Why does ice float in water? - George Zaidan and Charles Morton
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0:07 - 0:09Water is the liquid of life.
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0:09 - 0:09We drink it,
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0:09 - 0:10we bathe in it,
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0:10 - 0:11we farm,
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0:11 - 0:11cook,
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0:11 - 0:12and clean with it.
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0:12 - 0:15It's the most abundant molecule in our bodies.
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0:15 - 0:17In fact, every life form we know of
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0:17 - 0:18would die without it.
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0:18 - 0:20But most importantly, without water,
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0:20 - 0:22we wouldn't have
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0:22 - 0:23iced tea.
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0:23 - 0:26Mmmm, iced tea.
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0:28 - 0:30Why do these ice cubes float?
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0:30 - 0:32If these were cubes of solid argon
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0:32 - 0:34in a cup of liquid argon,
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0:34 - 0:35they would sink.
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0:35 - 0:38And the same goes for most other substances.
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0:38 - 0:40But solid water, a.k.a. ice,
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0:40 - 0:43is somehow less dense than liquid water.
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0:43 - 0:45How's that possible?
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0:45 - 0:47You already know that every water molecule
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0:47 - 0:49is made up of two hydrogen atoms
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0:49 - 0:51bonded to one oxygen atom.
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0:51 - 0:53Let's look at a few of the molecules
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0:53 - 0:54in a drop of water,
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0:54 - 0:58and let's say the temperature is 25 degrees Celcius.
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0:58 - 0:59The molecules are bending,
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0:59 - 1:00stretching,
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1:00 - 1:01spinning,
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1:01 - 1:03and moving through space.
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1:03 - 1:05Now, let's lower the temperature,
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1:05 - 1:07which will reduce the amount of kinetic energy
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1:07 - 1:09each of these molecules has
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1:09 - 1:12so they'll bend, stretch, spin, and move less.
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1:12 - 1:13And that means that on average,
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1:13 - 1:15they'll take up less space.
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1:15 - 1:17Now, you'd think that as the liquid water
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1:17 - 1:19starts to freeze,
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1:19 - 1:20the molecules would just pack together
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1:20 - 1:22more and more closely,
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1:22 - 1:24but that's not what happens.
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1:24 - 1:25Water has a special kind
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1:25 - 1:27of interaction between molecules
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1:27 - 1:29that most other substances don't have,
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1:29 - 1:31and it's called a hydrogen bond.
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1:31 - 1:33Now, remember that in a covalent bond
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1:33 - 1:35two electrons are shared,
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1:35 - 1:36usually unequally,
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1:36 - 1:38between atoms.
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1:38 - 1:39In a hydrogen bond,
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1:39 - 1:42a hydrogen atom is shared, also unequally,
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1:42 - 1:43between atoms.
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1:43 - 1:46One hydrogen bond looks like this.
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1:46 - 1:48Two look like this.
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1:48 - 1:49Here's three
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1:49 - 1:50and four
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1:50 - 1:51and five,
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1:51 - 1:51six,
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1:51 - 1:52seven,
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1:52 - 1:52eight,
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1:52 - 1:53nine,
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1:53 - 1:53ten,
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1:53 - 1:54eleven,
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1:54 - 1:54twelve,
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1:54 - 1:56I could go on.
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1:56 - 1:58In a single drop of water,
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1:58 - 2:00hydrogen bonds form extended networks
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2:00 - 2:02between hundreds, thousands, millions,
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2:02 - 2:04billions, trillions of molecules,
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2:04 - 2:08and these bonds are constantly breaking and reforming.
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2:08 - 2:10Now, back to our water as it cools down.
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2:10 - 2:12Above 4 degrees Celcius,
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2:12 - 2:14the kinetic energy of the water molecules
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2:14 - 2:17keeps their interactions with each other short.
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2:17 - 2:18Hydrogen bonds form and break
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2:18 - 2:20like high school relationships,
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2:20 - 2:23that is to say, quickly.
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2:23 - 2:24But below 4 degrees,
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2:24 - 2:26the kinetic energy of the water molecules
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2:26 - 2:28starts to fall below the energy
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2:28 - 2:30of the hydrogen bonds.
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2:30 - 2:32So, hydrogen bonds form much more frequently
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2:32 - 2:34than they break
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2:34 - 2:35and beautiful structures start to emerge
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2:35 - 2:37from the chaos.
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2:37 - 2:39This is what solid water, ice,
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2:39 - 2:42looks like on the molecular level.
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2:42 - 2:44Notice that the ordered, hexagonal structure
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2:44 - 2:47is less dense than the disordered structure
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2:47 - 2:49of liquid water.
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2:49 - 2:51And you know that if an object is less dense
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2:51 - 2:52than the fluid it's in,
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2:52 - 2:54it will float.
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2:54 - 2:56So, ice floats on water,
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2:56 - 2:57so what?
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2:57 - 3:00Well, let's consider a world without floating ice.
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3:00 - 3:01The coldest part of the ocean
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3:01 - 3:03would be the pitch-black ocean floor,
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3:03 - 3:05once frozen, always frozen.
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3:05 - 3:07Forget lobster rolls
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3:07 - 3:08since crustaceans would lose their habitats,
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3:08 - 3:11or sushi since kelp forests wouldn't grow.
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3:11 - 3:13What would Canadian kids do in winter
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3:13 - 3:15without pond hockey or ice fishing?
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3:15 - 3:17And forget James Cameron's Oscar
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3:17 - 3:20because the Titanic totally would have made it.
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3:20 - 3:22Say goodbye to the white polar ice caps
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3:22 - 3:23reflecting sunlight
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3:23 - 3:25that would otherwise bake the planet.
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3:25 - 3:27In fact, forget the oceans as we know them,
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3:27 - 3:30which at over 70% of the Earth's surface area,
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3:30 - 3:32regulate the atmosphere of the whole planet.
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3:33 - 3:34But worst of all,
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3:34 - 3:37there would be no iced tea.
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3:37 - 3:40Mmmmm, iced tea.
- Title:
- Why does ice float in water? - George Zaidan and Charles Morton
- Description:
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View full lesson: http://ed.ted.com/lessons/why-does-ice-float-in-water-george-zaidan-and-charles-morton
Water is a special substance for several reasons, and you may have noticed an important one right in your cold drink: ice. Solid ice floats in liquid water, which isn't true for most substances. But why? George Zaidan and Charles Morton explain the science behind how how hydrogen bonds keep the ice in your glass (and the polar ice caps) afloat.
Lesson by George Zaidan and Charles Morton, animation by Powerhouse Animation Studios Inc.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 03:56
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