The Strange and Unexpected Reason Ice is Slippery


Thank you to Nord VPN for supporting PBS

[chatter in background and ice skates scraping]


Hey smart people Joe here. I'm willing to bet you're familiar with this stuff -- ice, good old frozen H2O

If you ever really stop to think about it

I mean sure we put it in our drinks, we slip on it, we slide on it, we skate on it...

And we're worried climate change is gonna melt every last bit of it

But even though ice is such a common and ordinary substance

It still has a few weird properties and mysteries that we're just now figuring out

And one of those that we're just now learning the secrets to

is one of the most obvious questions about ice -- why is ice slippery?


The most intuitive explanation for ice's slipperiness is that there must be a thin layer of liquid water on the surface of the ice

This is an idea that goes back to the 1840s when the physicist Michael Faraday did this

He took two ice cubes -- when they were placed in contact for just a few seconds

They became one ice cube.

Now Faraday thought that there must be a layer of liquid water between them that froze

But I don't see a layer of liquid water, but maybe something has to create the layer of liquid water...

This is what's called a phase diagram, and this one is for water

for a particular temperature and pressure it tells you which phase -- solid, liquid or gas

the substance will be in. Looking at the diagram

We see water at minus five degrees Celsius and 1 atmosphere of pressure

... like your typical ice rink will be solid ice. This phase diagram also shows us something else

There's more than one way to melt ice

You can raise the temperature

We're all familiar with that way of melting. But water is special because you can also melt it by increasing the pressure

And for most substances their solid form is more dense than in the liquid form -- think of candle wax

so putting the solid form under more pressure only makes it more solid.

But water ice is different and weird

Its liquid form is more dense than the solid form

and that's why ice floats

So if we take solid ice and squeeze it under a lot of pressure -- when we force it to be more dense

It can become a liquid again. Even at much colder temperatures where we'd usually expect it to be a solid

[ice skates scrape]

So the idea is this:

when you step onto the ice, you exert a pressure on the area under your feet that lowers the melting point of the ice and

creates a thin layer of liquid

-- slippery water. Now pressure does lower the melting point of ice. For a long time people thought that this "pressure melting"

was the reason ice was slippery -- a person's weight, concentrated on the area under their feet, melted the top layer of the ice

But something doesn't add up here

I weigh about 88 kilograms and the area under these skates is like

900 square millimetres

-- you plug that into the appropriate equations and it turns out it only lowers the melting point of ice by a

fraction of one degree Celsius and if you're wearing regular shoes

you'd lower the melting point by even less. Basically for pressure melting to work, you'd have to be an elephant in stiletto heels

Look at that phase diagram again. To get the kind of pressure on ice skates needed to lower the melting point of water by any

significant amount, I need to weigh over 3,000 kilograms

That's only half as heavy as most elephants.

There's another answer that feels closer to the truth -- friction

My ice skates are gliding across the ice

They create friction. That friction creates heat and that can create a thin layer of liquid water on top of the ice

Right, we know this does happen -- ice skates gliding across ice can create friction that can melt the ice beneath them

But it also can't be the whole story

Because anyone who's tried skating for the first time knows you don't have to move very much for your feet to slip out from under you

and it also doesn't explain why you don't have to move for ice to be slippery. So does friction explain it?

Well, maybe in part, but it doesn't explain the whole story of why ice is slippery

There's a fundamental issue here that we haven't talked about

Imagine a layer of water on some hard surface like a puddle on a smooth floor

It's pretty obvious that ice is actually way more slippery than just water

There must be something else going on. Several theoretical and experimental

In the last few years have found the real cause of ice's slipperiness -- that thin layer of liquid water on the surface>

It isn't really liquid water at all. But it's also not solid ice

It's something very different and very strange. Okay. Do you remember the weirdest property of solid water?

It floats

Water solid form is less dense than its liquid form. I mean think about it. That is weird solid rocks

Don't float on lava

But now let's think about this on the molecular level -- water is a polar molecule

The slightly positive hydrogen's are attracted to the slightly negative oxygens

It's known as hydrogen bonding -- a single molecule in liquid water will only be hydrogen bonded to a few other water molecules

But a single water molecule in ice will have more molecules hydrogen bonded to it than in liquid water

This makes ice a crystalline solid with water molecules bonded together basically trapped in a very consistent repeated pattern

But what about a molecule on the surface of the ice?

Those may be bonded to as few as just one other

Molecule in the crystal and it's this lack of structure that allows the surface molecules to basically tumble around in disorder

Detaching and then reattaching themselves to the surface and to each other

Think about it like this:

These water molecules down here represent those deeper in the ice

They're held in place pretty rigidly by all the hydrogen bonds surrounding them. But up here on the surface

These water molecules don't have as many hydrogen bonds holding them in place and they're able to sort of roll around

This creates something on the surface that isn't really a liquid because water's phase diagram says that technically speaking

Water is solid under these conditions, but it's not totally a solid either -- at least not the way that we normally think of solids

Scientists call it a quasi-liquid layer

one researcher who studied its properties compared it to

Marbles scattered across a dance floor. You just take one step and the marbles roll out from under you

Scientists still have a lot of questions about the properties of this quasi-liquid layer

Since the border between it and ice is really hard to see

We're not even sure exactly how thick it is

All that it's somewhere between the size of a single bacterium and a thousand times smaller than that. We're talking like a few molecules

So ice is slippery because of a non-solid quasi-liquid layer of water on the surface. It's not caused by pressure

It's not totally caused by friction

It's caused by the H2O molecules coming loose on the molecular level, and those H2O molecules

They don't act like liquid water. They act like something much weirder.

[ice skates scrape]

I know it's strange to say here in the 21st century that we don't know everything that there is to know about something as ordinary

as frozen water, but I think this demonstrates something really fundamental about science

-- that the more resolution they're able to gaze into the universe whether it's on the biggest or the smallest little molecular scales

Well, we'll always keep uncovering new discoveries and we'll always keep updating the things that we knew yesterday

Even if it's in the places that we've been looking all along

Stay curious.

We'd like to thank Nord VPN for supporting PBS

Imagine a VPN as a secure encrypted tunnel for online traffic to flow

No-one can see through the tunnel and get their hands on all your internet data

[cat screeches in background]

Nord VPN allows you to use public Wi-Fi in coffee shops or hotels or anywhere else that you're on someone else's network

That way you can access personal and work accounts on the road or just simply keep your browsing history to yourself

They don't log your activity while you're online. They've got thousands of servers in 60 countries and

24/7 customer support

To learn more about Nord VPN, just check out the link in the description and a special thanks to these galaxy brain patrons

[vocalising] ah-ah-aaah...

[vocalising] ...ah!

Hey, how's it going cool cats?

[ice skates scrape, fumbling noises]

This intro is gonna kill me

[ice skates scrape, fumbling noises]

[tragic music]

[indistinct] Did you get it?