Hello there. Here at Crash Course HQ we like to start out
each day with a nice healthy dose of water in all it's three forms. It's the only substance on all of our planet
earth that occurs naturally in solid, liquid, and gas forms. And to celebrate this magical bond between
two hydrogen atoms and one oxygen atom here, today, we are going to be celebrating
the wonderful life sustaining properties of water. But we're going to do it slightly more clothed. Much better. When we left of here at the Biology Crash
Course, we were talking about life and the rather important fact that all life
as we know it is dependent upon there being water around. Scientists and astronomers are always looking
out into the universe trying to figure out whether there is life elsewhere because that is kind of the most important
question that we have right now. They're always getting really excited when
they find water someplace particularly liquid water. And this is one reason why I am so many other
people geeked out so hard last December when Mars' seven-year-old rover, Opportunity, found
a 20-inch long vein of gypsum that was almost certainly deposited by long-term liquid water
on the surface of mars. And this was probably billions of years ago,
and so it's going to be hard to tell whether or not the water that was there resulted
in some life. But maybe we CAN figure that out and that
would be REALLY exciting! But why?! Wy do we think that water is necessary
for life? Why does water on other planets get us so
freaking excited? So let's start out by investigating some
of the amazing properties of water. In order to do that we're going to have to
start out with THIS The world's most popular molecule -- or
at least the world's most memorized molecule. We all know about it. Good old H2O. Two hydrogens, one oxygen. The hydrogens each
sharing an electron with oxygen in what we call a covalent bond. So as you can see, I've drawn my water molecule
in a particular way and this is actually the way that it appears. It is V-shaped. Because this big old oxygen
atom is a little bit more greedy for electrons it has a slight negative charge whereas this
area here with the hydrogen atoms has a slight positive charge. Thanks to this polarity, all water molecules
are attracted to one another -- so much so they actually stick together, and these are
called hydrogen bonds. We talked about them last time. Essentially what happens is that the positive
pole around those hydrogen atoms bonds to the negative pole around the oxygen atoms
of a DIFFERENT water molecule. And so it's a weak bond. But look, they're bonding! Seriously though, I cannot overstate the importance
of this hydrogen bond. So when your teacher asks you, "What's important
about water?" Start out with the hydrogen bonds and you
should put it in all caps and maybe some sparkles around it. One of the cool properties that results from
these hydrogen bonds is a high cohesion for water, which results in high surface tension.
Cohesion is the attraction between two like things, like attraction between one molecule
of water and another molecule of water. Water has the highest cohesion of any non-metallic
liquid, and you can see this if you put some water on some wax paper or some Teflon or
something where the water beads up. Some leaves of plants do it really well. It's
quite cool. Since water adheres weakly to the wax paper
or to the plant, but strongly to itself, the water molecules are holding those droplets
together in a configuration that creates the least amount of surface area. It's this high surface tension that allows
some bugs and I think one lizard and also one Jesus to be able to walk on water. The cohesive force of water does have its
limits of course. There are other substances that water quite likes to stick to. Take glass
for example. This is called adhesions and the water is
spreading out instead of beading up because the adhesive forces between the water and
the glass are stronger than the cohesive forces of the individual water molecules in the bead
of water. Adhesion is attraction between two different
substances. In this case the water molecules and the glass molecules. These properties lead to one of my favorite
things about water: the fact that it can defy gravity. That really cool thing that just happened
is called capillary action, and explaining it can be easily done with what we now know
about cohesion and adhesion. Thanks to adhesion, the water molecules are
attracted to the molecules in the straw. But as the water molecules adhere to the straw,
other molecules are drawn in by cohesion, following those fellow water molecules. Thank
you, cohesion! The surface tension created here causes the water to climb up the straw.
And it will continue to climb until eventually gravity pulling down on the weight of the
water in the straw overpowers the surface tension. The fact that water is a polar molecule also
makes it really good at dissolving things, which makes it a good solvent. Scratch that. Water isn't a GOOD solvent,
it's an AMAZING solvent! There are more substances that can be dissolved
in water than in any other liquid on Earth. Yes, that includes the strongest acid that
we have ever created. These substances that dissolve in water -- sugar or salt being
ones that we're familiar with -- are called hydrophilic, and they are hydrophilic because
they are polar, and their polarity is stronger than the cohesive forces of the water. When you get one of these polar substances
in water, it's strong enough that it breaks all the little cohesive forces, all those
little hydrogen bonds, and instead of hydrogen bonding to each other the water will hydrogen
bond around these polar substances. Table salt is ionic, and right now it's
being separated into ions as the poles of our water molecules interact with it. But what happens when there is a molecule
that cannot break the cohesive forces of water? It can't penetrate, and come into it. [Seriously...?] Basically, what happens when that substance
can't overcome the strong cohesive forces of water? Can't get inside of the water? That's when we get what we call a hydrophobic
substance, or something that is fearful of water. These molecules lack charged poles,
they are non-polar and are not dissolving in water because essentially they're being
pushed out of the water by water's cohesive forces. Water: we may call it the universal solvent,
but that does not mean that it dissolves everything. There've been a lot of eccentric scientists
throughout history, but all this talk about water got me thinking about perhaps the most
eccentric of the eccentrics -- a man named Henry Cavendish. He communicated with his female servants only
via notes and added a staircase to the back of his house to avoid contact with his housekeeper.
