When we add a compound (salt) to a solvent, the boiling points rises. But, what could we say about the speed the solvent reaches the boiling point? It's better to add salt before or after boiling water while making pasta?
Thermodynamics – How Salt Impacts Boiling Speed and Phase Transition
phase-transitionthermodynamics
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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.
From the book Freeze Drying by Georg-Wilhelm Oetjen:
Oesterle showed that not only can tBA speed up the sublimation of ice from amorphous freeze-concentrated mixtures, but also similar effects can be achieved with volatile ammonium salts such as ammonium acetate, bicarbonate and formate.
In other words, a scientist showed via experimentation that tBA (also known as tetrabutylammonium hydroxide, which is a type of ammonium salt) and other ammonium salts can speed up the sublimation of ice.
Here is some more information on ammonium salts, and here is some more information on tBA.
So, yes, certain salts can speed up the sublimation of ice.
Sources:
I am unable to access Oesterle's full paper, but here is a link to its abstract (the paper was entitled The influence of tertiary butyl alcohol and volatile salts on the sublimation of ice from frozen sucrose solutions: implications for freeze-drying).
Here is a link to a pdf of the book Freeze Drying; the quote above is from page 87.
(I assume you met sublimation as in the first sentence of your question it says "water ice will, over time, sublimate to vapor.")
Update:
Doing some further research, I've found salts that slow down the sublimation of ice (such as NaCl). What I'm finding is that it depends on the salt.
While I'm not sure, I believe I found a sort-of explanation of why salt sometimes affects sublimation - according to a chemistry.SE question (here):
In wood, most or all of the bonds between the individual atoms and fibre units are covalent; making them very strong...In ice, on the other hand, the units of the crystal is held together with comparatively weak hydrogen bonds, meaning that not a lot of energy is required for a surface molecule of water to escape...
In other words, perhaps salts of different kinds do one of the following:
- Don't do anything
- Weaken the bonds between the atoms, therefore speeding up sublimation
- Strengthen the bonds between the atoms, therefore slowing sublimation
From my research, I think that NaCl is one of the third type and ammonium salts like ammonium acetate are of the second type.
Please note that this explanation of the phenomenon is mostly speculation based on what I've found. It is mostly to show where I'm headed in my research on this topic.
Hope this helps!
Best Answer
You should add the salt befor you start heating the water.
For the technical details of why this is have a look at Why does salty water heat up quicker than pure water? and Why does adding solutes to pure water lower the the specific heat?. Salt lowers the specific heat of the water so for a given rate of heat input e.g. a given setting on your electric hob, salt water heats up faster than pure water. The reasons salt has this effect are a bit obscure, but are discussed in a paper I reference in my answer to the first question I've linked to.
There's a little more subtlety to the issue than this though. Adding salt increases the boiling point, so you need to get the salt solution hotter to make it boil. However for saturated salt solution the specific heat falls by about 22% while the boiling point only goes up by 8%, so the salt solution still boils faster even taking into account the elevated boiling point.
However salt solution is denser than pure water. Saturated salt solution is about 20% denser than water. So if you take a fixed volume of solution, rather than a fixed mass, the total heat capacity doesn't change much. For saturated salt solution the 22% decrease in specific heat is almost exactly compensated for by the 20% increase in density. Add the 8% elevation in boiling point, and for the same volumes saturated salt solution takes slightly longer to boil.
All of this is fun to calculate, but the small amount of salt used when cooking pasta is unlikely to have much effect.
See http://www.engineeringtoolbox.com/sodium-chloride-water-d_1187.html for various properties of salt solutions.