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Title says it all. If hydrogen and helium are lighter than air, why won't liquid hydrogen and liquid helium defy gravity?

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Gaseous hydrogen and helium are lighter than air. Hydrogen, helium and air are close approximations to ideal gases, and for an ideal gas the volume of one mole of gas is about 22.4 litres. That means the density of an ideal gas is proportional to its molecular weight, so hydrogen ($M_w = 2$) and helium ($M_w = 4$) are lighter than air (average $M_w = 28.8$).

However you're asking about liquid hydrogen and helium, and liquids are much denser than gases because the molecules are much more tightly packed. For example the density of liquid hydrogen is around $68 \,\mathrm{kg/m}^3$ compared to air at about $1.3 \,\mathrm{kg/m^3}$. That's why liquid hydrogen doesn't float in air.

Incidentally, the density of liquid nitrogen (a close approximation to liquid air) is about $800 \,\mathrm{kg/m^3}$ so liquid hydrogen would float on liquid air.

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Because liquid hydrogen and liquid helium are much heaver than air.

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    $\begingroup$ please explain why. $\endgroup$
    – Lucas
    Jan 22 '13 at 7:22
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    $\begingroup$ Well, this is an experimental fact - it is easy to look up their densities and the density of air. Or do you want some theoretical explanation why liquids typically have higher densities than gases? $\endgroup$
    – akhmeteli
    Jan 22 '13 at 7:28
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you may as well ask why water wells on the earth, as the air we breathe is only basically water in a gaseous form which means that any gas is lighter than the liquid it derives from. If you study the natural cycle of the atmosphere you will find that Temperature, Heat, Pressure, and molecule size determines the balance of any mater whether in solid, liquid or gas state. Generally when in a gaseous state it is generally lighter than air depending on the density of the air.

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First of all, no mass 'defies' gravity. Second, the normal state of these two is air so they cannot be 'lighter than air' which would in effect be lighter than themselves. If you were meaning to ask why the liquid forms of these elements won't float, the answer is simply because liquids don't float. This is one of the properties that defines a gas. The molecules must overcome the IMF's and have a high enough KE but liquids do not. The density of the liquid would have to be lower than that of the gas and no liquid has a lower density than a gas. Although hydrogen has a low gaseous density, it's liquid density is still considerably greater than any gas.

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  • $\begingroup$ Actually, hydrogen and helium are lighter than air. Air is made up of other gases as well, and H and He will sit at the top of a room filled with air. $\endgroup$
    – Kitchi
    Feb 24 '13 at 13:25
  • $\begingroup$ I hope you aren't insinuating that no liquid is ever lighter than any gas... $\endgroup$
    – user121330
    Aug 8 '16 at 14:16

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