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Name: Isabelle
Status: student
Grade: 9-12
Country: Canada
Date: Fall 2009


Question:
Are hygroscopic salts hydrates?



Replies:
Isabelle,

Hygroscopic salts absorb water from the surroundings. Hydrated salts already have water of crystallization incorporated in their crystalline structure. It is normally anhydrous (salts with no water in their crystalline structure) that are more likely to be hygroscopic as they still have the ability to take in water. However, it is still possible for hydrated salts to take in more water, depending on how much water is in the salt to begin with and the affinity of the ions for water molecules. Indeed, some very hygroscopic salts will continue to take in more and more water from the air until a solution is formed, rather than simply a hydrated crystal.

The reason salts do this is normally explained by the strong interactions between the ions and the water molecules. This strong interaction means that heat energy (enthalpy) is released when the salts become hydrated. In fact, if you want to predict if a salt will be hygroscopic you need to take into account the entropy (randomness) of the salt and water separately compared to the hydrated salt (or solution) formed afterwards as well as the enthalpy released or taken in if it were to absorb water. So it is quite a complex question. Also, we have not talked at all about how FAST they take in water, rather than whether they take it in at all, which is another question!!

Best wishes,

Tom Collins


Not necessarily. By common definition a "hygroscopic salt" is only a loosely defined salt -- one that absorbs atmospheric water vapor. There is also an ambiguity with the term "hydroscopic salt" which I take to mean the same thing -- others may differ. Common "hydroscopic salts" for example NaOH and KOH certainly absorb water vapor from the atmosphere but they are not hydrates in the usual sense of the term.

Underlying your inquiry is a larger "philosophical" issue that confuses the teaching of chemistry. That is: So much emphasis of putting substances / reactions / phase changes /... and the list goes on and on. We end up making the classification of substances and processes more important than the substances themselves. A silly example is arguing whether the correct term is "elephant" or "elefant".

What we name it is not the important point -- it is what it is. Do not get hung up on naming "stuff".

The same applies, in my opinion, to hygroscopic and hydroscopic. I know you may not have wanted to open that "box", but how we name things often determines how we think about those things.

Vince Calder



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