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I would like to have a good understanding of what is happening when you add salt to boiling water.

My understanding is that the boiling point will be higher, thus lengthening the process (obtaining boiling water), but at the same time, the dissolved salt reduce the polarization effect of the water molecules on the heat capacity, thus shortening the process.

Is this competition between these two effects real ? Is it something else ?

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Perhaps a perfect example of a physics/chemistry crossover question that we should allow. –  Nick Nov 3 '10 at 16:31
    
We did this this in first-semester chemistry. I'll try to come back when I have opportunity. –  Mark C Nov 7 '10 at 3:56
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4 Answers

up vote 2 down vote accepted

I think the dominant effect might actually be the fact that the salt you add might not be at boiling temperature. But this is just based on the fact that the boiling-point elevation due to salt in water is actually quite low for typical amounts of salt used in cooking, say. I'm not too familiar with the second effect you mention though.

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This is not the dominant effect. –  Ron Maimon Jul 15 '12 at 8:14
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Okay, first we have the phenomenon: Yes. adding salt increases the boiling point of water, which means that you have to input more energy to get the water to boil, but your egg or pasta will cook faster once you do, because the water will be hotter.

Then there's the why. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is the same as the atmospheric pressure above the liquid. If we can artificially increase the vapor pressure of the liquid, we decrease the boiling temperature. If we can artificially decrease the vapor pressure of the liquid, we increase the boiling temperature. So the question has now become: why does the vapor pressure of water decrease when we add salt to it?

So imagine a pot of water. At any given temperature there will be some water molecules in the gas phase above the pot (that's the origin of the vapor pressure), and some in the liquid phase in the pot. The proportion in the two phases is determined by the interplay of lowering potential energy (by decreasing elevation in gravity, by forming hydrogen bonds, by lining up the polar ends of the molecules, etc.) and increasing the entropy (there's more accessible states in the gas phase, most liquids are incompressible, etc.). The potential energy part favors the liquid phase, while the entropy part favors the gas phase. The real requirement here is to minimize the free energy, F = U - TS, with F the free energy, U the potential, T the temperature, and S the entropy. Since S is paired with the temperature, increasing the temperature increases the impact of the entropy part, which is why the vapor pressure increases as we increase the temperature.

So now we toss in some salt, while keeping the temperature fixed. The volume fraction of the water decreases, and suddenly there are new accessible states for the water molecules in the liquid phase -- so the vapor pressure decreases. We keep adding salt and the vapor pressure keeps decreasing. If we keep going, eventually there's no vapor pressure.

Raoult's law says that the vapor pressure of a solution is proportional to the vapor pressure of the pure solvent (basically that there is a straight line between the pure vapor pressure and zero, when we've buried it in salt). That's taken as the definition of an ideal solution. Real solutions have a curved functional form between the two boundary conditions, with the deviations from linearity coming from interactions between the solute (the salt) and the solvent (the water). Those interactions might be things like breaking up the network of hydrogen bonds in the water, disrupting the polarization arrangement (both of which will favor gas phase), or bonding/pairing up with water molecules (which will favor liquid phase). At relatively low concentrations of solute the interaction effects are pretty small, so the dependence of vapor pressure on solute concentration remains roughly linear. The cool observation though is that at most temperatures and for most solvents, it doesn't matter what solute you use (as long as the solute itself doesn't have a vapor pressure), the vapor pressure of the solvent is still decreased by adding solute (which indicates that the entropic contribution is the most important part, and the interactions don't play a big role).

Now to sum up: for a given concentration of salt dissolved in water, there are more states accessible to the water molecules in the liquid phase than there are in pure water. So at every water temperature as we pour in energy to make it boil, there will be a lower vapor pressure than there would have been without the salt, and thus we won't get to the boiling point until the water has reached a higher temperature (until we've poured in more energy than we would have had to). Salt does disrupt the network of hydrogen bonds in the water molecules, but the effect isn't very big at reasonable concentrations of salt, and it's never big enough to counteract the entropic effect.

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good theory

how about a test

my niece just did 3 trials each on 2 cups of water and varied the number of tablespoons of salt

0 and 1 tablespoons boiled at about 10.5 minutes 2 tablespoons boiled at about 9.3 minutes 3 about 7.5 minutes and 4 boiled at about 6 minutes.

and now she wants to know why she got those results

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Nice experiment--- this is a shocking property of salt in water. –  Ron Maimon Jul 15 '12 at 5:54
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The competition is real, and it's no contest. The reduction in specific heat from the polar effect swamps the miniscule elevation of the boiling point.

When you dissolve salt in water, it makes ions in solution, and the ionic atoms trap a cage of water around them immobile. The net effect is that you reduce the number of degrees of freedom, and you reduce the specific heat. So a given amount of heat energy is more effective at heating salt water than ordinary water. The effect is large, see kwinb's answer for a qualitative experiment.

This means that when you add salt to boiling water, the act of dissolving the salt (which keeps the internal energy fixed, or releases internal energy), is more than enough to heat the water to the new boiling temperature. I have often added salt to boiling water, and I used to expect it to stop boiling momentarily, to catch up to the new boiling point. Instead, I consistently noticed hyper-boiling where I added the salt, and no time-lag. The reason is the heat released as the salt is dissolved.

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Just a thought - if you're adding salt to boiling water, the crystals will be providing nucleation centres before they dissolve. I doubt this is the main cause of the increased boiling you see, though. –  Benjamin Hodgson Sep 24 '12 at 10:44
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