Ask A Scientist© General Science Archive

name         Dr. Phillip R.
status       other
age          40s

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
========================================================
The pure water will boil first.  The salt will raise
the boiling temperature.

BTW: There are ways to harness the power from the sun
by running tubes of pure water through ponds with
heavily salted water.  The ponds be heated by the sun,
to temperatures well above 100 deg Celsius.  The pure
water in the tubes would be heated by the pond, and
then is used to run turbines and generators; I believe
that that pure water could actually boil inside the
tubes while they are inside the ponds.

-Wil
========================================================
Dr. Phillip -

Salt water will boil at a higher temperature - all else being equal - than 
fresh
water.  For the chef, this is an advantage.  It  means he can get the 
temperature
of the water higher before it boils and leaves the pot thereby increasing the
possible cooking temperature and cutting down the cooking time.  The actual
cooking time will be effected by the rate at which you can apply heat.  I 
would
expect that with large amounts of heat available... like on a big stove... 
this
would likely cut down cooking time.

This is not an answer from a chef, but seems scientifically logical.  I 
wonder if
you will find a chef among the Newton Scientists?!

Larry K.
========================================================
Given these new constraints, it is difficult for me to see what
process(es) would be happening that would make one or the other pot boil
faster. The three tablespoons of NaCl in 2 gal of water gives a weight
percent of about 1-2 %. This would not significantly alter the vapor
pressure, the heat of vaporization, the heat capacity, or the heat transport
properties of the water significantly.

I would like to know if anyone has done the measurement in a controlled way.

Vince Calder
=========================================================
Hi Dr. Phillip !
see, it is not a matter of  water with salt to boil slower or faster...
it is a matter of at what temperature the water will boil ...
Solutions have higher boiling points (and lower freezing  points)
than the corresponding pure solvent. So the  pure water in an open pan
has a boiling point lower than the water with salt. The exact value
depends upon the local atmospheric pression.
That is  also why salt is thrown  on icy sidewalks and streets because the
salt dissolves in the ice and lowers its freezing point. Therefore, the ice melts
because the outdoor temperature is no longer low enough to maintain it as a solid..
The extent to which freezing and boiling points are affected by solutes is
related  to the number of solute particles present in the solution. The higher
the concentration of solute particles, the more pronounced the effect.
Colligative properties of solutions are those properties, like boiling point
elevation and freezing point depression that depend on the number of solute
particles present in solution.
And thanks for asking NEWTON! Tell your friends about it!
Mabel
(Dr. Mabel Rodrigues)
=========================================================
Dr. Philip R.,

The water with the three tablespoons of salt in it will
have a higher boiling point when compared to the pure
distilled water.  In reality, the chef's are right in
the fact that the water will boil slower if one places a
handful of salt in it. This phenomena is a colligative
property of liquids in chemistry. I hope that this
answers your question.

Sincerely,

Bob Trach
=========================================================
Adding anything to pure water will both lower its freezing point and raise
its boiling point. (Which is the reason we add antifreeze to car radiators,
instead of using pure water). So, salt in water should raise the temperature
at which it starts to boil. Since the heat input is the same in both pots,
the pure water should reach its boiling temperature (100 C) before the salted
water reaches its boiling temp (>100C). I believe (guessing here) chefs add
the salt in order to achieve a higher temp, so that the food itself will cook
faster. So in a way you were on the right track but for the wrong reason....
The salted water starts boiling a little later, but the pasta is done sooner?
  Bon appetite!

Paul Mahoney, Ph.D.
=========================================================