Re: Cobalt (II) chloride hexahydrate:heated&turned from pink 2 blue;howDOiUSEit

Kimberly,
the easiest way to test whether a compound is a hydrate is to heat (dry) 
it at temperatures above the boiling point of water. If you were to weigh 
the hydrated material before drying and after, and with a knowledge of the 
material (so that you had the molecular weight), and expecting that all 
water was driven off (you can continue to dry until the weight stops 
changing), then the final weight of the material allows you to calculate 
how many molecules of the material are left (in a unit called "moles", 
akin to "dozen" or "gross", or any such "collection" unit.  Let's label 
this X).  The difference in weight before and after drying can be used 
with the molecular weight of water to find how many molecules of water 
were driven off (again, in units of moles.  Let's label this Y).  Your 
hydrated formula is then:

desicant molecule_X (H2O)_Y
(reported with X and Y as the lowest whole numbers possible, see for 
example cobalt chloride hexahydrate)

Because water is weakly bound to compounds such as cobalt (II) chloride, 
these materials serve as water "traps".  The water can be driven off and 
the material reused to trap water again (these naterials are collectively 
called "desiccants").  There is a maximum amount of water which may be 
taken up by any particular substance, which depends upon the ability of 
the water molecules to pack closely enough to the substance so that they 
experience an attractive interaction with its electron "cloud".  In the 
case of CoCl2, you can only pack 6 water molecules (the hexahydrate) in 
before there is no more physical room left.  This means that you can use 
these materials to collect up to some particular level of water, which can 
be predicted by stoichiometry and mass of the desiccant.  As such, they 
are often used to keep an enclosed volume free of water vapor and the 
items contained in this volume "dry".


Using these materials to predict weather (humidity), estimate water 
concentrations in air, etc. is much more difficult.  Knowledge of the 
uptake rates for water within the desiccant (which change with how much 
water is already there), the surface area (average) of the desiccant, and 
other such properties would be necessary to extract an estimate of the 
water concentration.  Instead, humidity probes often depend upon physical 
phenomena to measure water vapor concentrations.  One such method is with 
a strain gauge, which involves a change in the shape of a small piece of 
material with differing amounts of water "stuck" to it.  This amount of 
water is "equilibrated" with the surface (as one molecule leaves, another 
sticks), rather than the case with a desiccant where the water binds 
tightly enough that it takes heat to remove.

I hope this information is helpful.
Please feel free to email me with further questions (weibelm@battelle.org)

Bets Regards,
Mike