The density of ice increases upon warming up to 70 K

Scientific Explanation

Most solids behave predictably when cooled: their atoms draw closer together, and density increases. Ice, however, defies this expectation. When warmed from temperatures near absolute zero, ice density actually increases up to about 70 Kelvin (approximately minus 203 degrees Celsius). Only above this temperature does density begin to decrease as expected for a normal solid.

This behavior stems from the open tetrahedral crystal structure of ice Ih. At extremely low temperatures, quantum mechanical zero-point vibrations cause the oxygen atoms to oscillate around their equilibrium positions more strongly than classical physics would predict. These vibrations inflate the already open ice lattice. As temperature rises toward 70 K, thermal vibrations become more uniform and the structure compacts slightly before normal thermal expansion takes over at higher temperatures.

This phenomenon is known as “negative thermal expansion” at low temperatures and has been observed in various ice phases, with the magnitude depending on the specific crystal structure.

Everyday Relevance

While this phenomenon occurs at temperatures far too cold for everyday experience, it matters for space science and planetary research. Ice exists on comets, moons like Europa and Enceladus, and in interstellar clouds at exactly these temperatures. The unusual expansion behavior affects the mechanical properties and behavior of ice crusts on these celestial bodies.

For fundamental physics, this anomaly provides a striking example of how quantum effects can manifest even in a substance as familiar as water.