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The positioning of the thermometer

  1. Apr 3, 2013 #1
    Hello, I am instructed by my chemistry professor to design an experiment to measure the temperature at which a saturated solution of Sodium Nitrate will begin to boil.

    If the reagents are pure this temperature will be constant .

    Can someone tell me why does the lab instructor suggests that the thermometer be placed on the surface of the liquid and not completely inserted in it. (IE he advises that it is essential to place the bulb, half immersed and the other half not immersed in the solution).

    Is this necessary ? Why ?
     
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  3. Apr 3, 2013 #2
    I don't know either. If you put the thermometer at the top, convection will create a hot top effect, and the temperature at the top will be higher than that at the bottom. What should be important is the temperature at the bottom though, since boiling starts at surfaces. Maybe the thermometer acts like the surface required for boiling and only first bubble matters?
     
  4. Apr 3, 2013 #3

    Borek

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    Think about pressure.
     
  5. Apr 3, 2013 #4
    A thermometer doesn't "measure" pressure so i am guessing the key point over here might be that the thermometer ( which is in fact a glass thermometer) might break ?

    But if that were true, why the need for partial immersion ?
     
  6. Apr 3, 2013 #5

    Borek

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    What is the pressure at the liquid surface? At the bottom of the vessel?
     
  7. Apr 3, 2013 #6

    While the solution is boiling, the pressure at the surface of the liquid will be greater than the atmospheric pressure (14.7 PSI).
    The pressure at the bottom of the surface will be the sum of the pressure due to the hieight of the liquid and the vapour pressure above the liquid.

    Will that be enough to break the bulb ?
     
  8. Apr 3, 2013 #7

    Borek

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    Why should it?

    Sigh.

    How does the boiling temperature depend on the pressure?
     
  9. Apr 3, 2013 #8

    chemisttree

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    Does the thermometer have a line completely around the circumference? If present it is generally below the lowest indicated temperature on the scale and above the bulb. It is called the immersion depth and the thermometer is calibrated to read most accurately when immersed to that line. If it isn't there you have a total immersion type. Putting those in halfway might be a good compromise between accuracy and keeping the bulb from breaking on the bottom of the boiling solution.
     
  10. Apr 3, 2013 #9
    Simple, since the air just above the surface will be most saturated with the vapors (water) which would exert a pressure along with the atmospheric pressure. Thus the total pressure would just be the sum of the two pressures . (first by vapors and the second by atmosphere).

    Let me convince you ... if u boiled pure water in a closed container , wouldn't u agree that the pressure right on the surface would be greater than the atmospheric pressure. (it is obvious that it would).

    Now moving on to a similar situation, where u remove the lid and then boil pure water. Would the pressure still not be slightly higher than the atmospheric pressure ?

    As the pressure increases the Boiling temperature of any liquid would tend to increase . (I really can't see where are you trying to take me :D )
     
  11. Apr 3, 2013 #10

    DrDu

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    No, the partial pressures of water, oxygen and nitrogen (+rest) will add up to atmospheric pressure. If there is more water vapour at the surface, then there is less air.
    However I also don't get the point of Borek. While water well below the surface may be under higher pressure and thus not boiling, it should have the same temperature as the boiling surface in thermal equilibrium.
    I would measure inside the liquid to optimize heat exchange between the thermometer and the liquid.
     
  12. Apr 3, 2013 #11

    Borek

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    I don't think it can be at equilibrium, it is heated from the bottom and evaporates on top, so I would expect a slight thermal gradient (even after taking mixing into account).

    At least that's how I interpret the idea of measuring temperature at the surface.
     
  13. Apr 3, 2013 #12
    I find that plausible, but would you care to share some experimental results to back this hypothesis ?

    If i understand your claim correctly: (correct me if i misunderstood)

    The temperature at the surface is the true boiling temperature of a solution while that deep inside the liquid is subject to inaccuracy due to the strong heat from the bunsen burner .
     
  14. Apr 3, 2013 #13

    DrDu

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    Yes, this is probably one of the main problems but I don't think it is related to pressure differences.
     
  15. Apr 3, 2013 #14

    Borek

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    To some extent it is - as the pressure inside of the liquid is higher, the pressure difference allows this gradient. Or at least it gives the upper limit for the temperature gradient.

    No, that's not what I am thinking about. Liquid below surface can be slightly hotter not because of the strong heating, but because additional pressure keeps it from boiling.

    To exaggerate: imagine a huge and deep tank 10 meters high, filled with water. If it is boiling at 100°C at the surface, at the bottom it can be even 120°C and not boiling yet.

    Sure, it is an unstable situation, as it would probably flash boil when moving up due to convection, I am just trying to show what I am referring to.
     
  16. Apr 4, 2013 #15

    chemisttree

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    Never heard of this depth vs bp in a lab experiment. Boiling always starts at the bottom because that's where the heat is strongest. If this effect were significant, boiling would occur only on the surface... never seen in my experience. All instructions I've ever seen state to completely immerse the bulb in solution to determine the solution bp.

    Half immersion is bad technique which will give arbitrary errors on the low side. Either OP is misremembering or the teacher doesn't know what he/she is doing. Could be both.
     
  17. Apr 17, 2013 #16
    Guessing - the vapour is formed at the bottom of the vessel, increasing slightly the concentration of the salt which will start to form a supersaturated liquid near the bottom, although it is likely to diffuse rapidly. Best results are more likely to occur where the liquid is only saturated near the surface. Vapour at the surface will be at equilibrium temperature with the saturated boiling liquid. Try both ways and see what you get.
     
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