Re: Why are the two compounds CaSO4 and CaCO3 insoluble in water?

Dear Sonya,

It is a quite strange question actually, just like "Why does glue stick?" 
!.
I can give you a definition and an genearl idea of the mechanism of 
solubility, which can be helpfull, but to know why CaCO3 is far less 
soluble than KCl in water, we can write a book on it! 

I found this definition in a General Chemistry Book :"Solubility is the 
maximum amount of material that can dissolve in a given 
amount amount of solvent at a given temperature to produce a stable 
solution"

Chemists often refer to substances as soluble, party soluble or insoluble. 
These terms have no precise meaning, however. Usually what they mean is 
that the substance is soluble if a good deal of solid has visibly 
dissolved, when only a modest amount of solid has been added to the 
solvent. If only a small amount of solid has visibly dissolved, the solid 
is partly soluble. If no solid can be observed to dissolve, the substance 
is called insoluble or poorly soluble. 

We can resume the mechanism as followed: When a solute is dissolved in a 
solvent, the attractive forces betwwen solute and solvent particles must be 
great enough to overcome the attrcative forces (intermolecular forces) 
within the pure solvent and within the pure solute. When solutes are 
dissolved, they become solvated if solvent molecules are sufficiently 
attracted to solute molecules or ions (usually by dipole or ion forces)

You will find such definitions in every book about general chemistry.
Tables are also available, showing you a list of different compounds and 
their solubility properties. Indeed, we can make general statements about 
which types of ionic compounds are water soluble.

Quick information about this two compounds:
CaCO3 is the chief constituent of limestone and of corals, sea shells, 
marble, etc. CaCO3 is poorly soluble in water (0.0062g/L)
Gypsum CaSO4,2(H2O) is an important calcium mineral. It is slightly soluble 
in water and ground water that comes into contact with gypsum often 
contains some dissolved calcium sulftae.

If you want to learn more about solubility of compunds, I advice you the 
book "Water Chemistry" by Snoeyink and Jenkins, that you can surely find in 
every scientific library.

I hope I've been helpfull enough in order to help you.
Keep being curious and interrested in sciences.

Yan G.