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  • Water is the liquid of life.

  • We drink it,

  • we bathe in it,

  • we farm,

  • cook,

  • and clean with it.

  • It's the most abundant molecule in our bodies.

  • In fact, every life form we know of

  • would die without it.

  • But most importantly, without water,

  • we wouldn't have

  • iced tea.

  • Mmmm, iced tea.

  • Why do these ice cubes float?

  • If these were cubes of solid argon

  • in a cup of liquid argon,

  • they would sink.

  • And the same goes for most other substances.

  • But solid water, a.k.a. ice,

  • is somehow less dense than liquid water.

  • How's that possible?

  • You already know that every water molecule

  • is made up of two hydrogen atoms

  • bonded to one oxygen atom.

  • Let's look at a few of the molecules

  • in a drop of water,

  • and let's say the temperature is 25 degrees Celcius.

  • The molecules are bending,

  • stretching,

  • spinning,

  • and moving through space.

  • Now, let's lower the temperature,

  • which will reduce the amount of kinetic energy

  • each of these molecules has

  • so they'll bend, stretch, spin, and move less.

  • And that means that on average,

  • they'll take up less space.

  • Now, you'd think that as the liquid water

  • starts to freeze,

  • the molecules would just pack together

  • more and more closely,

  • but that's not what happens.

  • Water has a special kind

  • of interaction between molecules

  • that most other substances don't have,

  • and it's called a hydrogen bond.

  • Now, remember that in a covalent bond

  • two electrons are shared,

  • usually unequally,

  • between atoms.

  • In a hydrogen bond,

  • a hydrogen atom is shared, also unequally,

  • between atoms.

  • One hydrogen bond looks like this.

  • Two look like this.

  • Here's three

  • and four

  • and five,

  • six,

  • seven,

  • eight,

  • nine,

  • ten,

  • eleven,

  • twelve,

  • I could go on.

  • In a single drop of water,

  • hydrogen bonds form extended networks

  • between hundreds, thousands, millions,

  • billions, trillions of molecules,

  • and these bonds are constantly breaking and reforming.

  • Now, back to our water as it cools down.

  • Above 4 degrees Celcius,

  • the kinetic energy of the water molecules

  • keeps their interactions with each other short.

  • Hydrogen bonds form and break

  • like high school relationships,

  • that is to say, quickly.

  • But below 4 degrees,

  • the kinetic energy of the water molecules

  • starts to fall below the energy

  • of the hydrogen bonds.

  • So, hydrogen bonds form much more frequently

  • than they break

  • and beautiful structures start to emerge

  • from the chaos.

  • This is what solid water, ice,

  • looks like on the molecular level.

  • Notice that the ordered, hexagonal structure

  • is less dense than the disordered structure

  • of liquid water.

  • And you know that if an object is less dense

  • than the fluid it's in,

  • it will float.

  • So, ice floats on water,

  • so what?

  • Well, let's consider a world without floating ice.

  • The coldest part of the ocean

  • would be the pitch-black ocean floor,

  • once frozen, always frozen.

  • Forget lobster rolls

  • since crustaceans would lose their habitats,

  • or sushi since kelp forests wouldn't grow.

  • What would Canadian kids do in winter

  • without pond hockey or ice fishing?

  • And forget James Cameron's Oscar

  • because the Titanic totally would have made it.

  • Say goodbye to the white polar ice caps

  • reflecting sunlight

  • that would otherwise bake the planet.

  • In fact, forget the oceans as we know them,

  • which at over 70% of the Earth's surface area,

  • regulate the atmosphere of the whole planet.

  • But worst of all,

  • there would be no iced tea.

  • Mmmmm, iced tea.

Water is the liquid of life.

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B1 US TED-Ed hydrogen water iced tea iced liquid

【TED-Ed】Why does ice float in water? - George Zaidan and Charles Morton

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    Halu Hsieh posted on 2014/07/05
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