Why does ice float in water? - George Zaidan and Charles Morton

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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.

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.