CaCO3

CaCO3 = calcium carbonate.

Key quick notes for exams:

• Formula and molar mass

  • CaCO3
  • Molar mass ≈ 100.09 g/mol

• Structure and forms

  • Carbonate ion: CO3^2− is planar and trigonal (sp2).
  • Calcium ions: Ca^2+ lattice with carbonate groups.
  • Polymorphs/minerals: calcite (trigonal/rhombohedral, the most common), aragonite (orthorhombic, less stable at room temp), and a less common vaterite form.

• Important properties

  • Very low solubility in water.
  • Common minerals: limestone (rock), marble (metamorphosed limestone), chalk.
  • Reactivity with acids: effervesces with HCl and other acids.

• Solubility and equilibria

  • Dissolution: CaCO3(s) ⇌ Ca^2+(aq) + CO3^2−(aq)
  • Ksp at 25°C: about 3.3 × 10^−9
  • In pure water, an approximate solubility would suggest [Ca^2+] ≈ [CO3^2−] ≈ sqrt(Ksp) ≈ 5–6 × 10^−5 M, but actual dissolution is affected by carbonate speciation (CO3^2− ↔ HCO3^− ↔ H2CO3/CO2) and CO2 in solution.

• Reactions you should know

  • With acids: CaCO3 + 2 H^+ → Ca^2+ + CO2 + H2O (effervescence)
  • With strong acids in water: same net reaction as above; dissolves readily.
  • With CO2 in water: CaCO3 + CO2 + H2O ⇌ Ca(HCO3)2 (calcium bicarbonate) — this is the dissolution mechanism in natural waters and weathering; Ca(HCO3)2 is soluble.
  • Thermal decomposition (calcination): CaCO3(s) → CaO(s) + CO2(g) at high temperature (~900–1000°C); endothermic; used to make lime.
  • Basic lab/industrial uses: formation of CaO for cement/lime, neutralization of acidity (soil, flue gas), filler in paints/paper, antacid (calcium carbonate tablets).

• Occurrence and uses

  • Abundant in rocks (limestone, marble), shells, teeth of marine organisms.
  • Major industrial uses: cement and concrete (as limestone and as a source of CaO after calcination), dietary/calcium supplement, agricultural lime to neutralize soil acidity, filler/piller in various products.

• Quick exam-style practice

  • Write the balanced equation for CaCO3 reacting with hydrochloric acid.
    • CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
  • What happens when CaCO3 is heated?
    • It thermally decomposes to CaO and CO2: CaCO3(s) → CaO(s) + CO2(g)
  • If a bottle contains only CaCO3 and water with air containing CO2, what species primarily forms in solution?
    • Calcium bicarbonate, Ca(HCO3)2, via CaCO3 + CO2 + H2O ⇌ Ca(HCO3)2 (soluble)
  • The solubility product (Ksp) indicates what about CaCO3’s solubility?
    • Very low solubility; at 25°C Ksp ≈ 3.3 × 10^−9. In pure water, approximate solubility would put [Ca^2+] around 10^−5 M in the absence of CO2, but actual solubility is higher in the presence of CO2 due to bicarbonate formation.

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