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In most cases, the water can be removed from the material by heating (sometimes under vacuum or under a flow of dry gas such as nitrogen).
In the laboratory, anhydrous hygroscopic substances such as calcium chloride (CaCl2), magnesium sulfate (MgSO4), and sodium sulfate (Na2SO4) are used to remove residual water from organic solutions. In this application, the solid is added to the solution. After a few minutes, the solid desiccant (now partially hydrated) is removed by filtration or decanting.
Hygroscropic drying agents are also used in desiccators, sealed jars that have a layer of desiccant in the bottom. They are also used in drying tubes which are stuffed with the desiccant material and placed over the opening of a chemical apparatus to keep atmospheric moisture out.
Therefore, always be sure to clean up any spills of hygroscopic materials right away. Also be aware that hygroscopic materials typically release a large amount of heat when mixed with water.
Always store hygroscopic materials in well-sealed containers (or under vacuum or an inert atmosphere). Know their physical properties so that if you open a container you can tell if the material has been contaminated with water (i.e. that jar of calcium chloride, CaCl2, should be a solid, not a liquid).
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