Boiling Salted Water Diatribe

April 22, 2010

Please excuse my tone when I speak about the following subject: 
Salted water does NOT make water boil faster! No, No, NO!
Kindly, do not site the numerous (and erroneous) internet sources which insist that it does.  It does not.   These oratory fables passed down from generation to generation are wrong.  Yes, great-great-nanna just threw up in her grave a little.

As a saving grace, salting boiling water is the right thing to do, but just for the wrong presumptive reason. In reality, salting water helps to raise the boiling temperature of your pot of water.  Thus, you can cook (commonly) pasta at higher temperatures.  This helps to produce al dente noodles, not soggy ones.

The boiling point elevation is a colligative property, defined as a property that depends on the concentration of the solute (in this case, salt).  Loosely speaking, the salt helps to stabilize the water molecules.  When water is more stable, it is less likely to get up and go dancing out the door, or boil. Thus it takes even more heat to boil the water, thus increasing the boiling point.

Strangely enough, salt also decreases the freezing point.  Thus it is possible to have salty water at subzero temperatures. That is why people salt ice in the winter.  They want to lower the melting point, so that it will be prone to melt at ambient temperatures.

In the graph below, we can see the behavior of pure water (purple) vs. salty water (blue). The left 'prong' is the line that separates the solid phase (to the left) and the liquid phase (to the right).  The right 'prong' is the line that separates the liquid phase (above the line) and the gas phase (below the line).  ** Notice that the blue fork is wider than the purple fork.  That means that salty liquids can exist as liquids for a larger range in temperatures than can pure water.


How much will the boiling point be raised? Well, in reality its peanuts. Bar nuts.

Roughly...
1 teaspoon salt/ 1 L water will get you a .1C increase in temperatures
1 tablespoon salt/1L water will get you a .3C increase in temperatures.
It is recommended that we add 1 heaping tablespoon salt to 1 gallon of water.  That's like a .08C/gallon. Pennies, really.

So, while salty water may raise your boiling point only a smidgen pigeon, it will ensure that your noodles are  well salted and flavorful.  However, it will not make your water boil faster.  If anything, it will make it boil slower.

2 comments:

Marc @ NoRecipes said...

LOL so scientific. I've never heard the myth that salting water makes it boil faster. The lower freezing point is also why you add salt to old fashioned ice cream makers. It allow the ice to melt but still be below freezing so it freezes the cream in the canister (which doesn't have salt in it)

Jonny said...

as an interesting (or not) corollary, I was recently watching BBC's "The Blue Planet" and was wrinkling my brow when narrator David Attenborough informed me that sea water at the poles can be liquid at below 0 degrees Celsuis. now I understand why. culinary inspiration and sources of related information seem to be able to come from anywhere, it's just a question of ordering the collected details appropriately in one's brain. thanks for the lesson!

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