|MadSci Network: Chemistry|
The simple answer to your question is: use liquid helium. Hydrogen freezes at about 14 degrees above absolute zero (14 Kelvin), while helium doesn't even turn to a liquid until 4.2 Kelvin. (See: Why does hydrogen freeze at a higher temperature than helium?)
So, once you liquify helium, you can freeze anything else using the helium. Heike Kammerlingh Onnes first liquified helium in 1908 at the university of Leiden, The Netherlands. (I had to look that up; it's not a name I keep in my head!) That achievement allowed him to perform the first experiments looking at the strange properties of liquid helium, and also led to his discovery of superconductivity. Kammerlingh Onnes won the Nobel Prize in physics for all this work in 1913.
The trick to liquifying helium is a process called "throttling." It's also called Joule-Thompson expansion, after the physicists who first proposed it. The idea is that you pressurize a gas, cool it down close to its liquid temperature, and then squeeze it through a very small tube into a low-pressure chamber where it expands. For the right starting pressure and temperature, the gas that goes through the tube will lose energy and drop in temperature. (If the starting pressure or temperature is too high, this same process will instead heat the gas up!)
By the mechanical process of throttling, you can lower a gas temperature below the temperature of the equipment you're using. If the pressurized gas is cold enough, then it will even liquify when it expands into the low- pressure chamber.
So, Kammerlingh Onnes used liquid hydrogen to cool helium gas, then throttled the helium to liquify it. (Hydrogen had been liquified in 1896 by Sir James Dewar of London, who invented the double-walled vacuum cannister for keeping the cold liquid insulated. The modern Thermos bottle is a type of dewar.) Now there are systems that can liquify helium starting with just liquid nitrogen, which is cheaper and less dangerous than liquid hydrogen.
As I said at the beginning, once you have the liquid helium, it's straightforward to freeze hydrogen, or any other gas.
Good question. Stay curious.
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