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Need help !

  1. Feb 8, 2005 #1
    ok in this lab, we heated a plastic pipet in water so that it expandes. then we covered the top so that the gases are trapped inside the pipet. then we inverted the pipet into cold water, and releases our thunb that covered the hole of the pipet. thus water got sucked in...but why? :uhh:
    thank you in advance :cool:
     
  2. jcsd
  3. Feb 8, 2005 #2

    Moonbear

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    Here are some hints:
    When you heat a gas (such as air inside the pipet), what happens to it?
    If the container it is in (the pipet) is open to the outside air, what direction does the gas flow, in or out, of the container when it is being heated?
    When you cool this previously heated pipet, what happens to the air inside it?
     
  4. Feb 9, 2005 #3

    GCT

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    Moonbear, have you done this experiment before?
     
  5. Feb 10, 2005 #4
    well, the volume of the pipet expands, and when it is inverted into cold water, it shrinks again...
    i still dont know how that caused the water to be sucked in
     
  6. Feb 11, 2005 #5

    Gokul43201

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    What about the air in the pipet ? What happens to it ?
     
  7. Feb 11, 2005 #6

    Moonbear

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    I'm a little confused on this. Did you see your pipet visibly change shape/size? Or are you just assuming the pipet expanded? If you did not see the pipet actually change shape, don't assume it did.
     
  8. Feb 11, 2005 #7

    Moonbear

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    Not this one, but the concept is common to several experiments (when I was in school, we did something similar but sucked a hardboiled egg into a flask instead of water into a pipet...all fine and good until the teacher forgot to clean up the egg before leaving for the weekend :yuck:)
     
  9. Feb 11, 2005 #8

    Gokul43201

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    The boiled egg is nicer with a clear plastic bottle and some liquid nitrogen.
     
  10. Feb 11, 2005 #9

    dextercioby

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    I would honestly doubt that the volume of the pipet (made outa plastic,if i'm not mistaking) would change such as to produce visible effects.
    One can then assume that the gas from the pipet undergoes an isochore transformation...

    Daniel.
     
  11. Feb 11, 2005 #10

    GCT

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    I had seen a similar case where low proof ethanol was poured on to a plate, a lighted match was thrown on top, and was concealed with a drinking glass.......the liquid rose up instantly almost to the top. Certainly this is not a case where the volume expands and such, however it seems to have the same concept in mind.

    A rather pathetic suggestion of mine is in terms of adhesivity and cohensivity of water; the increase in temperature decreases the surface tension of water and allows the water to climb up the glass from the sides somewhat engulfing the air inside the container.

    I tried searching for this experiment on the net to no avail.
     
  12. Feb 11, 2005 #11

    Moonbear

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    That's my initial assumption. Your typical cheap plastic transfer pipet is what I'm guessing was used. But it would be good to clarify this so we don't lead MercuryRising astray with an incorrect assumption.

    If this assumption is correct, then my understanding of the experiment is that the pipet was left open at one end, heated, then sealed while still hot (stick your finger on the end), and not cooled until placed in the water and seal (finger) removed. If that's true, then there's a simple explanation to consider. The answer lies in the effect of temperature changes on the air inside the pipet (or any vessel). My guess at this point is the experiment is intended to demonstrate Charles' Law, or generally ideal gas law.
     
  13. Feb 11, 2005 #12

    Moonbear

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    If the intention of the experiment were to demonstrate adhesivity/cohesivity properties, it would have been better done with an open, glass capillary tube (narrow diameter). A plastic pipet with a closed bulb at the top wouldn't demonstrate this property well.

    I can't envision the experiment working at all if the pipet bulb expanded during heating.
     
  14. Feb 11, 2005 #13

    Gokul43201

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    This is a very simple experiment with a very elementary explanation.

    Let's not confuse things for Mercury, and bring in second (or higher) order effects. Mercury, have you heard of Gay Lussac's Law (or even the Ideal Gas Equation) ?
     
  15. Feb 11, 2005 #14

    cronxeh

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    I'm assuming you allowed the inside of the pipet to 'start' release the gas right?

    This way the gas pushed out the outside pressure and once you put your thumb on the hole you essentially closed the contaire - which was now at a lower pressure than the outside world. Once you opened the pipet the water should rush in under air's outside pressure until the two reach equilibrium (there is an equation for this, but essentially it should reach about 80 percent of the pipet with water)

    PV = nRT - by keeping n and T constant you get
    P1V1 = P2V2

    So in outside you have p (atm) * V1 (a lot of air) = P2 (about 0.8 of atm) * V2 of bottle
    Once you opened the bottle the two will try to reach equilibrium, and so you should end up with water inside the bottle
     
  16. Feb 12, 2005 #15

    Gokul43201

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    Ouch ! This needs to be cleared up.

