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I and three friends did an experiment where we investigate the salt effect, also known as the uncommon ion effect.

The specifics are that we made a $\pu{1L}$ solution saturated with $\ce{CaCO3}$ and $\ce{NaCl}$, filtrated it, and measured the $\ce{Ca^2+}$-ion concentration and compared it with a pure solution saturated with only $\ce{CaCO3}$. The $\ce{Ca^2+}$-ion concentration was measured using AAS.

We expected the solution with the $\ce{NaCl}$ to have more $\ce{Ca^2+}$-ions than the one without, but that wasn't the case. The opposite was true. The concentration dropped by about 15% when the salt effect was supposed to occur.

Is it possible that there were so many $\ce{Na+}$ and $\ce{Cl-}$ ions that there weren't enough $\ce{H2O}$ molecules anymore to surround the $\ce{Ca^2+}$ and carbonate ion, causing their concentration to drop? Or is it far more likely that we simply made a mistake somewhere.

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