[lordsumesh1]

Not a homework question, but as I have found here http://en.wikipedia.org/wiki/Magnesium_sulfate), here http://www.sciencelab.com/msds.php?msdsId=9927218, and in my lab work, MgSO₄ is more soluble in colder water. My question: why? What physical/chemical property of MgSO₄ allows it to be more soluble in cooler water. I can't for the love of chemistry find this anywhere. Thanks for any guidance in advance!

For anyone curious, I'll give a little context to where I ran into this. Was doing a freshmen biology lab and my labmates and I wanted to test the effect of magnesium ions on fermentation. We were given solid MgSO₄, and then had to dissolve it to known concentrations. As we sat there grumbling about the lack of prepared materials for such a short lab, we did a little math and found how many grams of MgSO₄ per litre of water and thus measured and added it to water. We were having trouble dissolving it all the way, so we set up a hot plate to heat it up, but I decided to do a little research out of curiosity. As I open Wikipedia, low and behold, we need to actually cool our solution rather than heat it. We got ice and we got it to dissolve fully as the solution was cooled.

[Corribus]

Solubility typically increases as temperature increases.  However water is a bizarre substance and its properties change in complex fashion as a function of temperature.  Some substances get less soluble in high temperature water because the polarity of water actually changes as temperature changes. 

Strange solubility patterns are often manifested in ionic salts in aqueous solution.  Sodium chloride, for example, shows only a very small temperature dependence of its solubility, and as you've pointed out, sulfates usually display complex patterns - solubility of sodium sulfate raises steeply over low temperatures, reaches a maximum at around 32 or so degrees, and then displays weak retrograde dependence thereafter.  From what I understand, it seems to have something to do with the associated hydrate portion of the crystals, which turn into a biphasic mixture at a specific temperature due to melting.  (Sodium sulfate is a decahydrate complex, with 10 water molecules cocrystalized with each single sodium sulfate unit - I'm assuming your magnesium sulfate is something similar.) 

Anyway, that's the best answer I could come up with after 10 minutes of research.  The simple explanation is I guess that sulfates are complex hydrated salts, so complex relationships between temperature and solubility might be expected.

[Babcock_Hall]

I am mediocre at thermodynamics, but dissolution of calcium sulfate (and barium sulfate) in water occurs with a decrease in entropy, unlike potassium chloride for example, which occurs with an increase in entropy.  A quick look at calcium, barium, and sodium sulfates (Chemistry, E. L. King, 1979) indicates that each one has a different behavior of solubility versus temperature.  Calcium sulfate increases in solubility; barium decreases; and sodium has a discontinuity (because the solid phase changes IIUC).  Hmm... sounds as if this is more complicated than I thought, but I think one could start by looking up ΔS and ΔH.

[AWK]

Solubility of magnesium sulfate^.7H₂O (anhydrous, g/100 g of water)
10 C 30.9
20 C 35.5
30 C 40.8 and so on (still increase with temperature)
About 50 C hexahydrate exists. Its solubility increases from 50.4 at  50 C to 73.9 at 100 C.