Boiling Saltwater: Part Two!

I don't even know if the last post was that interesting, but it's got a second part now.  Oh well.

Another picture of water boiling (I don't know if there's salt in this one...).

So we know that adding salt to water increases its boiling point -- that is, the temperature at which water boils (which it cannot exceed in its liquid form).  Presumably, this would mean that raising the boiling point of water with salt would make it take longer to boil.

But this is not so!

Rather, salt water reaches a boil faster than an equal volume of regular water.  Oh, sweet counter-intuition.  Here's why:

There's this thing called specific heat, which is the amount of energy required to raise a certain amount of a certain substance by a certain temperature.  Typically it's one gram of something by one degree Celsius.  Water (H20) has a specific heat of 4.184 g/J*ºC, which means that it takes 4.184 Joules (a unit that measures energy) to raise the temperature of one gram of water by one degree Celsius.  Water has an unusually high specific heat compared to other common substances.

Dissolved salt (that is, sodium and chlorine ions) has a negligible specific heat compared to water.  This means that all the sodium and chloride ions heat up really quickly -- so quickly, in fact, that we can basically ignore the energy that they absorb.  Therefore, salt water consisting of 70% water and 30% salt will heat up approximately 30% faster than 100% water.  Pretty awesome.*

 *If you think stuff like that is awesome, anyway...

Boiling Saltwater

Life takes its toll, even for the halophile...

But here comes another blog post.

Adding salt to water will increase the water's Boiling Point.  What this means is that it takes a higher temperature to make the water come to a boil.  At sea level, water boils at 100 ºC (212 ºF), though at higher altitudes the boiling point of water decreases, and vice-versa, due to some other non-salt-related phenomena.

The reason that salt increases the boiling point of water is fairly straightforward.  Salt (NaCl), as I've mentioned previously, is an ionic compound.  Many ionic compounds (particularly those including sodium) are soluble in water, which means that they dissolve.  When salt, composed of sodium ions and chlorine ions, dissolves, it breaks down into its individual ions.

These ions are nonvolatile, which means that they do not evaporate.  Instead, they just absorb heat energy which would otherwise be absorbed by the water (H20).  Now, you might think that this means that you can add any old thing (like a shoe) to boiling water to "steal" its heat, and that's true... but all you'd accomplish is to make the water to take longer to boil.  Once it reaches 100 ºC (or a slightly different temperature depending on where you live), it's going to boil.  Adding salt to water makes it boil at a higher temperature, which has its benefits.

This guy knows.

 

Whatever temperature water boils at is as hot as that water is going to get (in its liquid form, anyway).  Gaseous water can get pretty dang hot, but unless you're cooking with a pressure cooker (which also raises the boiling point of water, by the way), all the steam that your boiling water produces is just going to cool down right away.  The benefit to increasing the boiling point of water is that it allows you to achieve higher temperatures, which means that you can cook your food faster or more thoroughly. 

If you're at a high altitude and your water boils at a temperature that's too low to adequately cook your food, you can add salt to raise the boiling point.

To give an idea as to the effect of salt on the boiling point of water, here's a data table produced by a fourth grader from Mankato, Minnesota six years ago (I like to have credible sources):