The melting point of a substance is the temperature at which it changes from a solid to a liquid state. For almost every substance, cooling it down will eventually cause it to freeze into a solid. Helium, the second lightest element, is the exception to this rule. Even when cooled to temperatures just fractions of a degree above absolute zero, helium remains liquid at standard atmospheric pressure. Unlike all other elements, helium does not possess a melting point under normal conditions.
The Critical Conditions for Freezing Helium
To force helium into a solid state, simply lowering the temperature is not enough; extreme pressure must be applied. This unique behavior puzzled scientists for decades after helium was first liquefied. The standard phase diagram that governs the state changes of most matter does not apply to this element, as helium has no triple point at atmospheric pressure.
Solidification only occurs when the atomic structure is physically squeezed together, overcoming its natural resistance to forming a lattice. The minimum pressure required to solidify helium is approximately 25 times the pressure at sea level, or about 2.5 megapascals. This pressure must be applied while simultaneously cooling the liquid to a temperature below about 0.95 Kelvin. If the pressure is released, the solid helium immediately melts back into its liquid form, even at these extremely cold temperatures.
The Quantum Reason Helium Stays Liquid
The resistance to freezing is explained by a concept from quantum mechanics known as zero-point energy. Classical physics suggests that all atomic motion should cease at absolute zero, allowing atoms to settle into a rigid solid structure. However, the Heisenberg Uncertainty Principle dictates that some movement must persist, defining this minimum, persistent quantum motion as the zero-point energy.
Helium atoms are extremely light, and the forces holding them together, called van der Waals forces, are very weak. Because of this combination of low mass and weak attraction, helium’s zero-point energy is comparatively high. This high energy prevents the atoms from settling into a fixed, ordered crystalline structure, maintaining the liquid state until external pressure compresses them.
Properties of Solid Helium and Cryogenic Uses
Once the required pressure is applied, solid helium forms a nearly transparent and highly compressible crystalline structure. Because it requires intense pressure to exist, solid helium is considered a quantum solid, where the atoms still exhibit significant zero-point motion within the lattice. Its high compressibility allows its volume to change by a large percentage, making it unlike any conventional solid.
The need for extremely cold temperatures makes helium uniquely suited for the field of cryogenics. Liquid helium is the coldest practical coolant available, routinely reaching temperatures of 4.2 Kelvin and below. Its primary use is cooling the superconducting magnets found in Magnetic Resonance Imaging (MRI) machines and large particle accelerators. Liquid helium provides the stable, frigid environment necessary for these powerful magnets to achieve zero electrical resistance.