Why are ice and snow slippery?

24 October 2011



Why are ice and snow slippery?


Chris - That's a very hard to answer question actually! In fact, it's the source of a lot of disinformation. If you have a look in many, many texts and books and things, especially historically, you will find they claim that when you have an ice skate, for example, on an ice rink, the pressure of the ice skate pushing down on the ice causes the ice to increase its temperature because it's being compressed and it melts a bit, and this puts a layer of water on the surface of the ice and that means the ice skate then slides along. This doesn't stand up to scrutiny though because people wearing shoes - where the same weight is distributed over a much bigger area therefore with much lower pressure - still slipover on the ice! In fact, if you plug the numbers into the calculations, you'll find it actually only changes the temperature of the ice by a fraction of a degree. So this cannot account for why ice is slippery. It can't be a melting phenomenon. What scientists actually think is going on is that probably because you have an ice surface and there'll be water molecules on the surface which are not tethered to other water molecules, they exist more as a liquid than as a solid. So, all ice is covered in a very thin, at a molecular level, layer of water. Michael Faraday showed this quite interestingly because he got two ice cubes which are both slippery, put them together and they stuck. Now, if there wasn't water there, then wouldn't have stuck together because obviously, then the water then froze onto both. So what scientists think is going on is that there is a very thin layer of water on the surface of any ice surface and it's that water that access the lubricant and reduces the level of friction at the surface. So, I think that's probably the best explanation I can come up with. Dominic - Does that depend on the temperature of the ice? Chris - Well, to a certain extent it will, but at the same time if you've got atoms which are molecules of water which are sitting on the surface of the ice which are untethered and unbonded to other surrounding molecules then it's easier for them to have energy and detach and form a water film than it is for molecules elsewhere in the ice to move around. So, it favours the formation of a layer of water at that point which could make them slippery.


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