oh snap, i forgot to write my weekly article about something or other.
So here’s the most important science thing you’ve never known why.
ADDING SALT TO WATER
So people add salt to water on three occasions for three different reasons.
adding salt to ice
So sometimes we add salt to ice when we’re making ice cream or chilling a bottle of champagne. The reason is pretty high-school chemistry.
Ice (unless it’s super cold) always has some liquid water (and then a layer of slush) on its surface. The molecules are always in the process of breaking off from the big ice crystal; and re-attaching themselves to the ice crystal. If you add salt to it, the salt will dissolve in the water; and the ions will prevent the water from forming into a nice crystal lattice. So the ice melts, and doesn’t turn back into ice; EVEN THOUGH it stays at the same temperature.
This is handy when you don’t want to slip and fall in the winter; and it’s also handy when you want to use ice to cool a bottle of wine. heat exchange is a matter of surface area. The more surface area is touching, the faster the heat will flow. So if you immerse a bottle in a pile of ice cubes and compare the surface area of the ice touching the bottle, with a bottle immersed in a icey slush. the slush bottle will cool faster.
Adding Salt to boiling water to cook an egg
this is fun. folk wisdom tells us to add salt to boiling water when we want to boil an egg.
why the hell do we add salt? people who know how salt DECREASES the freezing temperature of water suggest that adding salt will increase the boiling temperature of water. So that the water will boil at 110 degrees Celcius, instead of 100 C. This is nonsense.
the answer is kind of coolllll.
what happens is, you know how sometimes, in the boiling violence of the water; the eggs get cracked? and then you get long elephant trunks of cooked egg-white sticking out of the shell? you know how that sucks?
adding salt to the water keeps that from happening.
i don’t know how it works? maybe the salt increases the heat capacity of the water? *shrug* maybe it’s magic.
adding salt to boiling water to make pasta
this makes the pasta taste good.
I believe I can add some more to the “salt + boiling water” thing…
When you add salt to water, the salt dissolves into it’s respective ions (Na+ and Cl-). These ions form an electrolytic pathway in the dissolved solution.
You see, pure water DOES NOT conduct electricity, because there are no free electrons floating around. By dissolving some salt, you get some electrons to play with from the ions, which allows the flow of electricity.
…Now here’s where my memory goes a little flat…
I can’t seem to make a connection in my head between an increase in electrical conductivity and thermal conductivity, but I seem to remember my AP Chemistry professor telling us that that was the case.
I remember making a mental image at the time of the salt creating “energy conduits” into the water, that allowed faster transfer of energy (heat or electrical) into the fluid. That’s why salt “appears” to raise the temperature of water, even without stirring the water (because the water is presumably more thermally conductive, and is sucking more heat from it’s surroundings).
I dont know, that may be wrong, now that I have 6 years of physics experience to look back on it, but it seemed right at the time…
Hm? I’m not sure about that second one. Boiling-point elevation when a solute is dissolved in a solvent is a pretty well-documented phenomenon: http://en.wikipedia.org/wiki/Boiling-point_elevation or any chemistry textbook really.
So water with salt in it should boil at a higher temperature, and if you wanted that for one reason or another, salt would do the trick.
Or are you saying that the boiling point does increase, but it’s irrelevant because there’s no point in cooking the egg slightly faster, and the real benefit is the leaking thing? I wouldn’t know anything about the leaking, or how much time it takes to cook an egg and what kind of time savings you can get with salt, then.
http://www.newton.dep.anl.gov/askasci/gen01/gen01021.htm here’s what some professor has to say on the topic.
adding 58 grams of salt to a kilogram of water will increase the boiling temperature by 1/2 a degree Celsius.
a pot of water to boil an egg (at least the ones i use) will take about 2 kilograms. so if i wanted to increase the boiling point to 110 C, i’d have to use about 2.3 KILOgrams. that’s more than a box of salt.
my aim in claiming that it doesn’t really have an effect refers to how the effect on boiling water isn’t as dramatic as is it on ice.
the egg-white snakes that grow out of cracked eggs if you boil them DO get eliminated by adding salt to the solution.
Ah, I see.