    Here's what happens. First you heat up the pipet and the air in it, to some temperature T1. Now you put your thumb over it and contain this hot air. The volume of the pipet V is nearly constant. Next you lower the sealed pipet into cold water and reduce the temperature of the air to T2 (T2 < T1). Since the volume enclosed is roughly constant, the pressure drops (P1/T1 = P2/T2). But since the pressure of the water is atmospheric, and the air pressure is lower, water will rush into the pipet.

    Going further, there is no way to tell how much water will enter the pipet unless you know the volume of the pipet and the temperatures of the hot and cold water.

    If the volume of the pipet is V, and calling atmospheric pressure Po, the number of moles of air trapped in the pipet is given by :

    [tex]n=\frac{P_oV}{RT_1} [/tex]

    The pressure upon cooling to a lower temperature [itex]T_2~ (< T_1) [/itex] is given by :

    [tex]P_2 = \frac {nRT_2}{V} = \frac {P_oT_2}{T_1} < P_o[/tex]

    After water fills the pipet, the volume of air in the pipet (V') must be such that its pressure is atmospheric. So :

    [tex]P_o = \frac {nRT_2}{V'} = P_o \frac {T_2V}{T_1V'} [/tex]

    [tex]=> V' = V \frac {T_2}{T_1} [/tex]

    Thus the fraction of water in the pipet is simply :

    [tex]V_w = \frac {V - V'}{V} = 1- \frac {T_2}{T_1} [/tex]

    Even if you used boiling water (T1 = 373K) and icy water (T2 = 273K), you can only get about 27% of the pipet filled with water (neglecting the thermal expansion of the pipet material compared to that of the air). If you do get more than this amount of water into the pipet, it can only result from not being very careful (ie : allowing air to bubble out) or performing the experiment deep underwater. To get the pipet filled 80% with water, you will need

    [tex]V'/V = \frac {P_oT_2}{P_2T_1} < 0.2 [/tex]

    For this, you'd have to be under at least 120 feet of (icy) water.
     
  17. Feb 12, 2005 #16

    GCT

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    I don't believe that Pinitial is equal to the atmospheric pressure, as long as the temperature of the gas is greater within the container. When the container is exposed -an open system-to the atmosphere, depending on the shape/area of the opening, a flow of air will be established with the constant maintenance of the temperature; air will appear to be flowing out of the pipet to a degree.

    Anyways.......

    When you place any container in an inverted manner to a certain depth, depending upon the shape/volume of the container the liquid will rise up the container until the water encounters an pressure equal to the pressure at that specific depth of the water-the point of entry into the pipet (which takes into account both the atmospheric pressure as well as the gravitation force due to the upper mass of water). At greater depths you'll get more water into the pipet, even if w/0 heating the pipet. And even with a room temperature pipet, you'll have water move up the column to a certain degree in the case where the pipet were to be placed completely in the water

    So what caused the water to dramtically fill the pipet? Perhaps it's due to the shape of the pipet; the volume of the bulb at the end takes up a significant proportion.

    Despite all of this I am not able to see the utility of this experiment, it seems to be a very cheap way of explaining the ideal gas law.
     
  18. Feb 12, 2005 #17

    Gokul43201

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    Yes there will be some air that leaves the pipet as it gets warm. But it will equilibrate with Po in a fraction of a second (at least for any pipet that I'm familiar with). So, the initial pressure will be atmospheric. And even if it is not, there will be either a (i)pressure drop, if the pipet is stiff (isochoric reduction of pressure), or (ii) reduction in volume if the pipet has a soft rubber bulb (isobaric compression). Either way, equilibrium is restored by liquid entering the pipet.


    The hydrostatic pressure from a few inches of depth is negligible compared to an atmosphere.


    For a standard cylidrical pipet (one with a plunger, not a bulb) that's about 4 inches long, even if you completely submerge the pipet in water, the hydrostatic pressure will force the water up by only 1 millimeter. (Even with a bulb, it would go up only 2 or 3 mm.) If the diameter of the pipet is small (which is typical), capillarity will have a much greater effect than hydrostatic pressure.
     
    Last edited: Feb 12, 2005
  19. Feb 12, 2005 #18
    hey Gokul...what does urs signature statement mean.....what would appear as infinite/?
     
  20. Feb 12, 2005 #19

    GCT

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    At this point I believe that your conclusions is neither consistent with an open or closed system. It seems that you are assuming that the system (meaning the pipet and surroundings) equilibriates through a change in the number of moles (and thus your account of "some of the air" leaving as the pipet gets warm). But note that this is clearly an open system. And that there is a constant work input on the system, which makes it a dynamic system. I should review a standard text to clear things up conclusively , however no time right now.

    I'm not trying to undermine your final conclusion that you arrived at, certainly the above is just technical and your conclusion (the "clearing up" post) remains very helpful.

    I'll get back to this later, or perhaps you can perform a order of magnitude calculation.

    And thus the I don't really see the significance of this experiment.
     
  21. Feb 12, 2005 #20

    GCT

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    blurry :uhh:
     
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