Miscellaneous: Powdering calcium carbonate

Calcium carbonate is a compound which can be easily found in nature, in various forms. One of these forms is marble, which is almost pure calcium carbonate, even though it can come in various colors. Because of that, marble was also used in this experiment as a source of relatively pure CaCO₃

For this experiment, one needs to prepare a piece of marble, a few containers, and a tool which will be used to powder the marble. Here we used a file and a pneumatic grinder.

Powdering the marble with the file was very slow and exhausting, so the pneumatic grinder was used instead. The grinding produces a fine calcium carbonate dust, so it's a good idea to wear a respirator. Larger pieces of marble can also break off and fly around, so protective goggles and other safety equipment is also welcome.

On the image above, one can see the result of the grinding - almost pure powdered calcium carbonate, which can be used in various chemical reactions.

Although calcium carbonate may not seem as a very interesting compound, it can be used to synthesize many other calcium compounds which have various applications. The reaction with hydrochloric acid yields calcium chloride which is used for drying other compounds since it readily absorbs water; it is also used in the electrolytic production of calcium:

CaCO₃ + 2HCl -> CaCl₂ + H₂O + CO₂

The reaction with nitric acid yields calcium nitrate which is mostly used as a fertilizer, but also in pyrotechnics and for cooling (the reaction with water is endothermic):

CaCO₃ + 2HNO₃ -> Ca(NO₃)₂ + H₂O + CO₂

In a reaction with sulphuric acid, calcium sulphate is formed, which is also known as gypsum (in its dihydrate form). The anhydrous form is used for drying other compounds, similar to calcium chloride:

CaCO₃ + H₂SO₄ -> CaSO₄ + H₂O + CO₂

The reaction of calcium carbonate with acetic acid yields calcium acetate. The pyrolysis of calcium acetate yields acetone and calcium carbonate, which can be used for the synthesis of additional acetate or other calcium compounds:

CaCO₃ + 2CH₃COOH -> Ca(CH₃COO)₂ + H₂O + CO₂

Ca(CH₃COO)₂ -> (CH₃)₂CO + CaCO₃

Calcium carbonate is also used in the production of calcium oxide (quicklime) which is also an important compound of this element. Upon heating, calcium carbonate decomposes to calcium oxide and carbon dioxide:

CaCO₃ -> CaO + CO₂

The heating is usually done in lime kilns, which can be seen on the images below.

The one on the left is located in Croatia, Varaždin county, near Bela. There are also several abandoned quarries in that area. On the image to the right one can see a similar kiln located on the Sweedish island Gotland.

Calcium carbide is also an important compound of calcium, although it is not directly synthesized from calcium carbonate, but rather from the oxide:

CaO + 3C -> CaC₂ + CO