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Let’s look at the equilibrium that develops in aqueous solution of ZnCO3:
ZnCO3 (s) `stackrel(larr)(->)` Zn^2+ (aq) + CO3^2- (aq)
According to Le Chatelier’s principle, if we could somehow decrease either [Zn2+] or [CO3]^2- then this equilibrium would shift to the right and the amount of solid would decrease. (ie. the solubility would increase.). This phenomenon of altering the solubility of a species in equilibrium by altering the concentration of one of its constituent ions is known as common ion effect.
There is a way to decrease the concentration of an ion here. The carbonate ion happens to be easy to decrease. All we have to do is add some acid (say HCl) to the container. The following sequence of reactions explains how this works.
HCl`stackrel(larr)(->)` H+ + Cl-
CO3^2- + 2H+ → H2CO3
Carbonic acid, H2CO3 is unstable in water solution, readily decomposing into H2O and CO2(g), and then the CO2(g) escapes into the air.
H2CO3 → H2O + CO2(g)
Because the CO2 escapes, the reverse reaction in the solubility equilibrium does not have a chance to take place. As per Le Chatelier’s principle, this would shift this equilibrium to the right and dissolve more solid ZnCO3 thereby increasing its solubility.
As long as more acid is added, this process will continue until all the solid ZnCO3 has been dissolved.
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