The Strange and Unexpected Reason Ice is Slippery

published on July 13, 2020

Thank you to Nord VPN for supporting PBS 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 are just now figuring out and one of those that were just now

Learning the secrets to is one of most obvious questions about ice why is ice slippery the most intuitive explanation for ice is 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 it 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 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 they're 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 in 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 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 millimeters 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've lower the melting point by even less basically for pressure melting to work you'd have to be an elephant and 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 cream 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 Isis 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 his feet was just one

Other molecule in the crystal and it's this lack of structure that allows the surface molecules to basically tumble around and 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 a little 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 and 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'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 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 it's frozen water but I think this

Demonstrates something really fundamental about science but 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 will 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

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