Question

Why aren't the covalent compounds soluble in water but ionic are ?

Answer 1

Both your premises are incorrect..........

Explanation:

There are some covalent compounds that are soluble in water, for instance, `"methyl alcohol"`, and `"ethyl alcohol"`.

On the other hand, there are some ionic compounds, for instance, sulfides, phosphates, and (some halides) that are INSOLUBLE in water.

Many ionic compounds have some water solubility; the insolubility of other ionic compounds is evidence of their intrinsic insolubility not their identity as ionic compounds. Water, a protic, dipolar solvent, is a good solvent for solvating ions; it cannot dissolve ALL ionic compounds, especially heavy metal sulfates, carbonates, and biphosphates.

Just to add a bit more background to the answer on solubility, consider the difference between INTERMOLECULAR and INTRAMOLECULAR bonding with respect to solubility.

Chemistry is an experimental science. You do the experiment and then try to rationalize it on the basis of what you find in the experiment. To address your question, covalent bonds are VERY STRONG; they are probably stronger than ionic bonds on a mole for mole basis.

But bond strength or weakness does not confer solubility in water. Both oxygen and nitrogen gas have SOME water solubility (i.e. as the diatomic molecules); if these gases were insoluble in water, then the oceans would not be able to sustain life. Between oxygen and nitrogen MOLECULES, there is next to no interaction (of course not, the materials are gases at RT!). Between oxygen/nitrogen ATOMS within the molecules, there is strong interaction.

So under one umbrella, there is intrinsic bond strength; and under another umbrella there is the solubility of the molecule or ionic species in water. When ionic substances dissolve in WATER, bond breaking does indeed occur; certain salts are VERY insoluble. How do you know? How else but by experiment.

Let's try another example. Carbon dioxide has very strong INTRAMOLECULAR bonds (intramolecular means the bonds WITHIN molecules, here the `O=C=O` bonds). These bonds are probably stronger than `C−C` bonds (I think they are) in diamond or carbon. Nevertheless, carbon dioxide, WHICH IS MOLECULAR, with little interaction BETWEEN MOLECULES, has some solubility in water. On the other hand, diamond and graphite, which are NON-MOLECULAR, have NO SOLUBILITY in water or in any solvent. The solvent/solute interactions cannot break up the array of bonds in the solid. I hope you see from where I am coming.

Anyway, please read the section in your text that deals with this issue. If I can be of further help, I will.