I tried conducting an experiment I saw at school with some materials I had at home. The experiment consisted of mixing a solution of Acetic Acid with Calcium Carbonate to yield Calcium Acetate: $$\ce{CaCO3 + CH3COOH -> Ca(CH3COOH)2 + CO3 + H2O}$$ I'm providing as much detailed in a summarized form to show what I did step-by-step if I did anything wrong. I didn't have any lab grade source of $\ce{CaCO3}$, but I found that chalk has a large concentration of $\ce{CaCO3}$, and I used that instead. I worked out a small calculator on Spreadsheets and found the appropriate quantities of each reactant. I then combined both of the ingredients and left the reaction to settle. After a couple days, I stirred and saw little to no fizzing, however, there was a lot of precipitate at the bottom of the beaker which I assumed to have been non-$\ce{CaCO3}$ residue from the chalk. I filtered the solution through a two coffee filters and let the solution evaporate. I should note that the solution was crystal clear after it was filtered. I came back a week later only to see a hard, styrofoam look-a-like on the sides of the beaker, and a layer of cloudy crystals at the base of the beaker. At the center of the base there's a crystal structure sprouting separately from the rest of the crystal layer.

Inside the beaker

I initially though perhaps my calculations or initial data gathered were incorrect but even after triple-checking them there was no error. Looking for possible structures of Calcium Acetate there was only one image that looked somewhat like the crystal sprouting at the center of the beaker, but still it doesn't explain what all the other substances are. Does Calcium Acetate have multiple crystal structures? Or has there been some sort of contamination from the chalk. Any help would really be appreciated:)


1 Answer 1


Too long for a comment, hence an answer:

  1. What you are asking about is not crystal crystal structure, it's called crystal morphology. Crystal structure is a model describing atomic positions and overall symmetry within a unit cell, whereas crystal morphology is the appearance of the crystal on a macro-scale. And yes, crystal morphology is also affected by the conditions crystals are growing under (concentration and evaporation rate, in your case), so that the same substance can have crystals of different shape.
  2. The formula for calcium acetate and hence the equation is wrong, the correct one would be $(\ce{OAc-} = \ce{CH3COO-})$ $$\ce{CaCO3 + 2HOAc → Ca(OAc)2 + H2O + CO2}$$ and, since your reaction's stoichiometry suggested 1:1 ratio between calcium carbonate and acetic acid, I suspect you didn't use enough acid leaving $\ce{CO3^2-}$ in the solution. Then, as the primary product – calcium acetate $\ce{Ca(OAc)2}$ – has crystallized (white edgy crystals on the bottom), the remaining calcium carbonate $\ce{CaCO3}$ started to precipitate or co-crystallize on the walls of the beaker (what you call a "styrofoam look-a-like" phase), resembling how calcite grows in nature (see this and other images of natural calcite on MineralMiner's FTP server):
    Also note that there is no such thing as $\ce{CO3}$. It's either carbonate $\ce{CO3^2-}$ (solution) or carbon dioxide $\ce{CO2}$ (gas).

  3. Regarding the the impurities from the chalk, usually there are parts of shells of marine animals and silicates (can be eliminated by filtering), also minor amounts of $\ce{MgCO3}$. However, if you took a chalk bar from the classroom, this is a different story as it's often nearly 50% $\ce{CaSO4}$ (I hope you didn't do that).


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