Hygroscopic Salts and Hydrates
Date: Fall 2009
Are hygroscopic salts hydrates?
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!!
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.
Click here to return to the Chemistry Archives
Update: June 2012