We were taught these rules to predict the order of solubility in general cases.

This is the theory we were taught:

Case1) When number of cations is greater than number of anions,

Solubility increases down the group

Case2) When number of anions is greater than number of cations,

Solubility decreases down the group


Case3**) When number of cations = number of anions,

          Case3.1) Radius of cation> radius of anion (for anions like                        >.                                Chloride, oxide, fluoride, hydroxide, sulphide)

                           Solubility increases down the group.

           Case3.2) Radius of anion> radius of cation (for anions like        >.                                   Iodide, bromide and polyatomic anions**)

                           Solubility decreases down the group(but for                              >.                                  hydroxides, solubility increases down the group)

Case4) For heavier metal compounds, we always use the concept of polarization which in turn is directly proportional to "intensity of color" and inversely to "solubility" .

I haven't seen these rules in any book or elsewhere so I am assuming my teacher came up with these rules on his own after analyzing solubility order in many compounds.

Here are things I don't understand-

A) Why does solubility depend on the number of cations and number of anions?

B) Ionic compounds are soluble in water. For a general compound which doesn't show hydrogen bonding, its solubility should depend upon its percentage of ionic character. Comparison of percentage ionic and covalent character can be simply done using Fajan's rule. So is comparing solubility order by comparing their percentage ionic character wrong?

C) In case3, how does the radius of cation and anion affect solubility?

  • 1
    $\begingroup$ Solubility in what? Water, benzene? If you're talking about water then number 2 isn't right for group-2 elements. Eg. Solubility of $\ce{CaCl2, MgCl2}$ is in 100 mL 74.5 g & 54.3 g respectively. $\endgroup$ – Mockingbird Jul 18 '17 at 3:51
  • $\begingroup$ According to Fajan's rule even your case 3 is wrong. $\endgroup$ – Mockingbird Jul 18 '17 at 3:55

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