Calcium Carbonate
Properties
| State | Solid (white powder or crystalline) |
| Color | White |
| Solubility | Insoluble in water (0.013 g/L at 25°C); soluble in dilute acids |
| Melting Point | 825°C (decomposes to CaO + CO2) |
| Boiling Point | Decomposes before boiling |
About Calcium Carbonate
Calcium carbonate is the workhorse mineral of the planet's surface — limestone, chalk, marble, coral skeletons, and the inside of every clam shell are all CaCO3, mostly in the calcite polymorph but with significant aragonite contributions in marine biology. Its chemistry is dominated by two equilibria. First, it is a sparingly soluble salt with Ksp around 3.3 × 10⁻⁹ at 25 °C, which is why it precipitates out of hard water and forms scale in kettles and pipes. Second, it is the textbook example of acid–carbonate evolution: CaCO3 + 2 HCl → CaCl2 + H2O + CO2, which is the reaction that erupts dramatically when you drop a piece of limestone into dilute HCl, dissolves antacid tablets in stomach acid, and slowly carves karst caves and sinkholes wherever slightly acidic groundwater meets limestone bedrock. Industrially, calcium carbonate is the feedstock for the entire lime cycle: heat it past 825 °C and it loses CO2 to give CaO (quicklime), which on hydration becomes Ca(OH)2 (slaked lime), which on exposure to atmospheric CO2 reverts back to CaCO3. That cycle drives Portland cement chemistry, paper manufacturing, steelmaking flux, and agricultural liming. Ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) are also the cheapest white filler in the world, which is why a glossy magazine page can be 30% CaCO3 by weight.
Where you'll encounter it
If you have ever chewed a Tums tablet, sanded a piece of joint compound, or sprinkled garden lime to fix an acidic vegetable bed, you have handled calcium carbonate. In a paper mill it is the filler and coating pigment that gives bright, opaque sheets at lower cost than wood pulp. In a cement plant the rotary kilns are basically running the CaCO3 → CaO + CO2 calcination at industrial scale. In an analytical lab, primary-standard CaCO3 is what you use to standardize EDTA solutions for hardness titrations, and a dilute HCl bottle dripped on an unknown rock is the field geologist's quick test for limestone vs. dolomite — limestone fizzes immediately, dolomite needs warming or scratching first.
Common Uses
- Active ingredient in antacid tablets (Tums) at 500–1000 mg per dose
- Feedstock for Portland cement via calcination to CaO
- Bright, low-cost filler and coating pigment in paper, paint, and PVC
- Agricultural lime to raise soil pH on acidic farmland
- Mild abrasive in toothpaste and scouring polishes
- Primary standard for EDTA standardization in hardness titrations
- Flux in iron smelting to form a fluid CaO·SiO2 slag
- Calcium fortification of orange juice, soy milk, and breakfast cereals
Safety Information
CaCO3 is essentially non-toxic — it is GRAS for food use and the only oral hazard is the milk-alkali syndrome that develops if a patient swallows several grams a day for weeks alongside calcium-rich foods. The relevant occupational hazard is respirable dust: it is controlled as a nuisance particulate rather than a toxicant, and chronic high-dust exposure has been linked to mild pneumoconiosis. No fire or reactivity hazard at normal handling temperatures.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.