|MadSci Network: Chemistry|
Greetings, Tyrel: I'll begin by recommending you first study this earlier Answer to a closely related Question: http://www.madsci.org/posts/archives/2000-10/971726754.Ch.r.html As you will see, some of your own Question is entirely Answered there -- the part regarding the dissolving of ordinary salt in water and the cooling that results. ----------------- Dan Berger adds: There's one other thing to consider when adding salt to ice that's not mentioned in the linked answer: when you melt something, it absorbs heat. If you add salt to ice water, you lower the freezing point and some of the ice melts. But you've not added any heat! So the temperature of the ice/water mixture drops because of the heat used to melt the ice. ----------------- Regarding the other part of your Question, about the heating of water when salt is added, there is actually a kind of illusion at work! To see what I mean, set a pot of water to gently boil, and then stick in a wooden spoon and stir it. You will notice that this mechanical agitation of the water causes it to boil faster. Why? Because the process of agitating water mechanically is still a process that adds energy (heat) to the water, and since it is already at the boiling point, all it CAN do is boil a little faster! Well, when you dump a bunch of salt into boiling water, this mechanical-agitation phenomenon will be immediately apparent -- for a few seconds, just as you have already noticed. No chemical reaction of any sort is involved (it is illusory to think so!). THEN, with the salt dissolving in the water, an actual chemical interaction, heat is absorbed and the boiling stops for a time. So, why would your mom say adding salt makes her water hotter? Well, there are two factors at work. One is that impure water can have a higher boiling point than pure water. Obviously the dissolved salt makes the water impure, which means she can heat the water to a higher temperature before it starts to boil again. Now, ALLOWING the water to become hotter is not the same thing as MAKING it hotter (the stove will do that!), but since your mom CAN end up with hotter water, her casual interpretation of the details of the phenomenon is reasonable. The other factor behind your mom's statement is that the salty water can stay hotter longer than pure water. If you recall that hot water can dissolve rather more salt than cold water, then by adding a lot of salt to her boiling water, your mom is using the dissolving process to STORE heat. As the water cools, see, the salt will start to precipitate out of solution, and it can only do that by giving back the heat which had been absorbed during dissolving! Well, if the hot water stays hot longer, one might be tempted to think that it was hotter in the first place than usual -- certainly that must be the case for pure water only. Which again is why your mom can get away with saying that adding salt makes the water hotter. She is mistaken, true, but only in small ways.
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