MadSci Network: Chemistry

Re: Hydrated salts

Area: Chemistry
Posted By: John Christie, Faculty, School of Physical Chemistry, La Trobe University, Bundoora, Victoria, Australia
Date: Wed Jan 22 23:28:25 1997
Message ID: 831499414.Ch

There is such an activity series, at least in the following sense: It is 
possible to arrange hydrated salts in order of how strongly they hold on to 
their water of hydration. Some salts, like calcium sulfate dihydrate, hold 
on to their water of crystallization very strongly, and are quite difficult 
to dehydrate; others, like potash alum, tend to lose their water of 
crystallization to the air on dry days. For any hydrated salt, at a fixed 
temperature, there will be a vapour pressure of water vapour that is in
equilibrium with it. If the humidity of the air drops, so that the vapour 
pressure falls below this value, the hydrated salt will tend to lose its 
water, or "effloresce". the higher this equilibrium vapour pressure is, the 
less strongly is the hydrated salt holding its water.

In theory, if a hydrated salt A holds its water more strongly than a
hydrated salt B, then if hydrated B and anhydrous A are mixed in a closed 
vessel, anhydrous B and hydrated A should be produced.

There are many factors that make things more complicated than that. Here are 
a few of them:

1. Many anhydrous salts and oxides take in water by adsorption - sticking it
onto the surface - as well as or instead of absorption - a change in crystal 
structure to form a hydrated salt. e.g. aluminium oxide, calcium chloride.

2. Many salts and oxides do not stop at the hydrated salt level, but continue 
to take in water from the air or from another salt until they dissolve
themselves (deliquescent materials). e.g. lithium chloride, copper nitrate,
sodium hydroxide.

3. Many salts form several different hydrates, with quite different affinities
for water, e.g. sodium carbonate, sodium sulfate.

Your other questions are not really very directly related to this sort of 
area, because in aqueous solution there is plenty of water around, and so
there is no question of competition between salts for a limited amount of
water. A solution of a salt is identical whether it was made up from the 
hydrated or anhydrous solid, or prepared in some other way. Nevertheless,
here are some answers:

barium chloride dihydrate and sodium sulfate decahydrate (or heptahydrate)
can each be dissolved up in water, and the two solutions mixed to produce an 
anhydrous precipitate of barium sulfate.

calcium formate I think does not form a hydrate; nor does ammonium sulfate.
Aqueous solutions of these two salts can be mixed to produce a precipitate
of calcium sulfate dihydrate.

I think these two examples work to show that the answer to both of your 
later questions is "yes". If they do not quite fit there will be others.
But I really doubt that such examples have much significance, because there
is plenty of water around, not competition for a limited amount of water!

Hope I haven't missed the point.


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