Hydrogen is the most abundant element in the universe. While most people encounter hydrogen as a colorless and odorless gas, it must be subjected to some of the coldest temperatures achievable on Earth to change its state. The process of taking this gas through its liquid phase and finally into a solid requires removing nearly all thermal energy from its molecules. Achieving this solid state places hydrogen at the extreme limits of cryogenic science.
The Specific Freezing Point Value
To transition from liquid to solid, hydrogen must be cooled to its melting or freezing point at standard atmospheric pressure. This phase change occurs at a temperature of 14.01 Kelvin. This corresponds to -259.14 degrees Celsius, or -434.45 degrees Fahrenheit. This makes hydrogen’s freezing point the second lowest of any known substance, surpassed only by helium.
Understanding Solid Hydrogen
Below 14.01 Kelvin, hydrogen becomes a colorless, transparent, cryogenic solid. The hydrogen molecules arrange themselves into an ordered structure, typically taking on a hexagonal close-packed (hcp) lattice arrangement at low pressure.
Solid hydrogen is notable for its incredibly low density, measuring approximately 0.086 grams per cubic centimeter. It is classified as a molecular solid, held together by weak forces. This light mass and minimal attraction contribute to hydrogen’s unique designation as a quantum solid, where the atoms exhibit significant zero-point energy, meaning they vibrate even at temperatures approaching absolute zero.
Why Hydrogen Freezes at Extreme Lows
Hydrogen’s ultracold freezing point is due to its fundamental molecular structure and the forces that govern its interactions. Hydrogen gas exists as a diatomic, non-polar molecule (H2), meaning the electrons are shared evenly, and it lacks any permanent positive or negative end.
Because there are no strong attractions like ionic or hydrogen bonds, the molecules must rely on extremely weak intermolecular forces, known as Van der Waals forces. Specifically, these are London Dispersion Forces, which arise from temporary fluctuations in electron distribution that create momentary, induced dipoles that weakly attract neighboring molecules.
Since the H2 molecule is the smallest and lightest, it contains very few electrons. Therefore, almost all of the molecule’s thermal energy must be removed through extreme cooling to slow the molecules down enough for these weak forces to lock them into a fixed solid lattice. This requirement for near-zero thermal motion necessitates the extremely low temperature of 14.01 Kelvin for solidification.
Real-World Uses of Liquid and Solid Hydrogen
Controlling and maintaining hydrogen at these extremely low temperatures underpins several high-technology applications. The most prominent application involves liquid hydrogen, which exists just above the freezing point, between 14.01 K and 20.28 K. Liquid hydrogen is widely used as a highly potent rocket propellant in the aerospace industry.
When combined with liquid oxygen, it provides an extremely high thrust-to-weight ratio, making it the fuel of choice for launching spacecraft. This high energy content per unit of mass is a significant advantage over other fuels. Storing hydrogen as a liquid at cryogenic temperatures also allows for a much greater quantity of fuel to be packed into a smaller volume compared to storing it as a high-pressure gas.
Beyond rocketry, controlling hydrogen at cryogenic temperatures is central to energy storage and the development of a hydrogen economy. Liquid hydrogen is used to store energy from renewable sources, acting as a compact energy carrier that can be transported and later used in fuel cells to generate electricity with only water as a byproduct. The study of solid hydrogen and its behavior at extreme conditions is integral to cryogenics research, including the development of superconducting materials.