Regelation and Its Applications: Melting Ice Under Pressure

Regelation is the process in which ice melts under pressure and then freezes again when the pressure is removed. Pure ice normally melts at 0 degree Celsius, but extra pressure lowers its melting point slightly, so squeezed ice can turn into water even below 0 degree Celsius.

This matters because it explains everyday skating and also how heavy objects can slowly pass through ice. It connects pressure, melting point, and latent heat in one simple experiment that shows phase change is not fixed only by temperature.

Key Concepts

The melting point is the temperature at which solid and liquid coexist. For most solids pressure raises the melting point, but for ice it lowers it because liquid water is denser than ice. The Clapeyron relation gives dT/dP = T times change in specific volume divided by latent heat. Latent heat of fusion of ice L is about 3.34 x 10^5 joule per kilogram. Specific volume of water is about 1.00 x 10^-3 cubic metre per kilogram and of ice about 1.09 x 10^-3 cubic metre per kilogram, so the change is negative.

Because of this, increasing pressure by about 135 atmosphere lowers the melting point by roughly 1 kelvin. Regelation also needs heat flow: melting absorbs latent heat from surroundings, and refreezing releases it.

Formulas

Clapeyron equation: dP/dT = L / (T x delta v), where L is latent heat per unit mass in joule per kilogram, T is absolute temperature in kelvin, delta v is change in specific volume in cubic metre per kilogram. For ice to water delta v is negative, so dT/dP is negative. Approximate shift: delta T is about -0.0074 kelvin per atmosphere of extra pressure. This assumes pure ice, near 0 degree Celsius, and slow equilibrium.

Worked Example

A wire loaded with weights presses on an ice block at -2 degree Celsius. How much pressure is needed to melt ice under the wire?

Needed fall in melting point = 2 kelvin. Using 0.0074 kelvin per atmosphere, pressure = 2 / 0.0074 = about 270 atmosphere. 1 atmosphere = 1.01 x 10^5 pascal, so pressure = 270 x 1.01 x 10^5 = 2.73 x 10^7 pascal. A thin wire gives large pressure because pressure = force / area, so a modest weight can create this pressure and the wire slowly cuts through, with water refreezing above it.

Common Misconceptions

• Pressure alone always melts ice at any low temperature: Wrong. The lowering is tiny; at -20 degree Celsius you would need about 2700 atmosphere, far beyond skating.
• Skating works only by regelation: Incomplete. A thin liquid-like surface layer and frictional heating also help make ice slippery.
• Ice expands on melting: Wrong. Ice contracts on melting because water is denser, which is why pressure favours melting.

Real-World Applications

Ice skating and curling rely partly on pressure melting plus surface premelting. The wire-through-ice demonstration is a classic lab proof of regelation. In glaciers, pressure at the base lowers melting point, creating a thin water film that helps glacier sliding over bedrock. Snowball making also benefits because hand pressure melts surface ice which refreezes and binds snow.

Regelation shows that melting point depends on pressure, especially unusually for ice because water expands on freezing. Pressure melting followed by refreezing explains the wire experiment, skating aids, and basal glacier motion.