Sciencemadness Discussion Board

Solubility of anhydrous salts vs hydrates

Biochemscientist - 15-7-2012 at 14:59

When looking up the solubility data on a particular salt, different solubilities for the hydrated and anhydrous forms are often displayed. For instance, according to Wikipedia, magnesium carbonate has the following water solubilities for the anhydrous and hydrated forms:

0.0106 g/100 mL (25 °C, anhydrous)
3.75 g/L (20 °C, pentahydrate)


My question is, why does an anhydrous compound have a solubility that differs from the hydrated form? If an anhydrous compound is dissolved in water, doesn't the compound convert into the hydrated form automatically?

zoombafu - 16-7-2012 at 17:28

I tried doing some research into this, and i'm not sure why either. All I can say is that hydrated salts and their anhydrous forms have different chemical properties. And on another note these salts don't turn into hydrates upon being dissolved. They separate into cations and anions.

dann2 - 17-7-2012 at 07:52

MY GOD!!!! Even I know the answer to that.


The actual Magnesium Carbonate has the same solubility in both cases.
If you do the maths (I did not) you will find that the same amount of moles (or kg) of Magnesium Carbonate will dissolve in the same amount of water. In the case of the
penta hydrate you have LESS Magnesium Carbonate (per kg of stuff) + you are actually adding a
certain amount of water along with the Magnesium Carbonate (the water of crystallization).


Take some anhydrous Mg Carb. and if you were to pour water into this anhydrous stuff so that you now had the exact same amount of water in there as if you had the pentahydrate in the first place. Then the 'solubility' of this mixture would be the exact same as the pentahydrate stuff.

watson.fawkes - 17-7-2012 at 07:56

Quote: Originally posted by Biochemscientist  
If an anhydrous compound is dissolved in water, doesn't the compound convert into the hydrated form automatically?
I don't have specific knowledge on this, but I would guess that the answer is of the form "automatically, but not immediately". Since the lattice structure of hydrated and anhydrous forms are, in general, different, I find it plausible that there are substances whose anhydrous crystals have extraordinarily low solvation rates with water. It would not practical to cite solubility figures that take days to achieve.

dann2 - 17-7-2012 at 08:08


<strike> Magnesium Carbonate would have the same solubility anhydrous or with water of crystallization. Both would dissolve rapidly. </strike>



[Edited on 17-7-2012 by dann2]

zoombafu - 17-7-2012 at 11:04

Quote: Originally posted by dann2  

If you do the maths (I did not)


I did do the math, and it doesn't work out.

Biochemscientist - 17-7-2012 at 11:04

Quote: Originally posted by dann2  
MY GOD!!!! Even I know the answer to that.


The actual Magnesium Carbonate has the same solubility in both cases.
If you do the maths (I did not) you will find that the same amount of moles (or kg) of Magnesium Carbonate will dissolve in the same amount of water. In the case of the
penta hydrate you have LESS Magnesium Carbonate (per kg of stuff) + you are actually adding a
certain amount of water along with the Magnesium Carbonate (the water of crystallization).


Take some anhydrous Mg Carb. and if you were to pour water into this anhydrous stuff so that you now had the exact same amount of water in there as if you had the pentahydrate in the first place. Then the 'solubility' of this mixture would be the exact same as the pentahydrate stuff.



Incorrect. Maybe you ought to actually try doing the math first to confirm your conclusions before making assumptions.

The molar mass of anhydrous magnesium carbonate is 84.3g/mol

The molar mass of magnesium carbonate pentahydrate is 174g/mol (84.3g/mol + 5*18g/mol = 174g/mol)

Acrcording to the solubility data, the molar solubility of anhydrous magnesium carbonate would be ~0.0012mol/L and the molar solubility of magnesium carbonate pentahydrate would be ~0.022mol/L

Therefore, a saturated solution of magnesium carbonate pentahydrate contains significantly more magnesium and carbonate ions than does a saturated solution of anhydrous magnesium carbonate.


barley81 - 17-7-2012 at 11:52

The readings are at different temperatures! Though I don't think the solubility should change so substantially over 5 degrees. Perhaps the data is wrong. Do you believe that anhydrous magnesium carbonate could possibly precipitate out of a saturated solution of the hydrate?

dann2 - 17-7-2012 at 15:03

My GOD, I guess I don't know the answer!!!!!!!!!!:P


The solubility table here:
http://en.wikipedia.org/wiki/Solubility_table

gives solubility at 0.39 grams per liter @ 20C (Anhydrous)


The solubility seems to be very dependent on the presence of absence of Carbon Dioxide as described here:

http://pubs.acs.org/doi/abs/10.1021/j150053a004


And some more here:

http://www.newton.dep.anl.gov/askasci/chem07/chem07297.htm

All thanks to Dann2 in tandem with Google

[Edited on 17-7-2012 by dann2]