Difference between hygroscopic and moisture sensitive chemicals

The terms 'hygroscopic and moisture sensitive' are often used for one another and many are confused while using them. Although, they have one similarity of absorbing moisture when exposed to atmosphere, in reality, they are two distinct characters of a chemical compound. Now, lets know the difference between the two terms: Hygroscopicity is a physical phenomena and moisture sensitivity is a chemical reaction. To be clear, we need to explain it using an example; Potassium hydroxide is a hygroscopic compound, it would adsorb moisture from air, also it is highly soluble in water and upon drying, it can be reverted back to the original physical and chemical state without change in its chemical characteristics. According to 'Arrhenius theory of electrolytic dissociation': [1][2]

KOH(s) + H2O(l) ↔ K⁺ (aq) + OH⁻ (aq) + H2O(l)

n-Butyllithium is a hygroscopic as well as, moisture sensitive compound, which upon exposureto the to air would react with moisture leading to the formation of n-butane and lithium hydroxide, thus loosing its chemical property as a strong base or super base used for the generation of anion. The following is the reaction of butyl lithium upon exposing it to air or moisture.[3]

C4H9Li + H2O → C4H10 + LiOH + Heat

In short, Potassium hydroxide retains its chemical properties upon exposing or dissolving in water, whereas, butyl lithium reacts with water converting into a different chemical entity.

References

1. Cillispie, Charles, ed. (1970). Dictionary of Scientific Biography(1 ed.). New York City: Charles Scribner's Sons. pp. 296–302. ISBN 978-0-684101-125.
2. Harris, William; Levey, Judith, eds. (1975). The New Columbia Encyclopedia (4th ed.). New York City: Columbia University. p. 155.
3. "Preparation, Properties, and Safe Handling of Commercial Organolithiums: Alkyllithiums, Lithium sec-Organoamides, and Lithium Alkoxides". Organic Process Research & Development. 2014, 18, 10, 1192. doi/10.1021/op500161b

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