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I understand that both a desiccating solid and a deliquescent solid are "hygroscopic" in the sense that they bind to water.

But what is the difference? And what are the applications of a desiccant vs a deliquescent?

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A deliquescent solid will eventually dissolve in water it absorbed, like, for example, calcium chloride does. Molecular sieves are very good dessicants but they do not dissolve in water, so they are not deliquescent. As you can probably guess, operating with dessicants that are deliquescent may be more troublesome than with non-deliquscent ones, but it doesn't mean that they are inferior.

The choice of a proper drying agent depends on the material you want to dry. For example, non-deliquescent dessicants, like aluminium oxide, silicagel or molecular sieves are best to fill columns for solvent drying, because they won't clog when their drying capacity is exceeded. Deliquescent drying agents are perfect for the use in dessicators, because it is easy to remove them when they are exhausted.

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