Solid ice itself isn’t slippery. There’s a thin film of liquid water on its surface that the skaters are sliding on.
Solids in general aren’t slippery because their surface molecules are tied tightly together and can’t roll around like ball bearings. The molecules of liquids, on the other hand, are free to move around, so liquids are generally slipperier than solids. A little water on a tile or concrete floor can turn it into an accident-lawyer’s dream.
But what scientists can’t quite agree upon is exactly what creates the liquid film on the surface of ice. Obviously, must come from slight melting, but what makes the ice melt?
Two explanations, pressure melting and friction melting, have been slugging it out during the more-than-a-hundred years that people have been trying to explain this simple everyday phenomenon.
The pressure-melting camp maintains that it’s the pressure of the ice-skate blade on the ice (or the ski on the snow, for that matter) that does the melting. There’s no doubt that ice will melt if you apply pressure to it because solid ice occupies a bigger volume than liquid water does, and if you press hard enough on a piece of ice, you can force it to collapse down into its smaller-volume form: liquid water.
The weight of an ice skater, concentrated on a tiny area size of a skate blade, can amount to a pressure of thousands of pounds per square inch. The problem, though, is even this intense pressure isn’t enough to do the necessary amount of high-speed melting, especially when the ice is quite cold, because that’s when its molecules are most tightly set in their rigid ways.
But wait a minute. Rubbing any two solids together; even a skate blade and a piece of ice, is bound to create friction, and friction creates heat. According to the friction-melting camp, this frictional heat is enough to melt a continuous liquid streak as the skate blade or ski skims along the ice or snow.
The best evidence today seems to favor friction melting assisted by some pressure melting at temperatures that are not too far below the freezing point.
Using a towel to avoid melting it pick up an ice cube from your freezer compartment or take out a whole tray of ice. Gently feel the ice by passing a finger across its surface. Don’t rub too hard. You’ll find that the ice isn’t slippery at all until your body heat and friction heat have had a chance to warm up the surface and melt it a little.
Clean ice is not slippery.
Don’t actually try this test in a bar, however. The bartender’s ice probably isn’t cold enough; it will be wet and slippery from the outset.