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CHemistry H2O vapour question

  1. Oct 7, 2007 #1
    1. The problem statement, all variables and given/known data
    Water at room temperature is placed in a flask connected by a rubber tubing to a vacuum pump, and the pump is turned on. Ater several minutes, the volume of the water has decreased and what remains has turned to ice. Explain

    3. The attempt at a solution

    This is the answer from the solutions manual but i don't understand.

    "As the pressure over the liquid H2O is lowered, h2O vapor is removed by the pump. As h2o vapor is removed, more of the liquid h2o is converted to h2o vapor. this conversion is an endothermic process and the temperature decreases. the combination of both a decrease in pressure and temperature takes the system across the liquid/solid boundary in the phase diagram so the h2o that remains turns to ice."

    What exactly is a vacuum pump, does this just suck up all the air in the flask? are we supposed to assume that the flask is closed?

    The part i dont' understand is, how is the pressure over the liquid h2o lowered. where does the H2O vapor come from? I mean the water is at room temp so how come theres h2o vapor. ANd the part about "h2o vapor removed, more liquid h2o is converted to h2o vapor", how does this occur?
  2. jcsd
  3. Oct 7, 2007 #2
    a vacuum pump is just like your vacuum cleaner. Yes, the flask is closed and it is pulling the air from the flask. this lowers the pressure in the flask and makes it easier for water molecules to go from the liquid to gas phase. There is always water vapor in air (water evaporates from its surface) but when the pressure is lower, the water molecules don't have as much force keeping them in the liquid phase - makes it easier to evaporate, even boil. So the water boils at a lower pressure. (Pressure of water vapor in the bulk matches pressure in the atmosphere = boiling point)

    Think about how water is boiled on the stove. We have to heat (give energy to) the water so it can boil = since water needs energy to go from a liquid (condensed phase) to a vapor phase, this is an endothermic process. Well, if the pressure is low enough, the water can get this energy from the warm atmosphere and the bulk water molecules - therefore lowering the temperature. The flask where the liquid water is will feel cold. If the pressure is really low then the water is really sucking up all the thermal energy it can and can take the temperature low enough to go from water to ice.

    hope this helps
  4. Oct 7, 2007 #3

    that cleared it up a lot. But i just have one more question to clarify

    When u suck the air up from the flask, this is not equivalent to lowering the volume, right?

    because if u lowered volume, pressure increases.
  5. Oct 8, 2007 #4
    yes, if you look at it from the other direction where the air/water molecules are being removed from the flask, they are going from a larger space (flask) to a smaller space (tube or connection to vacuum). The pressure here is inverse tothe pressure left behind in the flask.
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