why some compounds are hydrated and some are not?

Question

compounds like hydrous copper (II) sulfate vs. anyhydrous copper (II) sulfate. why is one hydrated, why is one not hydrated.

Answer 1

Hydrated compound almong metals compound is quiet common.
This is due to ligand binding between the metal centre and the oxygen in water. The oxygen partially donates electrons, much like in hydrogen bonding in water but ligands instead of bonds are formed. This depends on a number of factors, the structure shape of the metal compound and if it is thermodymaicly stable to do so.

Such examples are

Al(NO3)3.9H20
CuSO4.5H2O
MgSO4.7H2O

The same can also happen in organic compounds as well
but this mostly centres about amine acid salts. Such an example is a well known drug paroxtine, which can exist as a monohydrate or dihydrate.

Hydrate compounds can be formed by carely heating the compound in question a solution that contians water. To form the non-hydrate form in most cases heating so that the water is driven is all that is needed. In the case of organic hydrates this becomes a little harder as the compound in question may decompese.

Answer 2

Some compounds have an affinity to water. When these are left in contact with the moist air, they will pull water from the air and hydrate, becoming hydrated crystals.

Copper (II) sulfate will become a pentahydrate (5 molecules of water), while Magnesium sulfate with become a heptahydrate (7 molecules). The anhydrous form of these is man-made (using the application of heat to drive off the water.

Answer 3

copper(II) sulfate is a compound that adsobs water when left under air exposure. It becomes copper(II) sulfate hydrate. As an analogy, people tend to perspire when under the hot sun. Thus, in some conditions, some people are "wet". Do you get it?

Answer 4

Hydrated means it has water in it, Anhydrous means it does not contain water. This is not the same as Dehydrated which means that the water has been removed.