The primary component of most seashells is calcium carbonate (CaCO3). You may have noticed bubbles forming when you initially covered the seashell with vinegar. The bubbles are carbon dioxide (CO2), which is created when the CaCO3 in the shell is exposed to an acid such as vinegar. The reaction that describes this process is:
CaCO3 + 2H+ ⇔ Ca2+ + CO2 + H2O
Sodium bicarbonate (NaHCO3) and calcium chloride (CaCl2) both dissolve well in water, and the solutions you made with them should have appeared relatively clear. These dissolutions can be expressed as:
NaHCO3 → Na+ + HCO3−
CaCl2 → Ca2+ + 2Cl−
In water, bicarbonate (HCO3−) is never present by itself, but exists in equilibrium with other forms of dissolved inorganic carbon: carbonic acid (H2CO3), carbonate (CO32−), and carbon dioxide (CO2). The balance between all of these species is shown by these equations:
CO2 + H2O ⇔ H2CO3 ⇔ HCO3− + H+ ⇔ CO32− + 2H+
When you mix the NaHCO3 and CaCl2 solutions together, the carbonate ion (CO32−) reacts with the calcium ion (Ca2+) to form CaCO3 through the following reaction:
CO32− + Ca2+ ⇔ CaCO3
The mixture should turn cloudy since CaCO3 is not very soluble in water and will precipitate out of the solution upon forming. You have just made little bits of shells!
When you add vinegar to this mixture, the excess hydrogen ions (H+) will dissolve the calcium carbonate (CaCO3) particles—just like it did to your seashell—and the solution should turn clear again. There should be many bubbles because the bicarbonate in solution will also react with the vinegar to form CO2.
Adding a base such as sodium hydroxide (NaOH) will then shift the equilibrium back to solid CaCO3, forming a cloudy precipitate again. This shows how sensitive CaCO3 is to the pH of its environment.