At what Pressure will Hydrogen start to Liquefy at Room Temperature?

Read all the factors that affect the boiling and melting temperature of liquid hydrogen and other related concepts.

Introduction:

Before we go into detail about the melting/boiling points, and temperature of liquid hydrogen, it is crucial to understand the basics of how & why different substances behave in different conditions. Two concepts are significant when it comes to liquefying of elements. First, a substance's natural melting and boiling temperatures are dependent on the strength of the forces that hold these particles together. In other words, the higher the forces, the higher these values and vice versa.

Second, the term critical pressure of a pure material is the pressure at which liquid and gas can’t coexist at any temperature. Similarly, the temperature at which a gas cannot condense into a liquid under any pressure is termed its critical temperature. Now, using these concepts, we can understand better how gases are turned into liquid states.

When the pressure applied to a gas is increased, its molecules start to move closer to each other, and dropping the temperature slows down their kinetic energy. Both these factors push the gas to become liquid. Some gases can be liquified by just dropping temperature, some by dropping pressure, and some by both.

It should be kept in mind that increasing pressure to turn a gas into liquid requires a lot of energy and in industrial practices, dropping temperatures is more practical to convert gases into liquid state.

Liquification of Hydrogen

Hydrogen, like any other gas, can be liquified when its temperature and pressure are altered. At atmospheric pressure, hydrogen needs to be cooled down to −240.18 °C to turn into to fully liquid state as this is where the critical temperature lies for hydrogen which is 33K. Its critical pressure is 13.1 atmospheres. Hydrogen at room temperature is always found in a gaseous state.

So, any temperature above -252.87 degrees will not liquify hydrogen at atmospheric pressure. There are now certain commercially available technologies through which pure liquid hydrogen is being produced in automobiles. One is liquifying the hydrogen by lowering down the temperature well below its critical point and the other keeps it in a high pressure at room temperature.

When this gas is in its liquid state, it behaves like a supercritical liquid, which is a gas that is highly compressed and combines both the properties of a gas and a liquid. What it means here is that H2 in this state behaves like a gas which is why it can be filled in containers, just like any other gas. In the meantime, it also acts as a liquid by having solvating powers and densities just like a regular liquid.

Can H₂ be Liquified at Room Temperature?

No, as mentioned above, any temperature above the critical temperature of liquid hydrogen won’t turn it into liquid form no matter how much pressure is applied to push the hydrogen atoms into each other. That is why, the containers used for transportation of hydrogen gas are quite cold. Practically speaking, the liquefaction of hydrogen is achieved by tweaking both pressure and temperature to attain a certain feasible point where both economic and convenience factors are checked.

One of the very common industrial practices to liquefy hydrogen gas is through the Hampson-Linde cycle. This method uses a series of processes in which hydrogen goes through compression, cooling, and expansion to achieve the low temperatures required for hydrogen to liquefy. Another point to clarify here is that by liquefaction of hydrogen, doesn’t mean that it will exist like a fluid with a surface. Instead, liquified gas is, as explained above a supercritical fluid behaving both as gas and liquid.

Wrapping Up:

To conclude the topic, it can be stated that a gas can be liquefied when room temperature and pressure are according to the critical values of that element. Elements like hydrogen that are naturally occurring gases on Earth cannot simply be cooled to become liquids; instead, they must first be transformed from gases into liquids to be transported and stored for use. Moreover, any temperature of liquid hydrogen above its critical point won’t turn it into a liquid state which includes room temperature.