http://www.newton.dep.anl.gov/askasci/gen01/gen01021.htm
Question - In a controlled situation, where the heat input is equal,
which will come to
a boil first.... two gallons or pure distilled water, or two gallons of
pure distilled water with approximately three tablespoons of salt thrown in
when the heat is turned on? As to whether the pots are open or closed, it
doesn't matter, as long as both are in the same condition. Finally, there is
no pasta in the water.
Almost all the chef's in the country believe that the water will boil SLOWER
if you put a handful of salt in the water. I believe it will boil FASTER.
Sorry to be a pest, but thanks. Feel free to ask me an veterinary question
if you have any.
-------------------------
Dr, Phil,
As soon as any of the salt dissolves in the water, the boiling point of
the water
will begin to rise -- by about one half degree Celsius for every 58 grams
of salt
dissolved per kilogram of water. In fact, any non-volatile soluble
substance will
raise the boiling point of water. That is why antifreeze (ethylene glycol)
provides boiling protection in winter as it simultaneously provides freezing
protection in the summer.
Referring to the specific situation you described: I will assume that the rather
small amount of salt added (relative to the much larger volume of water)will be
completely dissolved well before ebulliation commences. If so, the salted water
will require more exposure to the heat before boiling than would the distilled
water. So the salted water "boils slower" than the distilled water. Nevertheless,
under these real-world conditions of low salt concentration, it would be
difficult to tell which pot boiled first.
Consider this experiment -- just do not do it: Bring two pots of plain water to
near boiling and then toss salt in one of them. The pot receiving the salt will
likely explode into violent boiling because the salt crystals provided nucleation
sites that would allow the water to vaporize as the salt fell through the
superheated liquid. Same thing would happen is you used fine sand. Under those
conditions, the salted water wins. However, that is not (or should not be)
the way things are done.
Regards,
ProfHoff 298
=========================================================
Almost all the chef's in the country believe that the water will boil
SLOWER
if you put a handful of salt in the water. I believe it will boil FASTER.
Salt in the water should raise the boiling point of the solution somewhat,
meaning that it will need to reach a higher temperature before it begins to
boil. So the chefs are correct.
If undisturbed water is heated to above its boiling temperature
(superheated), adding salt will cause it to immediately boil over. This
phenomenon is easy to observe with liquids heated in a microwave oven.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois