Why can you skate on ice and not on other substances?

(Lansing State Journal, Nov. 20, 1991)

Ice skating involves contact between two surfaces - the metal blades of the ice skate and the ice - and the concept of friction. If a person is to glide over a frozen pond, there must be a minimal amount of friction between the skate blades and the ice. Friction is one of the least understood physical concepts and a lot of study has gone into answering the question of why the friction between skate blades and ice can be so small. Here's how most scientists explain ice skating: If you were to look very closely, you would see that the skate blade never touches the ice directly. A thin layer of water is formed between the two surfaces, so that the blades are actually gliding on the film of water. A skater exerts a pressure equal to her weight divided by the area of the skate blade on the ice. Since the area of each blade is very small, the pressure on the ice is very large. The large pressure melts a small amount of ice directly beneath each blade, forming a thin layer of water. Skate blades must be kept sharp, or the skater's weight will be distributed over too large an area, and the skater will be unable to exert enough pressure to melt any ice.

Skating is possible only on ice - and not on other substances - because of the unique properties of water. The density (mass divided by volume) of water depends on both the pressure and the temperature. Under standard atmospheric conditions, water is at its densest at 4 degrees Celsius (41 degrees Fahrenheit ). Ice, which forms at 0 degrees Celsius (32 degrees Fahrenheit ) is less dense than water. This is why a can of soda (which is mostly water) left in the freezer for too long will explode. The frozen soda has the same mass as the unfrozen soda, but frozen soda is less dense and must therefore occupy a greater volume. Most liquids contract when they freeze; water doesn't.

At a given temperature, increasing the pressure on ice causes it to melt. This is another unique property of water; most liquids solidify when place under higher pressure. The colder it becomes, the more pressure you need to exert on the ice in order to produce the film of water necessary for skating. This means that it can become too cold for skating. For ice skating to be possible, the temperature must be low enough for ice to form, but high enough so that the pressure needed to create a thin layer of water can be easily exerted by a person in ice skates.

Melting by the application of pressure is a property unique to ice which allows a skater to minimize the friction between the blades of the skate and the ice.

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