HYDRATED SALT
→ Water of
crystallization is the fixed number of water molecules present in one formula
unit of a salt.
→ In coloured crystalline & hydrated salt, the
molecules of water of crystallization account for their characteristic colour. Upon
heating a hydrated salt, its colour changes since the molecules of water of
crystallization are removed & salt becomes anhydrous.
Examples.
(i) CuSO4. 5 H2O → CuSO4
+ 5H2O
(blue)
(white)
(ii) Gypsum :─ CaSO4.2H2O → CaSO4.1/2H2O
Gypsum plaster
of paris
(iii) Sodium carbonated decahydrate Na2CO3.10H2O
→ Crystalline salts containing molecules of water of
crystallization are not wet.
Reason: The molecules of water of crystallization are
the part of the crystals. Upon heating there are released, making the salt
anhydrous.
PLASTER OF PARIS (CaSO4.1/2H2O)
→ It is obtained by heating gypsum at a temperature of
373K in a kiln.
Gypsum :─ CaSO4.2H2O CaSO4.1/2H2O + 3/2
H2O
Gypsum plaster
of paris
Temperature must be controlled because at high
temperature, plaster of paris gets completely dehydrated to form anhydrous
calcium sulphate.
CaSO4.2H2O
→ CaSO4
+ 1/2H2O
Calcium
sulphate
Or
dead burnt plaster
Uses:
It is used
(i)
for making toys
& materials for decoration,
(ii)
in surgical
bandages for setting fractured bones. White powder of plaster of paris, on
mixing with water changes to gypsum, giving a hard solid mass.
CaSO4.1/2H2O + 3/2H2O
→ CaSO4.2H2O
Plaster of paris Gypsum
(iii)
for covering the
walls & roofs & on making designs on them.
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