plaster of Paris /water /setted
plaster
salt13 Setting
of gypsum ("plaster of Paris"),
another chemical
reaction of a salt with water
Plastering of broken arms or
legs needs a special type of calcium sulfate which is also used in
the household.
* Photo: Mix 1 teaspoon
of plaster of Paris (CaSO4x0.5 H20)
and 1 teaspoon of water.
Observation
Photo right: After
1 hour the pulp has changed into a hard brittle substance.
Explanation:Setting
of gypsum
resembles the setting of lime mortar: both are
chemical reactions with water:
CaSO4
x 0.5 H2O +
1,5 H2O -----release of heat---->
CaSO4
x 2 H2O
In the previous experiment you
have heated glassy gypsum (CaSO4 x 2
H2O) to split
off crystal water. Heat was consumed. Setting of plaster of Paris is
about the reverse of this reaction.
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modification: 18.10.2001