Question

I do a lab where we mix a solution of sodium carbonate with a solution of calcium nitrate. As expected, calcium carbonate (white solid) precipitates out. However, if the concentration is high enough, when the solutions are first mixed a translucent, goopy substance forms. Continuing to stir that substance for another minute or two always produces the white solid. The goopy substance does not form if the solutions are dilute enough (~2 g or less of CaCO3 forming in ~100 mL of water) but always forms if the solutions are more concentrated (~4 g or more of CaCO3 forming in ~100 mL of water). What that goopy substance is? Also, it only forms once the two solutions are mixed.

Answer 1

The goopy substance is likely a hydrated intermediary phase of calcium carbonate, forming under high concentrations before transforming into the typical white solid precipitate during stirring.

Step-by-step explanation:

The translucent, goopy substance that forms when high concentrations of sodium carbonate and calcium nitrate solutions are mixed is likely a hydrated intermediary phase of calcium carbonate. This gel-like substance may involve clusters of CaCO3 molecules encased in a water layer, which, upon stirring, transform into the better-known white solid precipitate of calcium carbonate. The occurrence of such a phase is not typically noted at lower concentrations due to insufficient ion quantities needed for its formation or because it might instantly form the solid without the intermediate state being clearly observed. In contrast to the gel-like substance, the white solid as seen in the test where calcium carbonate is directly formed is the result of a straightforward precipitation reaction.

According to solubility rules, calcium carbonate is generally insoluble in water, which is why it precipitates. However, the initial gelatinous phase can happen due to supersaturation conditions, where the ions come together rapidly, trapping water molecules and not immediately forming the regular crystal lattice of calcium carbonate. Upon further stirring, water is expelled, and the regular crystalline precipitate forms. This process is analogous to the formation of soap where calcium stearate precipitates when hard water reacts with soap.