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I have a problem with a sample made from $\ce{CaCl2*2H2O}$ and $\ce{MgSO4*7H2O}$ in about equal amounts. I took the two salts, pressed them together, and the result was a weird brine that was very wet and almost seemed to turn into water before my eyes.

I am wondering what happened here, and was hoping that someone could walk me through it. As you can tell by the title, I believe that a reaction might be involved that I did not consider when making the sample.

As far as I know, the two salts are both very hygroscopic, so it could be that they simply adsorbed water and that is why the sample became wet. However, the effect seemed more severe after mixing them together, so I am thinking that a reaction might have been involved.

Could it be that the two salts reacted this way? $$ \ce{CaCl2*2H2O + MgSO4*7H2O -> CaSO4*2H2O + MgCl2*6H2O + H2O} $$ And if so, what happens with the leftover water that is not bound? Especially since $\ce{MgCl2*6H2O}$ is even more hygroscopic than any of the other salts, could it be that the water was attracted by the $\ce{MgCl2*6H2O}$, which then turned the sample into a brine?

I'm just trying to understand what happened, I did not add any heat, but I did add a lot of pressure.

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  • $\begingroup$ You really need what will be a very complicated phase diagram to figure this out. It would seem obvious that enough water came from the original salts to create a "brine". If there were any solids left then four possible salts with a number of different hydrates. $\endgroup$ – MaxW Mar 11 at 21:30
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You are correct.

In the system $\ce{Ca^2+*Mg^2+*Cl^-*SO4^2-*H2O}$, the least soluble phase is $\ce{CaSO4*2H2O}$.

Once you're mixing $\ce{CaCl2*2H2O}$ and $\ce{MgSO4*7H2O}$, your $\ce{CaCl2*2H2O}$ starts absorbing atmospheric moisture and then dissolving in it. Then you have a brine which dissolves the $\ce{MgSO4*7H2O}$, speeding up the kinetics of the thing. The solid that you precipitate by the recation has less $\ce{H2O}$ per mole of salt, so the excess $\ce{H2O}$ just ends up in the brine (the "water"). Pressing it speeds it up because there is more surface area of the two salts in contact, which is where the reaction happens.

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