[taliesin]

I've looked and looked, but I can only seem to find what a hygroscopic reaction means, not why it happens.  For example, why does calcium chloride draw water out of the air?  (hint, the answer is not "because it is hygroscopic" -- I've discovered that much)  

[mike]

Maybe try reading this one, any good?

http://www.newton.dep.anl.gov/askasci/chem00/chem00541.htm

[taliesin]

There's a lot of stuff in there that seems to explain what a hygroscopic material is with lots of examples, but the article basically boils down to, "because the hydrate has less energy and is therefore more stable".  That’s closer, but still not a very good explanation.  It's actually interesting, because I have been running into this problem a lot.  Earlier, I was trying to find out why, when an inverted capillary tube is immersed in a liquid which is heated, when the vapor pressure exceeds atmospheric pressure, air escapes the capillary tube.  (This was an experiment to determine the boiling point of an unknown liquid).  The only real answer I could find was, "because air escapes the capillary tube".  So it seems that there are a lot of these things floating around, where the answer is not generally known, and the person answering gives a definition instead of an explanation.  (Which in real terms amounts to saying, "because that's the way it is").  Maybe in some cases that is the best answer.  Like in physics, they don't really know what causes frictional forces exactly, but they can measure the effects, and are happy with that.  Anyway, if anyone knows, maybe they could answer this, "why is the hydrate more stable than the salt in a hygroscopic material?"

[mike]

Yes it is interesting isn't it.
 I think this is why scientists get into science because you always seem to be able to keep digging for an answer to something. Things can be explained at a variety of levels, so it depends on which one satisfys your curiosity.

Personally  I like the "more stable" / "less energy explanation" an d explanations in terms of increasing entropy because this makes sense to me. I am sure there are physical chemists, physicists and even matheticians who can answer the questions on all sorts of levels.

Maybe even a philosopher could answer the question of why does calcium chloride feel the nned to draw water form the air  

Maybe someone else has some thoughts...

[taliesin]

      Ahh, but the point of my post was that people give definitions because they don't know the real answer and don't care to say so.  It's fine if you like the answer "because it's just that way", but it's not really an answer.  Just because the reason is not know does not mean that there is no reason, just that the reason is unknown.  I like the "more stable" explanation too, but its still not an answer, but just a restating of the original question, really.  The question "Why does CaCl2 draw water out of the air" equals "because the hydrate is more stable" since it just leads to the obvious question, "why is the hydrate more stable?"  It's actually very close to just saying "because it's hygroscopic", since it's not telling you anything at all about the chemistry involved.  In other words, sorry to say, it's just another definition.  Are there compounds that are just magically hygroscopic, while others are not, and for some unknown reason, they are more stable when they can draw water to them from the air to make hydrates?  Is the only way to tell if a given chemical will be hygroscopic or not through empirical evidence, or is there a way to predict it?  And where did this energy come from that is making it less stable?  Is there something about the chemical bond of the anhydrous salt that make it attract water?

There was an article on the Newton website (mentioned earlier) which actually talked about this too (sorry, can't find it again).  Some students were convinced that gravity is caused by the spin of the earth, and that if the earth weren't rotating about it's axis, there would be no gravity.  The writer went on and on about how people like to think they know something, even if they find out they are wrong later, rather than admit they just don't know.  (Actually, he called it an American trait, which says more about what he thinks of Americans than the trait, which is ubiquitous among those who don't like admitting they don't know, no matter what their nationality).  So really it is about knowing what you know and knowing what you don't know, which is a very important trait for anyone to have.