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Name: Paul
Status: student
Grade: 12
Country: Finland
Date: Fall 2009

Why does the solubility of magnesium carbonate decrease with temperature until around 30 degrees Celsius and then increase rapidly after that?


I was not aware of this data for magnesium carbonate but changes in trend for solubility as temperature is increased are usually to do with a change in the species forming in solution, or a change to the undissolved solid species.

If solubility decreases with increased temperature, as is the case for most gases and a few solids such as some Cerium salts, this is usually explained by the dissolving process involving a decrease in entropy. This is disfavored and the entropy term becomes more important at higher temperatures, so the solute becomes less soluble. The route of this explanation comes from good old DG = DH -TDS, where the entropy term is multiplied by the temperature. So the initial decrease in solubility of MgCO3 would be due to this: either a more ordered species is formed in solution, by hydration of the ions, or the MgCO3 lattice itself is particularly disordered in this low temperature range.

After 30degC when the solubility starts to increase there must be a change, either in the species in solution or in the undissolved MgCO3 itself. If the ions are hydrated in a more disordered way above this temperature, that would account for this change in solubility trend. The other possibility is that the MgCO3 solid changes to a more ordered form above this temperature, making the dissolving more entropically favored. This could be explained by it forming a less hydrated or anhydrous form, I know that various hydrates of MgCO3 are known as is the anhydrous salt.

Sorry I cannot be more specific. These are only general points and I until your question came up I was not aware of this trend. I know that Na2SO4 has another complex solubility curve, and that is normally explained in a related way. Thanks for bringing up this interesting question.

Best wishes,

Tom Collins

"Solubility" is such a simple sounding property. What could be simpler? In fact, it can be very complicated, depending upon the physical and chemical variables involved. Especially so if the solvent is water. You heat "it" up and "it" becomes more soluble. WRONG!! Your example of magnesium carbonate (Mg(CO3)) is especially pathological because not only do you have the initial compound but (several) other "solubilities" are involved: Mg(+2), Mg(OH)(+1), CO3(-2), HCO3(-1), H2CO3(-2), CO3(-3), CO2(gas), and last but not least (H+). Each of these reactions has a different dependence upon temperature, and under some conditions some of the combinations form solids -- not always the same crystal structure. So it is no surprise that the temperature behavior is complicated. Sometime "simple" chemical systems are not so simple!!

Vince Calder

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