# Hygroscopic Substances and Drying Agents

My book says:

Most of the hygroscopic substances are drying agents

Based on what I read on several websites, I think dehydrating agents also act as drying agents. If so, all dehydrating agents would also be drying agents and that would contradict my book's statement. So, is there another type of hygroscopic substances or is the book wrong, or is my information or logic wrong?

P.S. I don't think the tags I've used are suitable. I just put the following tags as Wikipedia said that hygroscopy is caused due to either absorption or adsorption. I would appreciate if anyone could help me out.

• Things aren't "drying agents" if they aren't used as such... – Mithoron Oct 2 '18 at 14:42
• This question is about the language and logic in regards to chemistry terms that requires analysis of what those terms are. I don't think closing it and sending it off to the English and philosophy scholars would yield an answer other than ask a chemist. – A.K. Oct 2 '18 at 15:33

Most of the hygroscopic substances are drying agents

not

all drying agents are hygroscopic substances.

First Terminology:
Hygroscopic: Absorbs water from air
Drying agent: Removes water from organic substance

Your book did not exclude other materials from being considered drying agents. It just said many of the substances that are hygroscopic are also useful as drying agents.

Take for example anhydrous calcium chloride calcium sulfate hemihydrate (also called plaster or DrieriteTM) and phosphorous pentoxide. All of these are hygroscopic (sequester water from air)

\begin{align} \ce{CaCl2 (s) + H2O (fl) & -> CaCl2 (aq)}\\ \ce{CaSO4.\tfrac12 H2O (s) + H2O (fl)& -> CaSO4.2H2O (s)}\\ \ce{P2O5 (s) + H2O (fl) &-> HPO3 (cd)}\ \end{align}

Calcium chloride is deliquescent (hygriscopic to the point that it forms a liquid) and calcium sulfate hemihydrate is hygroscopic (stays solid but still absorbs water) and both can be used as drying agents for most solvents. Phosphorous pentoxide on the other hand cannot be used as a drying agent for some materials such as alcohols and styrene.

$$\ce{P2O5 (s) + EtOH -> EtH2PO4 (cd) + HEt2PO4 (cd)}$$ $$\ce{PhCHCH2 ->[P2O5] [PhCHCH2]_n \tag{explosive}}$$