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Thread: gas + gas = liquid

  1. #1 gas + gas = liquid 
    Forum Radioactive Isotope zinjanthropos's Avatar
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    A question* came up this weekend.... If hydrogen & oxygen are gases then how do they form a liquid (water)? Someone said that the shape of the H2O molecule itself is why. If so then can someone elaborate or explain how water is formed by two gases.

    Are there other liquids besides water that are formed by gases combining?

    *rather not Google


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    In this reaction H2 and O2 are small molecules with very weak intermolecular forces, this is why they are gases at room temperature as the kinetic energy they have is large enough to overcome the forces between molecules. When they react they form water, the H2O molecule is very polar (the O bit is slightly negative, the H bits slightly positive) this means the molecules are attracted to each other and the forces between the molecules are relatively strong - a hydrogen bond (this is why water has a relatively high melting and boiling point compared to other non-metal hydrides like H2S, CH4 etc). These relatively strong intermolecular forces mean at room temperature the kinetic energy the molecules have is not strong enough to overcome the forces of attraction between molecules so water is a liquid.

    How water is formed: The mechanism is quite tricky (as gas phase radical reactions often are - people have made careers studying a handful of reactions!) - my old tutor compared liquid phase chemistry to gardening and gas phase chemistry to a traffic accident as all the molecules are zipping around randomly. The first step is the breaking of the H-H bond in the H2 molecule, the H atom then attacks and O2 molecule then via all sorts of radical pathways water is formed.

    I can't think of any other examples off the top of my head at room temperature but I'll have a think.


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  4. #3  
    Forum Professor Zwolver's Avatar
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    After a reaction it doesn't form a liquid. It forms a gas. H2 and O2, when sparked, form H2O gas... not a liquid. The fact that it can be a liquid as well, if cooled down, is also true for the H2 and the O2. If cooled down enough, you will have liquids as well.

    At room temperature, and earth atmospheric pressure sure..

    But yes, water is quite unique. A similar substance is methanol. Methanol and methane are very similar in structure, with a very different boiling point. So it most likely the OH part of water.

    Look at methanediol.. with a boiling point of 194C
    methanol with a boiling point of 65C
    and methane with a boiling point of -162C

    funnily enough, methanetriol has a boiling point of 56C

    So i believe stability depends on this dipole, which water has.
    Growing up, i marveled at star-trek's science, and ignored the perfect society. Now, i try to ignore their science, and marvel at the society.

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    Bullshit Intolerant PhDemon's Avatar
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    It's to do with intermolecular forces (specifically hydrogen bonding) as I said above.

    (Methanetriol is complicated by steric factors)
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    Forum Professor Zwolver's Avatar
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    Quote Originally Posted by PhDemon View Post
    (Methanetriol is complicated by steric factors)
    I don't understand what you mean. Steric factors?
    Growing up, i marveled at star-trek's science, and ignored the perfect society. Now, i try to ignore their science, and marvel at the society.

    Imagine, being able to create matter out of thin air, and not coming up with using drones for boarding hostile ships. Or using drones to defend your own ship. Heck, using drones to block energy attacks, counterattack or for surveillance. Unless, of course, they are nano-machines in your blood, which is a billion times more complex..
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  7. #6  
    Bullshit Intolerant PhDemon's Avatar
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    https://en.wikipedia.org/wiki/Steric_effects

    There may also be other effects (electronic or inductive) that contribute to anomalous boiling points.
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  8. #7  
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    funny I've asked myselve the same question,
    Carbon:solid
    Oxigen:gas
    CO,CO2:gas
    Hydrogen:gas
    H2O:liquid
    CH4:gas
    heat,temperature,entropy,enthalpy,weak bonds, strong bonds, etc... all human classes derived from observation
    how to cramp all that information into an atom, so its behaviour can be observed as is, or how to take human observation out of the equation, mathematics?
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  9. #8  
    exchemist
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    Quote Originally Posted by perdurat View Post
    funny I've asked myselve the same question,
    Carbon:solid
    Oxigen:gas
    CO,CO2:gas
    Hydrogen:gas
    H2O:liquid
    CH4:gas
    heat,temperature,entropy,enthalpy,weak bonds, strong bonds, etc... all human classes derived from observation
    how to cramp all that information into an atom, so its behaviour can be observed as is, or how to take human observation out of the equation, mathematics?
    Ah well if you learn the quantum theory of atoms and molecules, and statistical thermodynamics, all is revealed! These are the two main pillars of physical chemistry, and they show that you can indeed derive all this from the behaviour of electrons and the various atomic nuclei of the different chemical elements. Which is pretty amazing and is what enthralled those of us who studied chemistry at university.
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