Hello there.

Here at Crash Course HQ we like to start out each day with a nice healthy dose of water in all its three forms. It is 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.

[Intro Music]

Ahhh, much better.

When we left off 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, you know, 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 and so many other people geeked out so hard last December when Mars's seven-year-old rover, Opportunity, found a 20-inch long vein of gypsum that was almost certainly deposited by, like, 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? Why 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, and we talked about them last time. But 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, 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 even, 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 adhesion 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, so 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 we call, it's 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. And 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. So 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. 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. So water, we may call it the universal solvent, but that does not mean that it dissolves everything.

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There have 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 air," to form water. Cavendish didn't totally understand what he discovered here, 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 not only to 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, like, Cavendish's 8th Law and Cavendish's 4th Law. So I, for one, am glad that he didn't actually get credit.

We're gonna 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 its solid form floats, is one of the reasons why life, on this planet, as we know it, is possible. And 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° F, or 0° C 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 its 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 then 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? [ding] 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's so much nicer in Los Angeles, where the ocean is constantly keeping the temperatures the same, than it is in, say, 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 bit of water in it and it takes forever to freaking boil. Oh, and if you hadn't already noticed this, when water evaporates from your skin it cools you down. Now, that's the principle 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 where 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 that 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 gonna 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.