[rwooduk1]

Hello, I am doing some work with gypsum on a surface and trying to understand the reaction more. I think this is how the reaction goes, but not sure and would appreciate any advice.

I think with the addition of water the gypsum (calcium sulfate dihydrate) becomes (calcium sulfate hemihydrate):

##CaSO_{4}\cdot 2H_{2}O \ + \ H_{2}O \ \rightarrow \ CaSO_{4}\cdot \frac{1}{2}H_{2}O ##

Then when it dries in air, or I put it in the oven, it loses it's moisture and becomes calcium sulphate. Is this correct?

Also, I am interested in the reaction of calcium sulphate with barium chloride solution. I have read that barium sulphate will precipitate out and then leave calcium chloride in solution. Would the solid on my surface then be barium sulphate?

Thanks for any help understanding the reactions more.

[AWK]

https://wiki2.org/en/Calcium_sulfate
see also anhydrite and gypsum

Make a clear solution of CaSO₄ (solubility approx. 2 g/L of water). The slurry must be stirred from time to time for several days, and then the solution above the sediment must be decanted. Add about 2 g of concentrated BaCl₂ solution to this. There will be pure BaSO₄ in the sediment.

[rwooduk1]

https://wiki2.org/en/Calcium_sulfate
see also anhydrite and gypsum

Thanks AWK this is a much clearer explanation than Wiki.

Make a clear solution of CaSO₄ (solubility approx. 2 g/L of water). The slurry must be stirred from time to time for several days, and then the solution above the sediment must be decanted. Add about 2 g of concentrated BaCl₂ solution to this. There will be pure BaSO₄ in the sediment.

Hmm, I was hoping that if I had a hardened layer of gypsum then I could pour barium chloride over it and it would react and convert the layer to barium sulphate, seems this is not the case. Many thanks for your help.

[Borek]

I think with the addition of water the gypsum (calcium sulfate dihydrate) becomes (calcium sulfate hemihydrate):

##CaSO_{4}\cdot 2H_{2}O \ + \ H_{2}O \ \rightarrow \ CaSO_{4}\cdot \frac{1}{2}H_{2}O ##

Looks like you are suggesting that adding water makes the final compound contain less water. That's against logic (and reaction balancing).