Q. 23

(i) Beryllium sulphate and magnesium sulphate are readily soluble in water whereas the sulphates of barium, calcium and strontium are only sparingly soluble. Explain.

(ii) Why is the temperature maintained around 393 K during the preparation of plaster of paris?

Answer :

(i) Beryllium sulphate and magnesium sulphate are readily soluble in water whereas the sulphates of barium, calcium and strontium are only sparingly soluble because as we move down the group the solubility of sulphates decreases.


On moving down the group the lattice dissociation enthalpy and hydration enthalpy both decreases but the decrease in hydration enthalpy is more than that of lattice dissociation enthalpy. So water molecules are not able to break the metal sulphate bond and thus on moving down the group the sulphates become less soluble.


(ii) The temperature is maintained around 393K during the preparation of plaster of paris because at a temperature higher than 393K the whole water of crystallisation is lost and the result anhydrous CaSO4 formed is called dead burnt plaster as it loses the properties of setting of water.


2CaSO4. H2O 2CaSO4 + H2O


(Plaster of pars) (Dead burnt plaster)


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