a partial charge,
so it is a polar molecule.
And so you can imagine when you put
a bunch of water molecules together
what might happen?
Well, the partially positive side
of one water molecule
where the hydrogens are
would be attracted to the
partially negative side
of another water molecule.
And so they would be attracted,
and this is known as a hydrogen,
hydrogen, hydrogen bond.
And I could keep drawing that.
This is going to be
partially positive here.
This is going to be partially negative.
They will attract.
This oxygen end is going to be attracted
to that hydrogen end.
This oxygen end is going to be attracted
to the that hydrogen end as well.
And so it's this hydrogen bonding
that gives water a lot of the properties
that make it special
that, as far as we know,
for harboring life or for even
allowing life to be possible.
Life as we understand it
needs a fluid environment.
Things move around and
bump into each other.
And it's these hydrogen bonds,
when the temperature and
conditions are appropriate,
that allow water to be in that liquid form
where they're strong enough
so that the water stays together,
but they're weak enough so that they allow
the water molecules to
flow past each other.
And not only does it provide
a good fluid environment,
it's a very good solvent.
Water is often known as the universal,
universal solvent, but it's
worth putting a disclaimer here.
Even though people say it
is a universal solvent,
that does not mean that
it dissolves everything.
Water does dissolve more
things in its liquid state
than anything else we know about.
But there are many molecules
that it cannot dissolve well.
The things that it does dissolve well
are polar molecules or
things that have a charge.
For example, when sodium
chloride dissolves in water,
a sodium ion is positive,
so that is positively charged.
And so you can imagine
it might be attracted
to the side of the water molecules
where the oxygen is.
But it dissolves well.
But things that don't have charge
don't tend to dissolve well in water.
But even the property
that there's certain things
that it does not dissolve
is also good for life.
Later on in biology we're going to study
phospholipid bilayers where
you have these molecules
where one end is hydrophilic,
which means it's attracted
to water molecules.
And then the other ends are hydrophobic,
which means they're not
attracted to water molecules.
And many evolutionary biologists believe
that this property of having one side
that's hydrophilic and one
side that's hydrophobic
would have allowed these molecules
to start collecting into membranes,
eventually forming these
spherical membranes
which could be the containers
for early cellular life.
Now, another property of water
which makes it very suitable for life
is it's high heat capacity.
Sometimes you'll hear people say
it has a high specific heat.
The specific heat is the
amount of energy needed
to raise one gram of water
by one degree Celsius.
And you might say why
does that matter for life?
Well, many life forms can only operate
within a certain range of temperatures.
And so if it was really easy
to raise the temperature
of water really high or very
low temperatures very fast,
well, that would make it much harder
for life to operate within water,
or even life to be made up of water.
A related idea to this
is that water also has a
high heat of vaporization.
We talk more about this
in detail in other videos,
but this is talking about how much energy
does it take for water to
go from its liquid form
to its gas form.
And this has proven
valuable in many life forms
for a form of cooling
where the vaporization
of water, evaporative cooling,
can take heat away from an organism
so that it doesn't overheat.
Other properties that
are important about water
include cohesion and adhesion.
Cohesion is the property
of water molecules
that is attracted to
other water molecules.
And you saw it here
with the hydrogen bonds.
But then when you look at a macroscale,
you'll see things like
water droplets form.
You've all seen water
droplets, or dew droplets.
These droplets couldn't form
if not for the cohesion of water.
And even one drop can be an environment
in which thousands of
microorganisms can live.
Adhesion is the property of water
where it can adhere to other things.
You might have seen this
in a glass test tube
where it looks like the water is
kind of crawling up the top of the sides.
And that's because some of the polarity
of the glass molecules of the test tube.
But this property,
along with the cohesion,
is what allows water
to transport nutrients,
say, from the roots of a tree
all the way to the top of a tree.
These properties are also an action
in our own blood vessels, when you get
to the really small blood
vessels, the capillaries.
And that is called capillaries
'cause you have capillary action of water,
which is due to its
cohesion and its adhesion.
A last property of water,
and this is not an exhaustive list,
is that it is less dense as a solid.
So another way to think about it is
ice, which is solid water, is less,
less dense than liquid water.
Now, you might be thinking
why does that matter for life?
Well, imagine the environments
where we think life first arose.
If you imagine some type of a pond,
and this is the cross-section of it,
if ice was more dense than liquid water,
and for many substances that is the case,
a solid form tends to be more dense,
then what would happen?
If it's cold up here in the air,
say, in the winter, then
this part would freeze.
But then as it got more dense
it would sink to the
bottom right over there.
Then the next surface water would freeze
and sink to the bottom.
And then over time, the
entire lake or the entire pond
would freeze over, and
life would not be able
to live in that pond.
Because when water freezes,
it breaks membrane-bound
structures as we know it.
And so that would not
be suitable for life.
But because ice is less dense than water,
what typically happens is
just that top layer freezes.
And then it'll freeze down as things
get colder and colder.
But you have an entire environment
where life can continue to thrive
even when the air is much colder
than what is suitable for life.
And because of water's high specific heat,
that temperature variation in that water
is going to be much less
than the temperature variation
outside of the water,
either in the air or on the land.
So this is just an introduction,
but hopefully it makes you appreciate
water a little more.
And remember, and I've
said this in other videos,
we are mostly water.
One way to think about it is
that each of us is made
up of trillions of cells
which are primarily made up of water
and exist in a water-based environment.
They coordinate with each other
and eventually have emergent complexity
that thinks that it is a
sentient being like each of us.
