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# how can hydrogen gas can be compressed to liquid form?..? Answered

actually i am trying to use a liquid  hydrogen as a fuel in a car ...........

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Use metal hydrides. Metals like lithium and alloys of iron can absorb large amounts of hydrogen. only a small amount of compression needed - hydrogen gas is released as the metal is heated (usually with the car exhaust).

You can't handle liquid hydrogen, forget about that.

L

No answer, but a question- so how does hydrogen- or any other gas behave above the critical point?? Can it be compressed more than what would be needed to make the fluid at lower temperatures? or is it just a matter that no fluid surface is formed??

In a supercritical CO2 you basicly get certain characteristics from a liquid (like ability to dissolve things) and certain characteristics of a gas (like viscosity).

This allows it to be used in many things such as for cleaning or extraction.

Supercritical fluid is reached above the critical pressure and temperature. Drop the temperature and it becomes a liquid. Drop the pressure and you get a gas. The critical point is usually present in phase diagrams of a specific substance.

For example:
http://www.wolframalpha.com/input/?i=phase+diagram+of+hydrogen

Liquid hydrogen can only remain liquid at temperatures lower than -240 C (33 K), and at pressures higher than12.8 atm (1300 kPa)
http://en.wikipedia.org/wiki/Critical_point_(thermodynamics)#Table_of_liquid.E2.80.93vapor_critical_temperature_and_pressure_for_selected_substances

So to make hydrogen liquid, you have to compress it, and cool it, and keep it cold

Because liquefying hydrogen is difficult, that's why people are working on other methods for storing it, as described in this article:
http://en.wikipedia.org/wiki/Hydrogen_storage

Hydrogen needs to be cooled down to liquify it.