Why do we slide on ice?



Have you ever wondered why we are able to slide on ice? For example why can we sky or skate? Why do we slide on ice and not on other smooth surfaces?

Well, the reason is quite simple and it is completely contained in the above image representing the so called water’s phase diagram.

icewaterPhase diagrams show the preferred physical states of matter at different Temperatures (abscissa – °C) and Pressure (ordinate – bar). Within each phase, the material is uniform with respect to its chemical composition and physical state. At typical temperatures and pressures on Earth water is a liquid, but it becomes solid (ice) if its temperature is lowered below 0°C and gaseous ( water vapor) if its temperature is raised above 100°C, at the same pressure. Each line (phase line) on a phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist in any relative proportions. Here, a slight change in temperature or pressure may cause the phases to abruptly change from one physical state to the other. Where three phase lines join, there is a ‘triple point’, when three phases stably coexist, but may abruptly and totally change into each other given a slight change in temperature or pressure. Under the singular conditions of temperature and pressure where liquid water, gaseous water and hexagonal ice stably coexist, there is a ‘triple point’ where both the boiling point of water and melting point of ice are equal.  A ‘critical point’ occurs at the end of a phase line where the properties of the two phases become indistinguishable from each other, for example when, under singular conditions of temperature and pressure, liquid water is hot enough and gaseous water is under sufficient pressure that their densities are identical. Critical points are usually found at the high temperature end of the liquid-gas phase line.

Analyzing a phase diagram it is generally possible to predict the thermodynamic behavior of the considered substance.

Water is a scientifically fundamental example of this kind of analysis. So, let’s think about what may happen on a skating rink. The temperature is obviously under 0°C. At this temperature and at the pressure of 1 bar the thermodynamically water stable phase is the solid one. There’s no doubt that there would be ice.

skierBut exactly when an hypothetical skater puts the blade of its runner over the surface of ice the situation changes. Or better the pressure conditions change. This pressure variation involves only the ice surface below the blade of the runner. Actually, the skater applies a pressure on the ground with its weight, determining a global pressure increase over the considered ice area.

Looking at the above water’s phase diagram, it is clear that if we increase the pressure the temperature of water solidification (temperature at which water is converted to ice) decreases under 0°C. The natural consequence is that the skater’s weight makes ice melt under the runner’s blade, as in that conditions of pressure and temperature the thermodynamically water stable phase is the liquid one. This means that, actually, the skater is not sliding over ice but over a thin layer of water between the blade and the below ice! That’s exactly what happens with a skier!

Cool isn’t it?


by Francesco Pochetti
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