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Name: Juan M.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 2000


Question:
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 solution)?


Replies:
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.
Assistant Director
PG Research Foundation, Darien, Illinois



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