Calcium Carbide Formula CaC2: Properties and Reactions
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Formula and Identity
The chemical formula of calcium carbide is CaC2. It is an ionic compound built from calcium cations and acetylide anions, not a mixture, and its industrial importance comes from what happens when the acetylide ion meets water. The table below collects the identity data used on labels, safety data sheets and transport documents.
| Item | Value |
|---|---|
| Chemical formula | CaC2 |
| Molar mass | 64.10 g/mol |
| CAS number | 75-20-7 |
| EINECS number | 200-848-3 |
| UN number | 1402 |
| Density | About 2.22 g/cm3 |
| Melting point | About 2,160 C |
| Appearance | Grey to grey-black lumps or powder |
Physical and Chemical Behaviour
Commercial calcium carbide is a grey-black solid with a hard, brittle texture and a characteristic odour that comes not from CaC2 itself but from traces of calcium phosphide and calcium sulfide, which release phosphine and hydrogen sulfide on contact with moisture. Pure CaC2 is colourless; the grey colour of technical material is caused by free carbon and iron-containing impurities. The material is stable while it stays dry, and it does not burn in air in the absence of water, but it reacts readily with atmospheric humidity, which is why drums are sealed and why stock in humid climates deteriorates measurably over months.
Reaction with Water and Acetylene Yield
The reaction that defines the product is CaC2 + 2H2O = C2H2 + Ca(OH)2. One mole of carbide yields one mole of acetylene, which means that the theoretical mass yield from pure CaC2 is about 0.406 kg of acetylene per kilogram, equivalent to roughly 350 litres of gas per kilogram at 0 C and 101.325 kPa. Real material never reaches that figure because technical carbide contains free lime, unreacted carbon and ash; commercial grades are therefore graded by measured gas yield, and good grades deliver between 260 and 300 litres of acetylene per kilogram at 20 C. The hydrolysis is strongly exothermic, and the calcium hydroxide by-product is produced as a slurry that must be handled as an alkaline waste stream.
Industrial Significance of the Formula
The CaC2 molecule is the entry point to acetylene chemistry. Acetylene produced from carbide is the feedstock for a range of chemical products and remains in use for oxy-acetylene welding, cutting and heating where high flame temperature matters. Calcium carbide is also a reducing and desulfurising agent in iron and steel, and it is charged into ladles and induction furnaces for that purpose, where the calcium and carbon act independently. In the foundry the same compound is used for desulfurisation of iron and for carburisation, and its by-product lime contributes to slag conditioning.
Quality Grading and Impurities
Calcium carbide is traded on gas yield, size fraction and impurity limits rather than on a chemical assay of CaC2. National specifications such as GB/T 10665 set the grading structure for the Chinese market, and customers typically order against a minimum gas yield plus a size class. The impurity that most often causes complaints is phosphorus, which forms phosphine in the generator; hydrogen sulfide from sulfide impurities must be scrubbed from the gas before use in chemical synthesis. Fines content is controlled separately because fine particles react too quickly in a generator and can cause local overheating.
Safety, Storage and Transport
Calcium carbide is a dangerous goods item. It is classified under UN 1402 in class 4.3, substances which in contact with water emit flammable gases, and transport and storage must follow that classification. The acetylene it generates in contact with moisture is flammable over a wide range, roughly 2.5% to 81% by volume in air, so the gas must never be allowed to accumulate in a confined space. Storage areas should be dry, ventilated, free of water lines and sprinkler systems, and separated from acids and oxidising agents. Drums are opened only as needed, with non-sparking tools, and the material should be kept away from sources of ignition. Small spills are swept up dry; water is never used for clean-up. Personal protective equipment includes eye protection, gloves and respiratory protection where dust is generated, and the atmosphere in enclosed handling areas should be monitored for acetylene and for phosphine.
Frequently Asked Questions
Q: What is the chemical formula of calcium carbide?
A: CaC2, with a molar mass of 64.10 g/mol. It is an ionic acetylide of calcium and is supplied as grey-black lumps or powder.
Q: What happens when calcium carbide is added to water?
A: CaC2 + 2H2O = C2H2 + Ca(OH)2. Acetylene gas is released and calcium hydroxide forms as an alkaline slurry; the reaction is exothermic and generates considerable heat.
Q: How much acetylene does a kilogram of carbide produce?
A: The theoretical maximum for pure CaC2 is about 0.406 kg per kilogram, roughly 350 litres at 0 C and 101.325 kPa. Commercial grades deliver 260-300 litres per kilogram at 20 C depending on their gas yield rating.
Q: Where does the smell of calcium carbide come from?
A: From impurities rather than from CaC2 itself. Calcium phosphide and calcium sulfide in technical carbide release phosphine and hydrogen sulfide when moisture is present, and those gases carry the characteristic odour.
Q: How should calcium carbide be stored?
A: In dry, sealed containers in a dry, well-ventilated building with no water piping or sprinkler coverage, away from acids and oxidisers. Keep quantities to the working minimum and open containers only when required.