Some believe he may have suffered from a form of autism, but just about everyone will admit
that he was a scientific genius. He's best remembered as the first person
to recognize hydrogen gas as a distinct substance and to determine the composition of water. In the 1700s most people thought that water
itself was an element, but Cavendish observed that hydrogen -- which he called inflammable
air, reacted with oxygen -- known then by the awesome name "dephlogisticated aire"
-- to form water. Cavendish didn't totally understand what
he discovered, in part because he didn't believe in chemical compounds and explained
his experiments with hydrogen in terms of a fire-like element called "phlogiston." Nevertheless, his experiments were groundbreaking,
like his work in determining the specific gravity -- basically the comparative density
-- of hydrogen and other gases with reference to common air. It's especially impressive
when you consider the crude instruments he was working with. This, for example, is what
he made his hydrogen gas with. He went on to not only establish an accurate
composition of the atmosphere, but also discovered the density of the earth. Not bad for a guy
who was so painfully shy that the only existing portrait of him was sketched without his knowledge. But for all his decades of experiments, Cavendish
only published about 20 papers. In the years after his death, researchers figured out that
Cavendish had actually pre-discovered Richter's Law, Ohm's Law, Coulomb's Law, several
other laws... that's a lot of freaking laws! And if he had gotten credit for them all we
would have had to deal with Cavendish's 8th Law and Cavendish's 4th Law. So I, for one, am glad that he didn't actually
get credit. We're going to do some pretty amazing science
right now. You guys are not going to believe this. Ok, you ready? It floats! Yeah, I know you're not surprised by this,
but you should be, because everything else, when it's solid, is much more dense than when
it's liquid, just like gases are much less dense than liquids are. But that simple characteristic of water: that
it's solid form floats, is one of the reasons why life on this planet, as we know it, is
possible. Why is it that solid water is less dense than
liquid water while everything else is the exact opposite of that? Well, you can thank your hydrogen bonds once
again. So at around 32 degrees Fahrenheit, or 0 degrees
Celsius if you're a scientist or from a part of the world where things make sense water molecules start to solidify and the
hydrogen bonds in those water molecules form crystalline structures that space molecules
apart more evenly, in turn making frozen water less dense than the liquid form. So in almost every circumstance, floating
water ice is a really good thing. If ice were denser than water it would freeze and then
sink, and then freeze and then sink, and then freeze and then sink. So just trust me on this one, you don't
want to live on a world where ice sinks. Not only would it totally wreak havoc on aquatic
ecosystems, which are basically how life formed on the Earth in the first place, but also
all the ice at the North Pole would sink and all of the water everywhere else would rise
and we wouldn't have any land. That would be annoying. There's one more amazing property of water
that I'm forgetting... Why is it so hot in here? Ah! Heat capacity! Yes, water has a very high heat capacity,and
probably that means nothing to you, but basically it means that water is really good at holding
on to heat. Which is why we like to put hot water bottles
in our bed and cuddle with them when we're lonely. But aside from artificially warming your bed
it's also very important that it's hard to heat up and cool down the oceans significantly. They become giant heat sinks that regulate
the temperature and the climate of our planet. Which is why, for example, it is so much nicer
in Los Angeles, where the ocean is constantly keeping the temperatures the same, than it
is in Nebraska. On a smaller scale we can see water's high
heat capacity really easily and visually by putting a pot with no water in it on a stove
and seeing how badly that goes. But then you put a little water in it and
it takes forever to boil. Oh, and if you hadn't already noticed this,
when water evaporates from your skin it cools you down. That's the principal behind sweating, which
is an extremely effective though somewhat embarrassing part of life. But this is example of another incredibly
cool thing about water. When my body gets hot and it sweats, that
heat excites some of the water molecules on my skin to the point that they break those
hydrogen bonds and they evaporate away. And when they escape, they take that heat
energy with them, leaving me cooler. Lovely. This wasn't exercise though. I don't know
why I'm sweating so much. It could be the spray bottle that I keep spraying myself with
or maybe it's just because this is such a high stress enterprise: trying to teach you
people things. I think I need some water, but while I'm drinking,
there's a review for all of the things we talked about today. If there are a couple
things you're not quite sure about just go back and watch them. It's not going to take
a lot of your time. And you're going to be smarter, I promise. You're going to do SO well on that test you
either don't or do have coming up. Ok, bye.
