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Name:   Dr. Phillip R.
Status:   other
Age:   40s
Location: N/A
Country: N/A
Date: 1999-2001


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.


Replies:
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.



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