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A water temperature of 10 °C (50 °F) can lead to death in as little as one hour, and water temperatures near freezing can cause death in as little as 15 minutes. [37] During the sinking of the Titanic , most people who entered the −2 °C (28 °F) water died in 15–30 minutes.
However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers. [6] There are two spin isomers of hydrogen; whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen. [5]
Phase I occurs at low temperatures and pressures, and consists of a hexagonal close-packed array of freely rotating H 2 molecules. Upon increasing the pressure at low temperature, a transition to Phase II occurs at up to 110 GPa. [3] Phase II is a broken-symmetry structure in which the H 2 molecules are no longer able to rotate freely. [4]
At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form. [30] The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol, [31] and it converts to the para form over the course of several minutes when cooled to low temperature. [32]
Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. Typically it will evaporate at a rate of 1% per day. [1] [42] The main danger with cryogenic hydrogen is what is known as BLEVE (boiling liquid expanding vapor explosion).
However, at low temperatures only the J = 0 level is appreciably populated, so that the para form dominates at low temperatures (approximately 99.8% at 20 K). [8] The heat of vaporization is only 0.904 kJ/mol. As a result, ortho liquid hydrogen equilibrating to the para form releases enough energy to cause significant loss by boiling. [6]
Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa, where hydrogen would be a liquid metal, even at low temperatures. [8] [9]
A low temperature (T°), thermal agitation allow mostly the water molecules to be excited as hydrogen and oxygen levels required higher thermal agitation to be significantly populated (on the arbitrary diagram, 3 levels can be populated for water vs 1 for the oxygen/hydrogen subsystem), At high temperature (T), thermal agitation is sufficient ...