Calcium Oxide
Properties
| State | Solid at room temperature |
| Color | White to pale yellow |
| Solubility | Reacts with water (forms calcium hydroxide) |
| Melting Point | 2613 °C |
| Boiling Point | 2850 °C |
About Calcium Oxide
Calcium oxide — quicklime — is the first step of the lime cycle, made by driving CO2 out of limestone in the calcination reaction CaCO3 → CaO + CO2 above 825 °C, an endothermic process that consumes about 178 kJ/mol and accounts for a meaningful fraction of global industrial CO2 emissions just from the stoichiometry, before you even count the fuel. The calcined oxide is reactive specifically because it has empty Ca²⁺ coordination sites and a strong thermodynamic drive to either pick up CO2 (reverse calcination) or, more aggressively, hydrate to Ca(OH)2: CaO + H2O → Ca(OH)2, ΔH ≈ −63 kJ/mol. The slaking reaction is exothermic enough to boil the slaking water, ignite paper, and historically powered the limelight stage lighting of 19th-century theater — a block of CaO heated to incandescence by an oxyhydrogen flame produced an intense thermoluminescent white at around 2400 °C, the brightest reliable point source available before electric arc lamps. Industrially, world production is roughly 350 million tonnes per year, the bulk of it routed straight to three uses: basic-oxygen steelmaking (where CaO fluxes silica and absorbs phosphorus and sulfur into a CaO–SiO2 slag), Portland cement clinker (where CaO and SiO2 react at 1450 °C to form alite C3S and belite C2S), and water and flue-gas treatment (where CaO/Ca(OH)2 captures SO2, HCl, and HF in scrubbers). The melting point at 2613 °C also makes single-crystal CaO a refractory ceramic for niche high-temperature crucibles.
Where you'll encounter it
On a steel mill floor, quicklime arrives by the truckload and gets dumped into the basic oxygen furnace at 5–8% of the iron charge — without it, sulfur and phosphorus would stay in the steel and embrittle it. In a cement plant, CaO is the dominant oxide in the kiln, around 65% of the clinker by mass. Field crews on civil-engineering jobs spread quicklime onto wet clay subgrade to dry and stabilize it before paving — the slaking reaction does the dewatering work mechanically and chemically at once. Around a chemistry lab, CaO appears as a desiccant for amines and alcohols where CaCl2 would form an adduct. In a self-heating MRE pouch, a sealed packet of CaO and water generates the heat through the same 63 kJ/mol slaking reaction that powered limelight. And every elementary school chemistry demo where someone adds water to 'quicklime' and watches steam erupt is showing the same exotherm.
Common Uses
- Flux in basic oxygen steelmaking at 5–8% of charge to absorb Si, P, and S into slag
- Primary oxide component of Portland cement clinker (around 65% of finished cement)
- Drying agent for amines and alcohols where CaCl2 would form Lewis adducts
- Subgrade soil stabilization on highway and earthworks projects via slaking dewatering
- Flue-gas desulfurization sorbent that captures SO2 as solid CaSO3/CaSO4
- Heat source in self-heating MRE meal pouches via the exothermic slaking reaction
- Refractory crucible material for high-temperature melts above 2000 °C
- Carbide manufacture by reaction with coke at 2200 °C to give CaC2
Safety Information
GHS H315/H318/H335. The combination of strong alkalinity (forms pH-12 Ca(OH)2 on contact with moisture) and a violent exothermic hydration is what makes CaO genuinely dangerous — splash on skin or in eyes generates heat and base simultaneously, and the resulting burns are deeper than a simple chemical exposure of either type alone. OSHA PEL is 5 mg/m³ total dust. Inhalation causes severe respiratory tract injury through the same mechanism. Full face shield, alkali-resistant gauntlets, and respirator (N95 minimum) for dusty handling. Keep dry until the moment of intended use; drums of partially hydrated CaO can self-heat dangerously.
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.