. . . .  
salt12 Water of crystallisation in gypsum and salts of cobalt You have already done experiments with red cobalt chloride.
A lamp shade of alabaster (Photo 1), desert roses (Photo 2) and glassy gypsum (Photo 3 and 4) are calcium salts of sulfuric acid with a specific quantity of water in a chemical bond (crystal water).
In the following experiment a small quantity of "Marienglas" is weighed and heated in an ampoule until no more water appears. After cooling the product (Photo 5) is re-weighed.
What you need
 2 Liquemin bottles, micro spirit burner, clothes peg to hold the bottle, some crystals of red cobalt chloride, a piece of glassy gypsum ("Marienglas").
Photo 4: Glassy gypsum allows to read the word "Marienglas" through it. Heat it in a horizontal bottle.
Observation
Photo 5: The glassy gypsum is not longer transparent and has a white colour, water vapour escapes.
Explanation
Red cobalt chloride is not only made of cobalt ions and chloride ions. Its crystals also contain water molecules bonded to the ions (crystal water).
Glassy gypsum also contains crystal water. During heating crystal water is split off, the new substances blue cobalt chloride and white gypsum are formed:                     CoCl₂.6H₂O   ---------------------------->       CoCl₂                                  +              6 H₂O
 red cobalt chloride with crystal water        blue cobalt chloride without crystal water          water
                      CaSO₄.2 H₂O  --------------------------->        (CaSO₄)                                +              2 H₂O
glassy calcium sulfate with crystal water     white calcium sulfate without crystal water         water

---

back........first published: 25.10.2001............last modification: 06.03.2007