Name: Juan M.
When we are evaporating a solution (lets say a salt in
water), the boiling temperature is increasing with the concentration. If
the evaporation takes place at atmospheric pressure what will happen to
the temperature of the vapor phase: will it be higher than 100C
(superheated vapor)? or shall it evolve at 100C all along the
evaporation process (eventually at a lower temperature than the boiling
The vapor will leave the liquid surface at the same temperature as the
liquid. If the boiling temperature is above 100 C, the temperature of the
evolved vapor will be too.
But this vapor is only "superheated" compared to what it would be if it came
from pure liquid water. The presence of the solute in the water reduces the
water's vapor pressure; the boiling temperature will then be the
temperature at which the vapor pressure is 1 atm. The vapor is not truly
"superheated;" it is actually at equilibrium.
Here's why: if heat is removed from the boiling solution, or if the
temperature is lowered by an infinitesimal amount, then the process will
reverse and vapor will condense back into the solution. How can this be,
you wonder, when vapor at atmospheric pressure and an elevated temperature
is superheated? Simple. Saying that the steam is superheated means only
that it is too hot to condense to the pure liquid. It is NOT too hot to
condense into the hot saltwater. The concentration (mole fraction) of water
in the saltwater is lower than the concentration in pure water at the same
temperature, so the water molecules are not so eager to get away from each
other. This changes the energetics of vaporization from both directions -
evaporation and condensation.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012