Chemistry 101

[rmyers@olie.wvitcoe.wvnet.edu (Robert Myers)]

I've been having mailer problems.  If this hits the list twice (or more?),
my apologies.



The effect of solutes on the boiling point of a solvent can vary depending
upon the particular solute involved.

In general, unless the solute is a quite low boiling point substance itself,
the boiling point of a solution is elevated above the boiling point of the
solvent.  Ethylene glycol (EG) by itself boils at 197 deg Celcius.  This
boiling temperature will assure that a mixture of ethylene glycol and water
will boil at a temperature above the boiling point of pure water and will
freeze at a temperature below the freezing point of pure water.  The
equations are DeltaTf = m*Kf and DeltaTb = m*Kb where Kf is -1.86 (the
cryoscopic constant) and Kb = +0.52 (the ebullioscopic constant) and m is
the molality of the EG.  Molality is defined as the number of moles of
solute per Kg of solvent.  A 50% solution (by weight not volume) of EG in
water will freeze at -24 deg F or -31 deg C.  (These solutions are usually
made by volume not weight and calculated temperatures will therefore be
different.)  These numbers assume that the solute is a non-electrolyte (they
don't form ions in solution).  The calculated temperature changes can
approximately double or even triple when dealing with solutes which are
electrolytes (the DO form ions in water).  The exact amount of this effect
depends upon the number of ions produced per mole of solute.  For NaCl it
will double.  For K2SO4 it will triple.  For Na3PO4 it will quadruple.  (As
a first approximation.)

The effect is that the liquid range of the water solution is extended at
both ends.  It freezes lower and boils higher.

Pressure increases the boiling point of the solution.  Boiling point is the
temperature at which the vapor pressure of the substance = applied pressure.
A pressure cooker keeps steam from escaping the cooking pot and results in a
higher boiling temperature.  This cooks the food faster.  A pressure
radiator cap has essentially the same effect.  Added pressure raises the
boiling temperature of the coolant and keeps it liquid at a higher
temperature.  That's why you don't remove the radiator cap from a hot
radiator.  If the coolant is hotter than the temperature at which it would
boil in an open container and you release the pressure the coolant will
suddenly boil and a spray of hot coolant will escape the radiator.

Heat transfer depends on several factors.  These include surface contact
areas, temperature gradient, flow rate and flow characteristics of the
coolant, specific heat of the coolant, amount of coolant, and thermal
conductivity of the materials of construction both for the engine and for
the radiator and probably several other factors I've omitted.  Solving this
problem is rather complicated and is perhaps best left for another exercise.

Bob
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   *  Robert L. Myers  <rmyers@olie.wvitcoe.wvnet.edu>      *
   *  Chair, Department of Chemistry, WVIT                  *
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