Ask A Scientist Chemistry Archive

Question:
Hello from Kennedy Jr. High.  We have been doing some experiments
with salt.  We know that salt will lower the temp of water by
3 - 5 degrees C when added to water.  We also know that salt
will lower the freezing point of water.  We are not sure why
What is happening? Is an endothermic reaction taking place?
Also, when adding salt to water, does the boiling point also
decrease?  Or increase or stay the same?  Our results seem
to indicate that the boiling point is reduced.  Is this true?
If so (or not) What is going on with the water and the salt molecules
to change freezing and boiling points?  Can you help?
Thanks
 
Answer 1:
When you add a salt to water and the container feels cold, yes, an
endothermic reaction is taking place.  The solution is grabbing heat from
the surroundings to get the salt to dissolve.  When you add a salt to
a pure solvent (say, water), the freezing point will go down and the
boiling point will go up.  That's because the vapor pressure of the pure
solvent is lowered (this would be easier to explain with a phase diagram).
Remember, the definition of BP is the temp at which the solution's vapor
pressure equals the external pressure.  By lowering the vapor pressure,
the whole phase diagram shifts...BP to the right (higher), FP to the left
(lower).  You may want to check out a intro level chemistry text or at
least draw out the phase diagram to see more clearly what gets lost is
just words here.  Did you ever try boiling water without heating it?
If you have the means, hook up a vacuum to a flask of water.  You don't
change the vapor pressure of the water, but you reduce the external
pressure enough so that the water does boil.  Good question!
-Joe Schultz

Answer 2:
One more thing...  The change in BP and FP is related to how much salt
is added to the solvent:  delta T (change in T) = Km  where K is either
what's called the molal freezing-point depression constant or molal
boiling-point elevation constant;  m is the molality of the solution
defined as moles of solute (salt) / kilograms of solvent.  By knowing
the constant for the solvent, how much solvent you have and how much
solute you add, you can predict by how much (in degrees C) the BP or FP
will change.  Hope all this wasn't too far out there for Jr. High?!
Good luck.
-JS

Answer 3:
Excellent question and answers...  I would just like to add
a couple of more comments...  The reaction that is endothermic
in this case is  NaCl(s) --> Na+(aq) + Cl-(aq)  ...  The
reason that the freezing point is lowered is as J. Schultz
said - the vapor pressure is lowered....  However there is a
simple analogy (that may be useful) which gives a simplified
picture...  Freezing involves the water molecules becoming
very ordered in crystalline lattice...  However when there are
foreign particles such as Na+ and Cl- ions in the water 
they hinder the formation of the crystalline structure because
they interfere with the intermolecular forces between the
water molecules.  Also, the freezing point depression and boiling
point elevation are called colligative properties...  that is, they
are properties that depend only on the number of solute particles and
not on their chemical properties...  The third famous colligative
property is osmotic pressure.
-Dr. Brown