Trouble understanding how volatility is measured

  1. I'm having a bit of trouble understanding how volatility is measured. I understand the units of volatility are mg/m3, but I don't know how a measurement of concentration relates to how volatile a liquid is. Wouldn't the vapour concentration of the liquid depend on the amount of time that it was allowed to sit an evaporate?
  2. jcsd
  3. Borek

    Staff: Mentor

    No. Once the vapor gets saturated, its concentration doesn't change any further, no matter how long you wait.

    If you take an empty bottle, add half an inch of water, close the bottle and leave it alone, no matter how long you wait there will be always the same amount of water resting at the bottom. That's because the vapor above is saturated. If you leave the bottle open, the water will eventually dry out, but that's because air is almost never saturated, plus it is exchanged, so the vapor can escape.
  4. Yeah, so in that case the units would represent the saturation concentration... But saturation concentration would be achieved eventually regardless of liquid, would it not? How does this concentration tell me how fast a liquid will evaporate?
  5. Borek

    Staff: Mentor

    It doesn't.

    Other than the obvious thing that the higher the saturated vapor pressure, the faster the evaporation.

    Trick is, there are many things that have to be taken into account when trying to evaluate evaporation speed. Heat transfer and air movement being probably the most important ones (and already difficult to describe).
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