Calcium Chloride and moisture

[Reef_Hobbyist]

I just dosed some BRS bulk calcium chloride for the first time. I made a little funnel out of a piece of paper so I could pour the crystals into a 1 gallon mixing jug of RODI. The BRS recipe calls for 2.5 cups dissolved in 1 gallon of RODI. Interesting that it got a little warm while mixing.

About 30 minutes later I walked back into my fish room and noticed that my paper funnel was soaking wet. It looks like the residue from the crystals must have absorbed moisture from the air? If so, that was quite interesting that it could be that wet after only 30 min.

 

[Randy Holmes-Farley Verified Community Expert]

Calcium chloride that is anhydrous (no moisture in the crystals) will get hot when it hydrates by adding water

The hydrate doesn't usually give heat and i thought the sold the hydrate, but perhaps they have changed that.

It is very hygroscopic and will suck water out of the air. It can also harden into a giant clod if a bag is open to the air. for this same reason.

 

[Reef_Hobbyist]

Calcium chloride that is anhydrous (no moisture in the crystals) will get hot when it hydrates by adding water

The hydrate doesn't usually give heat and i thought the sold the hydrate, but perhaps they have changed that.

It is very hygroscopic and will suck water out of the air. It can also harden into a giant clod if a bag is open to the air. for this same reason.

Label reads - This product is produced using pharmaceutical grade material and high purity crystalline dihydrate. It came in a plastic screw top container and is not clumpy. The jug was a little warm to the touch when mixing but only for about 30 seconds.

 

[Habib(Salifert)]

Calcium chloride that is anhydrous (no moisture in the crystals) will get hot when it hydrates by adding water

The hydrate doesn't usually give heat and i thought the sold the hydrate, but perhaps they have changed that.

It is very hygroscopic and will suck water out of the air. It can also harden into a giant clod if a bag is open to the air. for this same reason.

The dihydrate gives also heat (exothermic) when mixed with water.

 

[Randy Holmes-Farley Verified Community Expert]

The dihydrate gives also heat (exothermic) when mixed with water.

You're right, Habib.
Thanks

here's a table of the different heats of dissolution for the different forms:

http://www.osi-univers.org/IMG/pdf/CalciumChloridHandbook-2.pdf

 

[JimWelsh]

Another interesting thing about calcium chloride is that the heat of formation is negative, meaning that crystal formation is also exothermic. Which is fun, because as the solution cools, crystals want to form, but their formation generates heat, which slows the cooling, which then affects further crystal formation. I did a timelapse of CaCl2 recrystallization of a supersaturated solution that was seeded to initiate nucleation. You can see the heat that is being generated is lofting crystals in a central column of rising warmed solution in the middle of the flask. You can also see the heat induced refractive artifacts disturbing the background graduation lines. Best watched in HD and full screen:

 

[Habib(Salifert)]

Another interesting thing about calcium chloride is that the heat of formation is negative, meaning that crystal formation is also exothermic. .....

Nice video!

Heat of formation and heat of crystallization are totally different things.

If dissolution is exothermic then crystallization is endothermic.

Differences in refractive index can be induced by differences in temperature but I believe in this case far more so by induced changes in solution concentration.

 

[JimWelsh]

So, I clearly got the "Heat of Formation" part wrong. I took that from page 6 of this document.

But, I'm still correct to a certain degree: The crystals being formed at the concentration and temperature of the solution in the video are calcium chloride hexahydrate. As you can see from the linked document, the Heat of Solution for CaCl2*6H2O is endothermic, so the crystallization is exothermic, as the video shows.