The visual indicator that enough of a drying agent , such as anhydrous MgSO4 or CaCl2 has been added to properly dry an organic solution is that the drying agent will move freely like a powder around the solution .
Because the anhydrous form is hygroscopic ( readily absorbs water from the air ).
What is the work of anhydrous magnesium sulphate ?
Magnesium sulphate is frequently used in the laboratory as an indicator, especially after aqueous work-up . aqueous work-up is a common technique in the lab to get rid of residual impurities after completion of a reaction . for this the organic reaction is cooled to room temperature and mixed with water .specific impurities will then diffuse into the aqueous phase and can be separated with a separatory funnel .
unfortunately some residual water will stay in the organic phase and this can have negative impact for characterization of compound . therefore Magnesium sulphate is added ,which is able to catch the residual water in its crystal lattice .after filtration of the solid magnesium sulfate ,the water is reduced to non-significant amount and the compound ready for further investigation.
Similarly , calcium chloride is strongly hygroscopic ( absorbs water from the environment ) , so it removes moisture from the air ,making it dryer . this results in water in the substance to be dried to evaporate into the drier air and this cycle repeats until the system reaches an equilibrium.
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