Sciencemadness Discussion Board

Strange reaction with CaSO4 + CaCl2 + heat -> melted at "lower" temp than expected

RogueRose - 13-10-2018 at 05:33

I was drying some CuSO4 x 5H2O - about 1kg which I just recrystalized and had slight moisture left in it. I added about 1/3kg CaSO4 (hemihydrate or near anhydrous & I originally thought was anhyd CaCl2) and 2/3kg anhyd CaCl2 into a large desiccator, applied vacuum and dried for 7 days.

I used a little mineral oil for the seal and there were a few drops/drips that dripped down the inside and probably got to the desiccant.

The CaCl2 was layered on top of the CaSO4 and I could see that it had changed from a dry chalky looking appearance to a shiny/glossy look while the CaSO4 remained the same. I would estimate about 40-70g of water was absorbed into the desiccant.

I mixed the two desiccants together after cleaning out the container and dried in the microwave on high until I started noticing an acidic smell of what I thought was SO2 at first then thought it was HCl but may have been a mix of both. The smell came on very strong very quickly.

Upon inspection I noticed that there were a couple spots where the material has melted. CaCl2 has a MP of almost 1500F and CaSO4 near 2500F so I can't see that these temps were reached but I can't figure out how I got some melted spots along with the smell of acids.

Could the two salts interacted with heat to reduce the MP and release the HCl and or SO2? Are there any other ways this could have melted besides from heat?

Ubya - 13-10-2018 at 05:42

a microwave oven can melt glass, so it's reasonable to think that a few spots of dry salt could absorb enough radiation to heat to the melting point

RogueRose - 13-10-2018 at 06:52

Quote: Originally posted by Ubya  
a microwave oven can melt glass, so it's reasonable to think that a few spots of dry salt could absorb enough radiation to heat to the melting point


Hmm. The salts were on a ceramic plate so I guess it is possible that it got hot enough to melt the CaCl2 but I didn't think it was anywhere near that hot! That is pretty amazing IMO, I'll be interested to see what the plate looks like after I get the salts off.

unionised - 13-10-2018 at 06:56

CaCl2 .6H2O has a melting point of about 30C
Hot solutions of CaCl2 will give of some HCl.
It doesn't take much before you can smell it.

RogueRose - 13-10-2018 at 21:06

Quote: Originally posted by unionised  
CaCl2 .6H2O has a melting point of about 30C
Hot solutions of CaCl2 will give of some HCl.
It doesn't take much before you can smell it.


That is odd, I thought all the hydrates decompose, or loose their water, at the "MP" for each hydrate, but they didn't loose shape/or melt. I've dried wet CaCl2 many times and I don't ever remember them melting, just loosing water but retaining shape.

unionised - 14-10-2018 at 01:44

The "hot ice" videos show that hydrated sodium acetate "melts"
It's not unusual in that behaviour.

Hunterman2244 - 14-10-2018 at 06:36

Has anyone considered the possibility of it being a eutectic mixture?

[Edited on 14-10-2018 by Hunterman2244]

RogueRose - 14-10-2018 at 07:42

I'm having a hard time believing that the hydrates are melting and I have a harder time believing that there was ANY hexahydrate in this mix. I had 1kg of desiccant all anhydrous and it absorbed 80-100g MAX water. and the most of the surface of the CaCl2 had changed to a shiny look of a hydrate vs the chalky look of the anhydrous - so I doubt very much that any hexahydrate could have formed b/c it would have had to concentrate on a few spots instead of evenly over all the desiccant.

Secondly, I've dried CaCl2 that has been so moist that it was sitting in a pool of saturated solution (it started anhydrous and absorbed H2O over a year & sat in solution) so it had to be hexahydrate. Heated in an oven IIRC it dried in the same form (flakes) but bonded together in a large chunk. I also tried similar with MgCl2 which I think did melt and seemed much more hygroscopic than the calcium chloride..

unionised - 14-10-2018 at 07:53

Quote: Originally posted by RogueRose  
I'm having a hard time believing that the hydrates are melting and I have a harder time believing that there was ANY hexahydrate in this mix. I had 1kg of desiccant all anhydrous and it absorbed 80-100g MAX water. and the most of the surface of the CaCl2 had changed to a shiny look of a hydrate vs the chalky look of the anhydrous - so I doubt very much that any hexahydrate could have formed b/c it would have had to concentrate on a few spots instead of evenly over all the desiccant.

Secondly, I've dried CaCl2 that has been so moist that it was sitting in a pool of saturated solution (it started anhydrous and absorbed H2O over a year & sat in solution) so it had to be hexahydrate. Heated in an oven IIRC it dried in the same form (flakes) but bonded together in a large chunk. I also tried similar with MgCl2 which I think did melt and seemed much more hygroscopic than the calcium chloride..


"I'm having a hard time believing that the hydrates are melting and I have a harder time believing that there was ANY hexahydrate in this mix."
Yet "most of the surface of the CaCl2 had changed to a shiny look of a hydrate".
Well, which is it?
Is it mainly hydrated or not.
Also, it's possible that , during heating you got moisture to evaporate off the bits that warmed up first then condensed on the cooler bits.


I didn't make up the melting point of the hydrate- it's there in WIKI.

Incidentally, when you buy CaCl2 as a desiccant, it isn't usually anhydrous- I think it's typically the "trihydrate".