Sodium carbonate and calcium carbonate are both alkaline and metallic carbonates; both also react with various acids to produce salts. Still, there are a number of differences between them.
Sodium and calcium fall under different groups on the periodic chart. Sodium (atomic weight 23) is a very electropositive alkali metal. Calcium (atomic weight 40) is an alkaline earth metal and not as electropositive. Sodium forms ions of charge +1. Calcium forms ions of charge +2.
Sodium carbonate is Na₂CO₃. Calcium carbonate is CaCO₃. Sodium carbonate contains seven water molecules of crystallization; calcium carbonate has a maximum of six.
Solubility and Basicity
Sodium carbonate is very water-soluble. Sodium carbonate, with a pH of greater than 11 for a concentrated solution, is a much stronger base than calcium carbonate, which doesn't even dissolve in water.
Pure calcium carbonate can be taken as an antacid. Sodium carbonate, by contrast, except when used as a minor ingredient, is hazardous to humans. It is used in pickling, puddings, sauces, and baked goods.
Sodium carbonate is used to manufacture glass and in water-softening. It is used in laundering, going by the name "washing soda" or "soda ash." It is also used in food processing. Calcium carbonate is mined as limestone, chalk, and marble. It is used in concrete, in manufacturing glass, as an extender in paints, as a filler, in sealants, in ceramics, and in blast-furnaces.