Regelation is a phenomenon in chemistry and physics you’ve likely witnessed, even though you did not know its name. Learn about what regelation is and see examples in daily life.
Regelation is the phenomenon of ice melting under pressure and then refreezing once the pressure is released.
Expanding Upon Freezing
Regelation only occurs for materials that expand upon freezing so that the melting point decreases as external pressure increases. For example, for 1 atm of pressure applied, the melting point of (water) ice falls by 0.0072 °C. What this means is that regelation occurs in other materials, such as gallium and bismuth. But, usually a discussion of regelation pertains to water.
Examples of Regelation
Three common examples of regelation are glacier movement, pulling a wire through ice, and ice skating.
- Regelation occurs in glaciers. The mass of a glacier exerts enough pressure to lower the melting point of the ice at its base, melting the ice and letting the glacier slide over the liquid. Under the right conditions, liquid water can flow from the base of a glacier. Water behind the glacier refreezes.
- Another example of regelation is the ice on a wire demonstration. Loop a fine wire over an ice cube and attach a heavy weight to the wire. The pressure the wire exerts on the ice melts it, allowing the wire to pass through the ice. The water refreezes behind the path of the wire so you can pull the wire through the ice, while leaving the ice cube intact. While regelation occurs, some of the melting comes from heating of the wire under tension.
- Ice skating works because the pressure of the skater pushes down on the skate blade enough to melt some of the ice into water. The skate then slides over the water. If the temperature is too cold, the pressure is insufficient to melt the ice and skating doesn’t work. Additional factors play into ice skating, not simply the process of regelation.
Snowballs are said to stick together due to regelation of snow, but this is not the case. Forming a snowball does not involve enough pressure to melt ice. Water surrounding snowflakes sticks them together. Snow does not stick when you try to make snowballs in extremely cold weather.
How Regelation Works
Michael Faraday first described and named the process of regelation. The process is due to the special nature of hydrogen bonding. When ice is compressed, the O:H (nonbond) distance shortens while the H-O covalent bond elongates and weaken towards O:H. Melting point depression occurs as the H-O bond loses energy. The melting point is proportional to the covalent bond cohesive energy. Essentially, the phase boundary between the liquid and solid phase of water changes. Releasing pressure lets the O:H-O bond return to its original state, refreezing the water into ice. The process is an example of hydrogen bond memory.
A related effect is piezoelectricity. Piezoelectricity is the accumulation of electrical charge in solid materials when mechanical stress is applied to them.
- Drake, L. D.; Shreve, R. L. (1973). “Pressure Melting and Regelation of Ice by Round Wires”. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 332 (1588): 51. doi:10.1098/rspa.1973.0013
- Sun, Chang Qing (2014). Relaxation of the Chemical Bond. Springer. ISBN 978-981-4585-20-0.
- Sun, Chang Qing; et al. (2012). “Hidden force opposing compression of ice”. Chem Science 3: 1455-1460. doi:10.1039/c2sc20066j
- Zhang, Xi; et al. (October 2014). “A common superslid skin covering both water and ice”. Physical Chemistry Chemical Physics. 16 (42): 22987–22994. doi:10.1039/C4CP02516D