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salt12 Water of crystallisation in gypsum
A
lamp shade of alabaster (Fig. 1), desert roses (Fig.
2) and glassy gypsum (Fig.
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.
Masterials
2 ampoules (5 mL), ampoule burner, clothes peg to hold the bottle, 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
Fig. 5: The glassy gypsum is not longer transparent and has a white colour, water vapour escapes.
Explanation
Glassy gypsum contains crystal water. During heating this is split off, a new substance white gypsum is formed: