“Figure-skating blades are very narrow, so a skater’s weight applies great pressure to the ice beneath him. The pressure causes some of the ice under the skate to liquefy because ice’s melting point depends on its pressure. The metal blade glides with virtually no friction on this thin layer of water.”
The above narrative, or something close to it, is what you are likely to receive upon asking a random middle-school teacher to explain why figure skaters glide so easily, traversing the entire length of a rink with little effort. It also has practically no connection to reality.
One can hardly blame the teacher. Given the breadth of modern science curriculum, educators are at the mercy of their textbooks, and textbooks have parroted the aforementioned science myth for generations because it provides a real life context for the claim that pressure affects the melting point of a solid. The principle is valid-pressure does lower the melting point of ice-but it has no application to figure skating.
The textbook account demonstrates poor logic on its face. A skate’s blade can only apply pressure to ice immediately beneath it. Even if one were to accept that the weight of a standing figure skater could cause the ice below him to melt, it would hardly explain how a figure skater can smoothly glide forward onto ice his weight had not affected.
The problems with the standard explanation go beyond such big-picture concerns. Pressure cannot claim responsibility for the melting of ice under a figure skater’s blade; basic math disqualifies it. The ice in a typical rink is kept between 4Ã’Ò°F and 7Ã’Ò°F below freezing but the pressure exerted by even a husky ice skater (as calculated by the Clausius-Clapeyron equation) would not reduce the freezing point of ice by even 0.3Ã’Ò°F. An ice-skating hippopotamus might be able to melt ice based solely on its weight, but not a human.
A further problem with the textbook account is that ice skaters can glide on ice too cold to be melted by any amount of pressure. Even an elephant balanced on a thumbtack cannot lower the melting point of ice below -7.6Ã’Ò°F, yet ice remains slick far below that temperature.
While the slipperiness of ice is a matter of ongoing research, its basic cause is simple enough. Ice, whether under pressure from a skater’s blade or not, always has a thin sheath of water on its surface. Though we are taught the “melting point” of ice is 32Ã’Ò°F, textbooks misrepresent what the “melting point” is.
Practically all science textbooks say that the “melting point” is when a solid turns into a liquid, and a surprising number even say that the “boiling point” is when a liquid turns into a vapor. Of course, this latter definition is quite obviously wrong. Water will evaporate (going from a liquid to a vapor) at room temperature. What fewer people realize is that analogous definition for “melting point” is also wrong. The surface layers of a solid will liquefy at temperatures far below its melting point, a phenomenon analogous to evaporation that scientists call “pre-melting.”
The depth of this slippery film depends on the ice’s temperature. A standard ice rink may be covered with dozens of layers of water molecules; ice at -250Ã’Ò°F may just have one layer of liquid on its surface.
This film makes ice, all ice, slippery, but figure skaters are aided by an additional factor. A small amount of friction exists even for a blade gliding across an ice rink. The energy lost to this friction tends to heat the nearby ice, which raises its temperature and can substantially increase the depth of the liquid film nearby. Someone walking on ice might tread on water dozens of molecular layers deep, but the liquid cushion allowing a figure skater to glide by may comprise hundreds or thousands of such layers.
Thus we find that the ease with which skaters slide across ice can be attributed to a ubiquitous sheath of water covering all ice enhanced by additional melting caused by the skate’s own friction. The notion that pressure lowers ice’s melting point enough to liquefy it is a convenient-but false-application textbooks employ to keep their readers’ interest. Moreover, the whole mystery of why ice is slippery below its freezing point is an artifact of textbooks’ misrepresentation of melting. There would be little mystery at all if students were told of pre-melting when they studied evaporation, its analog.