B1 Intermediate US 8398 Folder Collection
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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.
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【TED-Ed】Why does ice float in water? - George Zaidan and Charles Morton

8398 Folder Collection
Halu Hsieh published on July 6, 2014
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