I was asked why ice is slippery and this simple question turns out to be rather more tricky than expected. The simple answer is that the slipperiness comes from a thin layer of liquid water molecules on the surface of the ice. Ice as a solid has a crystalline structure with very little movement possible between its molecules. The molecules in a liquid are still very close together but they are able to move freely passed and around one another. It is this movement of the molecules that makes the surface slippery – it can move at random beneath your feet.
The tricky part of the answer is to explain why there is liquid water on the surface of ice that might be several degrees below zero Celsius.
The first (and largely discredited) hypothesis is that the pressure exerted on the ice is sufficient to melt it at a local level meaning that you are actually standing on a thin layer of water. The problem with this is that the amount of pressure actually needed to melt ice seems rather greater than can be generated by a typical person standing on some ice.
The second hypothesis suggests that the frictional forces between your foot and the ice generate enough heat to melt the surface layer of molecules.
The third (and most recent) hypothesis is that ice always has a liquid phase of molecules on its surface but that it is so thin it has been overlooked in the past.
As I write this, I have not been able to find a recent and definitive answer to the question, but the third option certainly makes the most sense to me.
Questions…
- Suggest one phrase that could be used to describe the behaviour of particles in a solid.
- Why are (a) salt and (b) grit put on icy roads?
- How could you separate the salt from the grit?
- What two properties do you need to know to calculate pressure?
- What is the SI unit of pressure and how does it compare to N/cm2?
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