[studytogether]

why adding salt of good solubility can increase dissolution of bad Sy one?

can I explain as the same reason as salt dissolve in water?

Just like the water encircle salt to dissolve salt,

the salt of good solubility encircle the salt of bad solubility, thus increase its solubility?

is the description above properly?

[Sev]

Consider AgCl (sparingly soluble salt).

AgCl is in equilibrium: AgCl ↔ Ag⁺ + Cl^-

AgCl solubility can be increased by adding an ion that precipitates Ag⁺ or Cl^-.
eg. Pb(NO₃)₂ is soluble.  Pb²⁺ will form precipitate PbCl₂ with Cl^-.  [Cl^-] decreases, shifting AgCl equilbirum to the right.  So addition of Pb(NO₃)₂ has increased AgCl solubility.

[Borek]

Pb(NO₃)₂ is soluble.  Pb²⁺ will form precipitate PbCl₂ with Cl^-.  [Cl^-] decreases, shifting AgCl equilbirum to the right.  So addition of Pb(NO₃)₂ has increased AgCl solubility.

No. Concentration of Cl^- in saturated AgCl solution is way too low for PbCl₂ precipitation. It may work other way around - adding AgNO₃ to PbCl₂ solution. But I am not sure I will decide to call this situation "increased solubility". Depends how you define solubility - note that you don't have more PbCl₂ in the solution, just more Pb²⁺. That's not the same thing.

studytogether: adding an inert salt increases ionic strength of the solution, lowering ions activities. This will increase solubility for sure.

[Sev]

No. Concentration of Cl^- in saturated AgCl solution is way too low for PbCl₂ precipitation.

Is that because Q (IP) will be less than K_sp of PbCl₂?

[studytogether]

thanks for you both.

could I view the reason why we increase ionic strength of bad-soluable salt can increase its solubilty is same to the reason why hydration can dissolve salt?

I feel they are similar in certain degree..

[Borek]

No. Concentration of Cl^- in saturated AgCl solution is way too low for PbCl₂ precipitation.

Is that because Q (IP) will be less than K_sp of PbCl₂?

Exactly.

[Borek]

could I view the reason why we increase ionic strength of bad-soluable salt can increase its solubilty is same to the reason why hydration can dissolve salt?

I feel they are similar in certain degree..

You may assume that ions "shield" other ions which makes them "less visible" in the solution. But the real explanation (and the one that gave correct results when described mathematically) was different. Sorry, I have no time to delve deeper right now. Look for Debye-Huckel theory.

[Sev]

Exactly.

OK Thanks Borek 

[studytogether]

thx all