Question

why plaster of Paris should be stored in moist proof container

Answer 1

Because when it mixes or comes in contact of water it becomes a substance called gypsum.

Answer 2

hey dear ♡
here is your answer ☆☞

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POP is stored in moist proof container because as it comes in. contact to the moist air it turns back in gypsum i.e. regains it ater of crystalisation.

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hope it helped ^_^