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How does water rise to form clouds if oxygen is heavier than nitrogen?

  1. Feb 12, 2013 #1
    1 - how does water rise to form clouds if oxygen is heavier than nitrogen?

    when it turns into a gas its hydrogen and oxygen---why does it rise (to form clouds) if oxygen is heavier than nitrogen, which makes up 78% of the earths atmosphere?

    *please note that i'm not confused as to the radiative excitation of the molecules that allow them to break the surface that starts the evaporation process

    2 - is the evaporating gas always radiatively ionized or just sometimes?
     
  2. jcsd
  3. Feb 12, 2013 #2

    ehild

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    Re: how does water rise to form clouds if oxygen is heavier than nitro

    The water molecules rise together with warm air.
    The air warmed up near to the ground will rise up as the molecules have greater speeds as those of cold air. The warm molecules above a hot stop exchange momentum and energy with he cold molecules above, and that results a net upward velocity of the air column. At great height, where the temperature is low, the vapour condenses and clouds form.

    The molecules of a gas have so high speeds that the effect of gravity during collisions can be totally neglected. The molecules change direction in the collision, and the distribution of their velocities stays random. They do not settle down.

    By the way, oxygen molecules and nitrogen molecules make up the atmosphere, and water vapour is from H2O molecules (relative molar mass 18 ) , lighter either than O2 (32) or N2 =28) .

    Evaporation means that a molecule has so big KE that its energy is enough to overcome the attractive potential inside the fluid. It does not need excitation. Simply, as the velocity distribution is random, there is some probability that a molecule has high enough speed to leave the surface of the liquid.

    ehild
     
  4. Feb 12, 2013 #3

    Simon Bridge

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    Re: how does water rise to form clouds if oxygen is heavier than nitro

    This is a homework problem? So I won't just tell you the answer...

    Some concepts to help you wrap your head around this:
    1. What we usually think of as "water" is a mix of H+, OH- and H2O.

    2. Gas phase of water is "steam" ... it is H2O gas - it can go places because it has a lot of kinetic energy.

    3. Water in clouds is vapor - liquid water droplets. They condense on dust - which is composed of (small) solid lumps.

    4. a gas does not have to be "lighter than air" to rise, it just has to be less dense as a body. A balloon full of people is much heavier than nitrogen - yet it manages.

    5. the atmosphere has a lot going on in it - i.e. it has weather - also eg. you will have noticed other heavy things getting carried up high ... like eagles and kites.

    So ... considering all this, how does water get into the sky to make clouds?
     
  5. Feb 12, 2013 #4

    haruspex

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    Re: how does water rise to form clouds if oxygen is heavier than nitro

    A little out of date, I think. From http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/The_hydronium_Ion: [Broken]
     
    Last edited by a moderator: May 6, 2017
  6. Feb 12, 2013 #5

    Simon Bridge

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    Re: how does water rise to form clouds if oxygen is heavier than nitro

    @haruspex: erk - OK, that's not just being pedantic, thanks. Replace H+ with H2O-H+ or something in the list. I did know about it, just didn't realize it's importance. This is why it is important for scientists to talk to each other aye.

    Someone really pedantic could also point out the dissolved gasses and salts and so on and on in even very pure water. Just to head that one off: I wasn't making an exhaustive list ;)
     
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