Oh joy. After lurking for something like 6 or 7 years (OK, I read Cecil and haven’t been to the message boards since 2003) I have something constructive to add. Below is a column I wrote for a student newspaper in New Zealand about salt in cooking. To be posted in a few minutes once I get it all together is a series of letters I exchanged with a “natural salt” company about the presence and lack of impurities.
(The column was modeled after gurii like Cecil and ran for 3 years)
Q: Is it true that the reason we add salt to water when cooking is to make the water boil at a higher temperature?
Sounds good doesn’t it? If the salt water boils at a higher temperature then the food in the water will cook more quickly. Remember, as a rough rule of thumb, reactions occur twice as quickly for each 10 degree increase in temperature. (This doesn’t really apply to many of the changes that occur during cooking, but we’ll assume it is true for the moment).
It is true that dissolving things (like table salt) in a solvent (like water) increases the boiling temperature of the resulting solution (and decreases the freezing point). The reason for this isn’t easy to explain in the space I have, but, put simply: Boiling occurs when the vapour pressure of the liquid is greater than the vapour pressure of the air pressing down – this is why water boils at a lower temperature at altitude. Now adding salt (or sugar etc) to water decreases its vapour pressure so the solution must be more strongly heated for the vapour pressure to exceed that of the atmosphere. Adding salt decreases the vapour pressure because (crudely put) the same amount of energy now has to be spread around more molecules (the original water plus the added salt). Thus, we can see that the change in boiling temperature is proportional to how much salt has been added. But is it enough to make a difference when cooking?
In the case of water, the boiling point will be increased by 26 degrees times the fraction of added salt measured in moles. (The mole fraction of salt = (mass salt divided by 58) divided by (this same number plus mass water divided by 18). That is F = (mass salt/58) / [(mass salt/58 + mass water/18)]
Yeah, I didn’t think you’d understand. OK for …
1 litre of water the boiling point will increase by about 1/120th of a degree for each gram of salt you add. So If you add a heaped tablespoon of salt (lets say 60 grams) of salt to a medium pot containing 2 litres of water then the salt water will boil be elevated by 0.4 degrees to 100.4 (and the freezing point decreases by the same amount). This is nowhere near enough to make the slightest bit of difference to how quickly the food cooks, at best it will decrease the time of cooking pasta by around 15 seconds. So, the reason salt is added to water is for taste.
Oh. 'merkin readers should note that like 95% of the world I use degree C. (Actually I’m a chemist so I use K but you know what I mean).
I hope that explains it.