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. Dust exposure limits are set well below those for inert particulate. 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. 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.