# At what pressure will hydrogen start to liquefy at room temperature?

I want to increase a fixed-size object's internal gas pressure by generating hydrogen in it, but I could not find the proper phase diagram for it. So I am wondering how high pressures I can get.

• As far I know, on such extreme pressures, there is no such fixed boiling point any more. But possibly someone knows it better. – peterh - Reinstate Monica Sep 21 at 13:12
• Here's your phase diagram: engineeringtoolbox.com/hydrogen-d_1419.html – Ross Presser Sep 21 at 21:21
• The term "permanent gas" has been invented exactly to describe your situation. – fraxinus Sep 22 at 7:53
• Apparently, the provided answers are enough, yet just wondering, given the danger of hydrogen, why do you want to inject hydrogen? Can you consider an alternative, like nitrogen? – Stéphane Gourichon Sep 24 at 12:58

$$\ce{H2}$$ cannot be liquified at room temperature, whatever the pressure. Generally speaking, all gases can only be liquified when the temperature is under its critical value.

• The critical temperature of hydrogen is 33.20 K, and so it becomes a supercritical fluid if the pressure is great enough. – KingLogic Sep 24 at 0:38
• Yes. But a supercritical fluid is not a liquid. It has no surface, where a boat can float. – Maurice Sep 24 at 2:34
• Yes you're right – KingLogic Sep 24 at 2:43

Hydrogen critical temperature is $$\pu{32.938 K, resp. -240.21 ^{\circ}C}$$. Above this temperature, it cannot be liquified.

So to answer your question, you can get as high pressure as you can produce and the container can withstand, as there is no condensation reducing the pressure.

WARNING: An accidental explosive container rupture can easily cause severe injuries or even death.

Industrial liquifying process involves:

• Cooling down by liquid nitrogen. The purpose is to reach temperature below about $$\pu{200 K}$$ where Joule--Thomson-(Kelvin)-coefficient becomes positive, what enables cooling by Joule-Thompson effect via throttled gas expansion.
• Liquifying hydrogen by further cooling down by the Linde process ( Hampson-Linde cycle. For gases other than hydrogen and helium, this is possible even at room temperature ( neon is at the edge ).
• Does it turn into a solid eventually? – user253751 Sep 23 at 12:46
• Any gas does. Hydrogen would need hundreds of GPa, – Poutnik Sep 23 at 16:04

As others have said, hydrogen cannot be liquified above its critical temperature, which my source (Wolfram Alpha chemical database*) says is $$\pu{32.97 K} = \pu{-240.18 ^\circ C}$$

However, with sufficient pressure, the molecules can be squeezed together until they have a liquid-like density**, and are thus no longer considered a gas, but rather a supercritical fluid. The pressure required to reach this point is called the critical pressure which, for hydrogen, is $$\pu{1.239 MPa} = \pu{179.7 psi}$$.

Hence, while you can't change hydrogen into a liquid at room temperature, you can change it into a (supercritical) fluid.

Incidentally, there are two established storage technologies for pure hydrogen in vehicles. One involves cooling the hydrogen below its critical temperature and liquifying it. The other stores it at ambient temperature, at pressures of $$5\,000$$ to $$10\,000\ \mathrm{psi}$$. In the ambient temperature cases they are not storing compressed hydrogen gas, but rather compressed supercritical hydrogen.

**Achieving a liquid-like density is not the formal definition of the transition from gas to supercritical fluid, but it does, I think, paint a helpful physical picture.

• @Baris Note for the OP: liquid like density is still quite low, compared to the liquid at less extreme conditions. E.g. liquid water ( and water vapor as well ) near water critical temperature and pressure have both density about 200 kg/m3. For hydrogen, it would be much lower. – Poutnik Sep 22 at 6:15
• Hmm, formally, you are probably right, but I have never heard calling hydrogen with p>1.239 MPa as hypercritical fluid (HCF).( for 4He 0.227 MPa ). In my undestanding, speaking about HCF has IMHO sence for temperatures not too far from the critical one. Hydrogen at such a pressure and room temperature is much closer to ideal gas than to hypercritical fluid. – Poutnik Sep 22 at 6:21
• @Poutnik Regarding your second comment: I think that's precisely why it's interesting to note this, since no one does typically recognize that these ambient-temperature containers are holding hydrogen in the supercritical fluid state. – theorist Sep 22 at 6:27
• What I had in mind is 1000->200kg/m3 density drop for water, versus analogical density drop for liquid hydrogen. Hydrogen critical density is 30.3 kg/m3 engineeringtoolbox – Poutnik Sep 22 at 6:33
• I still insist if the state is not near enough to the critical point, speaking about supercritical state is very formal, not being related to specific supercritical properties found near the critical point, caused by very significant ideal gas deviation. – Poutnik Sep 22 at 6:41