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Water in an open container immersed in a heat bath at BP

  1. May 1, 2014 #1
    I realize this is probably pretty basic, but it has me stumped:
    An open test tube containing water is immersed in a heat bath of boiling water. The water in the test tube does not boil. Why?

    Would this be because the water in the test tube is at a temperature where it two phases (liquid and vapor) co-exist?
  2. jcsd
  3. May 1, 2014 #2


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    Gold Member

    What you have is a crude type of thermometer, and evidently this shows how when measuring temperature by any instrument you have to be sure it is indicating the temperature accurately.

    Primarily this a heat conduction problem ( and phase change for this particular case )at this temperature, although convection and radiation will play a part, more so at different temperatures, radiation most especially at higher temperatures.

    Heat flows from a hotter body to a colder body.

    For heat to flow into the test tube, it has to be at a lower temperature, however slight that may be, than the heat bath. The greater difference in temperature the more heat will flow, diminishing as the temperature difference decreases. This is one type of thermal lag effect.

    As the temperature of the water in the test tube increases, and turns into vapour, heat is taken away. There is also a heat loss from the test tube body exposed to the air.

    We thus have heat flow from the boiling water bath into the test tube through a barrier and heat being removed via the vapour and the test tube body. If the heat being removed is greater than that entering then the test tube water will not boil.

    If you immerse the whole apparatus in a room such tht suppose the steam collected from the bath remains at the same temperature of the bath, several of these sources of 'error' can be minimized or eliminated, you will then have the conduction from the bath through the test tube wall to test tube water being the 'source of error' for your crude thermometer. If then you wait long enough, the test tube water should begin to show signs of slight boiling if and when the two masses come to thermal equilibrium.
  4. May 1, 2014 #3
    Thank you very much! This answers my question perfectly!
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