A saturated $CaCO_3$ stock solution is at 25°C. Experiment(i) adds25 g $Na_2CO_3$;experiment(ii) adds25 g $Na_2SO_4$. Which describes the calcium concentration changes?
Adding carbonate supplies a common ion, shifting CaCO₃ dissolution toward solid and lowering free calcium. Adding a noncommon electrolyte raises ionic strength; reduced activity coefficients can increase dissolved concentrations at the same thermodynamic solubility product, the salt effect. The intended answer is **B**. This qualitative comparison assumes the usual salt-effect regime without a separate sulfate precipitate dominating; solution volume and full speciation data are not supplied.
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