CaCO3
Calcium carbonate (CaCO3)
Also known as: limestone, calcite, chalk, marble
Calcium carbonate is the mineral of shells, coral, chalk cliffs, marble statues and limestone mountains. It is an ionic compound of Ca²⁺ and the polyatomic carbonate ion CO₃²⁻. Heated strongly it decomposes into quicklime and CO₂ — one of humanity's oldest industrial reactions — and it fizzes satisfyingly in acid, the classic geology field test.
Molar mass breakdown
100.086 g/mol| Element | Atoms | Mass (g/mol) | % by mass |
|---|---|---|---|
| Oxygen (O) | 3 | 47.997 | 47.96% |
| Calcium (Ca) | 1 | 40.078 | 40.04% |
| Carbon (C) | 1 | 12.011 | 12% |
What calcium carbonate is used for
- Cement and lime production
- Antacid tablets (neutralises stomach acid)
- Filler in paper, paint and plastics
- Agricultural lime for correcting acidic soil
Key facts
- Molar mass ≈ 100.09 g/mol — a convenient round number for stoichiometry practice.
- Decomposes above ~825 °C: CaCO₃ → CaO + CO₂.
- Reacts with HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ — the fizz test.
Frequently asked questions
What is the molar mass of CaCO3?
About 100.09 g/mol: 40.078 (Ca) + 12.011 (C) + 3 × 15.999 (O).
Why does CaCO3 fizz with acid?
The carbonate ion reacts with H⁺ to release carbon dioxide gas — the bubbles you see.
Are chalk, marble and limestone the same chemical?
Yes — all are mainly CaCO₃ with different crystal sizes and histories: chalk from microorganism shells, marble from heat-transformed limestone.