I)plaster of paris means calcium sulphate hemihydrate (CaSO₄2H₂O)
ii)plaster of paris on mixing with water , it sets into hard solid mass due to the formation of gypsum (CaSO₄2H₂O)
   CaSO₄I/2H₂O+1 1/2H₂O--------------> CaSO₄2H₂O
iii)because of the above reason plaster of paris should be stored in air tight pakes or dry places and moister proof container

                  Plaster of Paris (CaSo4 . 1/2 H2O) in presence of Humidity (Moisture) will be formed as a hard rock like substance know as Gypsum(CaSo4 . 2H2O). Turning Gypsum into Plaster of paris (Powder) is highly difficult.
                    In order to prevent plaster of paris to the exposure of humdity it is being stored in Air-Tight Container or Dry places.
                      These Air Tight Containers do not allow any air to go out side or enter inside which prevents the exposure to humidity in its vicinity. In Dry Places No moisture is seen.
                        Therefore, Plaster of Paris has to be stored in Air-Tight Containers or Dry places.
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