How does vacuum form in a barometer?

  1. Hi, here is something I can't understand:

    When you fill a 100% tube with Mercury, close it in one end (the top) and stick it into a bowl of mercury, vacuum forms in the top of the tube. Where does the vacuum come from if it was not there to begin with? The tube was full of mercury to begin with, so I have a hard time understanding how vacuum - empty air - can form if there was no empty air to begin with.
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    A vacuum is not empty air. A vacuum is a volume of space devoid of any matter, including air. In this case the weight of the mercury is enough to cause a small vacuum to form at the closed end of the tube.
     

  4. So the weight of the mercury in the tube pushes the mercury out into the jar, and Volume devoid of matter just appears? Can you do this with a tube of water as well? and if vacuum forms in the top of the tube would that make water boil because vacuum exerts no pressure on water (thus lowering the boiling point?)
     
  5. Drakkith

    Staff: Mentor

    I believe that is mostly correct. I don't think there's enough vacuum to make the water will boil, but it should fill the vacuum with water vapor. Note that water has other gasses dissolved in it, so you may not get a true vacuum at the top.
     
  6. Chestermiller

    Staff: Mentor

    The space above is not pure vacuum. The space above is filled with mercury vapor at the equilibrium vapor pressure corresponding to the temperature of the liquid mercury. If you did the same thing with water, the space above would be filled with water vapor at the equilibrium vapor pressure corresponding to the temperature of the liquid water. The vapors in these spaces result from the evaporation of a tiny amount of the liquid. Since the vapors are in equilibrium with the liquids, the liquids can't boil.

    Chet
     
  7. SteamKing

    SteamKing 8,343
    Staff Emeritus
    Science Advisor
    Homework Helper

    With a mercury barometer, you have to use a tube greater than 76 cm in length to obtain a vacuum, because standard atmospheric pressure will support a column of mercury approx. 76 cm tall. If you use water, the tube must be greater than 10 meters tall, due to the difference in density of mercury versus water. This is why mercury is used in making a barometer: it makes for a more compact unit than if another fluid was used.
     
  8. Thank you so much for the great explanations.
    SteamKing: So I think i'll skip the water tube-vacuum experiment ^^ but it puts things in perspective - Thank
    you!

    Let's say we have a 80cm tall barometer full of mercury, (I guess we would get 4cm of vacuum? at atmospheric pressure?), if we placed the barometer in a large sealed off container and lowered the air density by 50% in the container would we then get double as much vacuum (8cm) because we have half the amount of atmospheric pressure pressing down on the mercury?
     
  9. jtbell

    Staff: Mentor

    No, the height of the mercury column would be reduced by 50%, to match the air pressure in the container. In your example, you begin with 76cm of Hg and 4cm of vacuum. You end up with... ?
     
  10. Ahh so the height would only be (1/2)*76cm = 38cm mercury and 38+4 = 42cm mercury :)

    Thank you again :)
     
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