Ask A Scientist© Chemistry Archive

 > >    name
 > >    status       student
 > >    age          12

 > >    Question -   I am doing a science fair project on microwaving
 > coffee
 > > and also water then adding sugar to cause a reboil.  so far my data
 > shows
 > > that when I put in the sugar the reaction occurs but the temperature
 > does
 > > not go up to 212F again.  I tried so far water heated to boiling
 > then
 > > adding salt also.  There is only a fizz reaction not reboil.  What
 > is
 > > going on here if the temperature is not at 212F?  Is this a chemical
 >
 > > reaction with released gases?  Is this because of the shape of the
 > > sugar?  I have been using a glass coffee cup.  Does that make a
 > > difference?  I have searched for information in the library and on
 > the
 > > internet but have found only one reference so far.  Do you know
 > where I
 > > can go look up more for my project.


Try colligative properties as a starting point in your search to
determine the boiling point of a liquid.
Important point, you have to take atmospheric pressure into account in
order to normalize the boiling points to STP.
H. Myron


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What you are doing is superheating the water or coffee, that is, heating it
above its boiling point.  In a glass vessel with no scratches, this is
fairly easy to do, because imperfections in the glass, extra particles,
etc. trap air and give steam bubbles a place to form.  If you don't have
any surfaces that help the steam to form, the water can be heated above its
boiling point, but not actually boil.  When a bubble manages to form,
however, the liquid will boil violently until its temperature drops to the
boiling point.  This process is called "bumping," and is of very practical
concern to chemists.  (Sometimes the bumping is so violent that the
containers are broken.)

When you add salt or sugar, you are providing lots of irregular surfaces to
the liquid, allowing the boiling to begin.  Other solids can do this as
well, such as powdered creamer, instant oatmeal, and toothpicks.

To find references, you may have some luck looking under "nucleation" or
"superheating."


        Richard E. Barrans Jr., Ph. D.
        Chemistry Division
        Argonne National Laboratory
        richb@anl.gov

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