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  • 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.

Hello there.

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B1 中級

水 - 液体オーサム。クラッシュコース生物学 #2 (Water - Liquid Awesome: Crash Course Biology #2)

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    Chi-feng Liu に公開 2021 年 01 月 14 日
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