Helium has the lowest freezing point and does not freeze under typical pressures due to its high zero-point energy. Solid helium only exists under high pressure and low temperature, and is virtually invisible. Helium is used as a cryogenic cooling agent and exhibits superfluidity at very low temperatures.
The material with the lowest freezing point is helium. Under typical pressures, it does not freeze at all, even at temperatures close to absolute zero. The reasons are dictated by quantum mechanics: the zero-point energy of a helium system is too great to allow for freezing. Zero-point energy is the minimum energy a particle or system always has, no matter what. Helium is the only substance that does not have a freezing point at ambient pressure, regardless of temperature.
A freezing point for helium exists only under at least 25 atmospheres of pressure and a temperature of 1.15 K. These conditions have been created in the laboratory through evaporative cooling. The result is a colorless, highly compressible, virtually invisible solid. Solid helium is so hard to see that the layers of Styrofoam are only used to tell where it is. The density of solid helium itself is only 66 times greater than air. By comparison, water is 1,000 times denser than air.
Helium was first liquefied in 1908 by Dutch physicist Heike Onnes, who cooled it to 1 degree Kelvin. To his surprise, the further cooling did not cause him to reach freezing point. It was only 18 years later, in 1926, that his student, Williem Keesom, managed to solidify helium by cooling it in a pressure chamber. Today, the liquefaction of helium is a critical step in extracting it from the earth and storing it.
Liquid helium is often used as a cryogenic cooling agent when liquid nitrogen is not sufficient. It must be kept under continuous high pressure and low temperature, otherwise it will expand rapidly and turn into a gas. Solid helium has no practical applications outside of scientific research.
Some of helium’s more unusual properties can be persuaded at temperatures close to absolute zero. At these temperatures, helium behaves as a superfluid, meaning it flows with zero measurable viscosity. It also has a tendency to creep into the walls of a container it is contained in.
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