Ask A Scientist© Chemistry Archive

 >
 >    name         Jim
 >    status       educator
 >    age          30s

 >    Question -   I am a junior high science teacher. Our staff is trying
 > to find an explanation to the following question:
 >Why does salt water boil at a higher temperature than tap water?
 >AND, what causes it to heat up more rapidly than tap water?
 >
 >What is happening on the molecular level?
 >
 >Thanks,
 >
 >The Science staff at Jefferson Junior High

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   You are talking about ``boiling point elevation,'' one of the
``colligative properties of solutions of nonvolatile solutes,'' about
which you can find information in a college-level physical chemistry
textbook, such as ``Physical Chemistry'' by Peter Atkins.  You may
also find information in a freshman (general) chemistry textbook, such
as ``Chemistry'' by Oxtoby and Nachtrieb.  These two titles are the
best of each kind of book.
   A given mass of water will not heat more rapidly if you dissolve
some salt in it.  If anything, it will heat more slowly, because you
must heat the salt as well as the water.
   The water boils at a higher temperature because the salt cannot boil
with it (the boiling point of sodium chloride is very, very high).
Hence in order for the water to boil the salt must come out of
solution: it must undissolve.  You know that dissolving is a favored
process, since it always occurs instantly when you mix salt and water.
If dissolving is favored, then undissolving must be unfavored -- you
never observe salt and water suddenly undissolving by themselves.
Therefore extra energy (as heat) must be supplied to the salt/water
solution in order to boil it, because extra energy is needed to
undissolve the salt.  I should say, extra energy *per atom or
molecule*, which is the same thing as a higher temperature.
   You may hear the argument that the salt atoms at the surface of the
water block the water molecules from leaving.  This is nonsense.  If
you put a thick layer of oil on the water, which blocks the entire
surface, you will not change the boiling point a whit.
Grayce
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 >>    name         Jim
 >>    status       educator
 >>    age          30s

 >>    Question -   I am a junior high science teacher. Our staff is
 >>trying to find an explanation to the following question:
 >>Why does salt water boil at a higher temperature than tap water?
 >>AND, what causes it to heat up more rapidly than tap water?
 >>
 >>What is happening on the molecular level?
 >>
 >>Thanks,
 >>
 >>The Science staff at Jefferson Junior High

I can answer your first question.

First, for reference, an attempt to "draw" a water molecule with its proper
shape:

   O
  / \
H   H

Water, as you may know, is a polar covalent molecule.  This means that the
electrons in it are distributed unevenly (they tend to hang out near the
oxygen atom).  Thus, one end of every water molecule (the hydrogen end) has
a positive "half-charge," and the other (oxygen) end has a negative
"half-charge."

As you may also know, when salt dissolves in water, it separates into Na+
and Cl- ions.  These ions are attracted to water molecules because of their
polarity.  This is why salt dissolves so well in water.  It also means
that, if a water molecule wants to escape a salt solution, as it does
during boiling, it has to overcome not only the attractive forces of the
other water molecules (as it would in pure water) but also the attractive
forces of the ions in the solution.  Since it has to escape stronger
forces, it needs to be at a higher temperature.

As to your second question, I'm not sure salt water does heat up more
quickly than tap water.  What evidence do you have to support this?

Hope this helps - it's quick and dirty.  Email back if you need part or all
of it clarified.

-Spence

Spencer Pasero   *"Must fight on...for everyone who has never seen _The
spaser19@idt.net *Octagon_!"
                  *                                             -The Tick
                  *
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The higher boiling temperature is a simple matter.  When a liquid boils,
what is actually happening is that the vapor pressure of the liquid equals
the atmospheric pressure, so that bubbles of the vapor can appear in the
liquid.  At 100 degrees C, the vapor pressure of pure water is 1
atmosphere.  So, when pure water is heated to 100 degrees C, the vapor it
produces has enough pressure to actually push back the atmosphere, which is
the process we observe as boiling.

If another substance, like salt, is added to the water, the vapor pressure
is reduced.  The amount that it is reduced depends on what fraction of the
molecules in the solution are the volatile component (water).  For
instance, if 90% of the molecules are water, the vapor pressure of the
solution will be 90% of the vapor pressure of pure water at the same
temperature.  So, when salt is dissolved in water, the vapor pressure of
the saltwater will be a little less ao 100 degrees C than the vapor
pressure of pure water.  To get the saltwater to boil, you need to raise
the temperature still higher, to get to the point where the vapor pressure
of the solution is 1 atmosphere.  Then it will boil.

As for saltwater heating up faster than pure water, does this really
happen?  I can't think of any obvious reason for it to.


                Richard Barrans Jr., Ph.D.
                Chemical Separations Group
                Chemistry Division CHM/200
                Argonne National Laboratory
                9700 South Cass Avenue
                Argonne, IL 60439
                richb@anl.gov
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