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Boiling Point Lab

  1. Oct 24, 2015 #1
    In class, we did a lab to determine the boiling point of rubbing alcohol. We used a thiele tube filled with water and place a test tube attached to a thermometer w/rubber band in it. However, we never saw any signs of boiling. We conducted several trials, the temperature went all the way above 93 degrees, but there was no sign of boiling.

    Our teacher said we overshot it..but I don't think that makes sense. Even if the temperature of the water exceeded the boiling point of the alcohol, the alcohol should have begun to boil.

    Also, I don't understand the point of using the water and thiele tube setup. Why not directly heat the rubbing alcohol and measure the temperature in it? The temperature will stop changing at the boiling point allowing for easy detection.
     
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  3. Oct 24, 2015 #2

    mfb

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    Did you use boiling chips?
    Superheating (if it happened) can be dangerous.
     
  4. Oct 24, 2015 #3
    No we did not. Do you know why we didn't see it boil? And why not directly heat the rubbing alcohol and measure the temperature in it? The temperature will stop changing at the boiling point allowing for easy detection.
     
  5. Oct 24, 2015 #4

    mfb

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    Superheating is an option. It also a reason why temperature doesn't have to stop at the boiling point.
     
  6. Oct 24, 2015 #5
    What does that mean?
     
  7. Oct 24, 2015 #6

    mfb

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    What is unclear? Did you look up superheating?
     
  8. Oct 24, 2015 #7
    I know what it is, but how and why does it happen. And what might have been the cause in this case?

    If it helps according to the procedure we were supposed to place a capillary tube in the test tube, but our teacher said that was not necessary.
     
  9. Oct 24, 2015 #8

    Borek

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    Capillary tube plays exactly the same role boiling stone does.

    Your teacher explanation as quoted ("you overshot") doesn't sound correct. Yes, it is possible the liquid was overheated, other than that it is not possible to miss the boiling point. Your thinking (boiling should keep the temperature "locked" at the boiling point) is right.
     
  10. Oct 24, 2015 #9
    So what happened? Superheating? And if so why?

    And what was the point of using the thiele tub setup? Why not just heat the rubbing alcohol directly in a beaker and wait till the temperature plateaus?
     
  11. Oct 24, 2015 #10

    Borek

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    Think about the size of the sample required for a direct heating, compared to a sample placed in a small tube.

    Any page explaining superheating will discuss why it happens. Have you read about it?
     
  12. Oct 24, 2015 #11
    But even if a small sample is used, the temperature will still plateau. Yes I did read about it but I don't quite understand it.
     
  13. Oct 24, 2015 #12

    Borek

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    How long till the small sample disappears? When using water bath changes of the temperature are much slower, which means it is much easier to control.

    What do you know? Which part you don't understand?
     
  14. Oct 24, 2015 #13
    It might take a very small unit of time, but while it is disappearing you can still read the temperature....correct?

    So I read that superheating occurs when surface tension added with atmospheric pressure creates the need for greater vapor pressure. But when and why does this happen? And why in particular must it have happened in this particular lab?
     
  15. Oct 24, 2015 #14

    Borek

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    Not necessarily. Thermometer doesn't react to temp changes infinitely fast, it has to be warmed up/cooled down.

    Whenever there are no places on which the bubbles can start to build. So called "nucleation sites". If the liquid is pure enough, if the walls of the container are smooth enough, vapor bubbles don't build up easily enough.

    To some extent it is random, sometimes it happens, sometimes it doesn't.
     
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