Why does Anhydrous Calcium Chloride absorb water?
I’d say you’re right about it having something to do with bonding. Calcium Chloride is quite often used as a dessicant, because it is that hygroscopic that it will dissolve in its own lattice structure of the resulting hydrate, leaving a puddle of liquid when left in contact with normal air (I think the term for this is deliquescence). The reaction to produce CaCl2.2H2O (calcium chloride.dihydrate) from anhydrous CaCl2 is really exothermic i.e. it produces a lot of heat, which means the dihydrate is much more thermodynamically favourable, and therefore more likely to exist. I think the hexahydrate is more thermodynamically favourable again. In terms of bonding, I believe this more favourable state probably comes from the electrostatic interactions between the lone pairs of the oxygen in the water with the positive state of the calcium.
