[soggybread]

If two chemicals are mixed together, say:



If I had varying concentrations of this solution, why is it that the higher concentration mixtures will precipitate at a higher temperature? and lower concentration chemicals will precipitate at a lower temperature?

P.S. -- Is it true that CaSO4 dissovles less when there's *more* heat? I found a claim to that on: http://www.scienceforums.net/forum/archive/index.php/t-6152.html

Thanks very very much for this *delete me*

[Yggdrasil]

Think of it this way.  Lets say you have a 100mL of a high concentration solution (100mg/mL) and 100mL of a low concentration solution (10 mg/mL).  In the high concentration solution, you have a total of 10g of your solute whereas in the low concentration solution, you have 1g of solute.

Now, you know that solubility of solids in liquids increases with increasing temperature.  Lets say 1g will disolve in 100mL at 25^oC.  This means that at 25^oC, the low concentration solution would be completely dissolved, but the high concentration solution would have precipitate in it (9g of precipitate to be exact).  At a lower temperature, both solutions would contain precipitate.  Now lets say 10g dissolves in 100mL at 60^oC.  This means that at and above 60^oC, both solutions would be completely dissolved, but below 60^oC, only the high concentration solution would have precipitate.

Therefore, if you heat both solutions to boiling and see when precipitate forms, you will see something like this:

Temp....100....60....50.....25.....10
High Conc.....clear....clear/ppt.....ppt....ppt....ppt
Low Conc.....clear...clear.....clear....clear/ppt...ppt

So, as you can see, the higher concentration mixture precipitates at a higher temperature.

[soggybread]

Thanks very much for clearing this up for me! But to clarify, is this caused by the higher level of entropy vs. lower level on entropy?

[Borek]

Now, you know that solubility of solids in liquids increases with increasing temperature.

Good rule fo thumb, but sometimes it doesn't hold. Li₂SO₄, MnSO₄.