MadSci Network: Chemistry

Re: what causes salt to make the tempture of ice water even colder?

Date: Thu Mar 31 18:58:10 2005
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 1112219598.Ch

Greetings, Tyrel:

I'll begin by recommending you first study this earlier Answer to a
closely related Question:

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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