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Salted water -- Boiling point elevation
Posted By: Wolfspirit, on host 64.229.210.43
Date: Tuesday, October 9, 2001, at 16:07:49
In Reply To: Re: 7th *Heaven*? posted by Don the Monkeyman on Saturday, September 29, 2001, at 00:34:47:

> > > > It couldn't possibly be Celsius. How would you, in that case, under normal air pressure conditions, aquire a water temperature of *over* 100 degrees?
> > >
> > > Salt the water.
> > >
> > > Don "Simple enough" Monkey
> >
> > I knew that salt would lower the freezing point of water. I didn't think that it also increased the boiling point.
> >
> > HEY WOLFSPIRIT! Is this true?
> >
> > Gri"not a scientist"shny
>
> AHEM. I am also a scientist, and yes, it is accurate. Adding a solute depresses the freezing point and raises the boiling point for a liquid.
>

That's true. The more solute (like salt) that one adds, the higher the boiling point will rise. Actually, adding any non-volatile soluble substance will raise the boiling point of pure water. This explains why having antifreeze (a mixture of 50/50 ethylene glycol and water) in your car radiator will prevent the water from boiling off in the summer, while simultaneously protecting it from freezing to ice in the winter.

> In fact, there is a common household application for this: pasta recipes (such as Kraft Dinner) often specify that you salt the water when boiling pasta. Some may think it is for flavor, but it is actually done so that the water boils at a higher temperature, which is, apparently, good for cooking.
>

Sorry, no. You add salt to the pasta water only for the flavour -- it's not enough to make the water boil at a higher temperature. Experiments at different atmospheric pressures show that cooking time is indeed reduced to a duration half as long if the water's boiling temperature is raised by 10-20°C. But to raise water temperature by this amount -- purely via addition of a solute -- would require a truly *enormous* quantity of additional salt.

For example, suppose you wanted to cook 900g of dry pasta in 4 liters of water, as fast as possible. Suppose you also wanted to increase the boiling temperature by 5°C, in order to cut down cooking time by a small amount. Unfortunately, you'd have to dump in 1140g of salt to raise the boiling point to 105°C. This works out to adding 56 Tablespoons of salt -- which, needless to say, would make the pasta rather inedible.

> (I said I was a scientist, not a cook.)
>
> Don "Miffed that Grishny would not accept MY word. :-p" Monkey

Wolf "Somehow kookily thought that you were an Engineer, not a Scientist" spirit