Chemically pure calcium carbide is almost colorless and transparent crystals, and extremely pure calcium carbide is large sky blue crystals, the same color as quenched steel. Industrial calcium carbide is divided into gray, brown or black according to its purity. Its crystal section is gray, and purple when the purity is high. The new section of calcium carbide has a shiny surface, and it will lose its luster and become grayish white when exposed to air due to the absorption of moisture in the air. Calcium carbide is actually an impure calcium carbide, containing lime and a small amount of impurities such as silicon dioxide, aluminum oxide, calcium sulfide, and calcium phosphide.
The chemical composition of calcium carbide with a CaC2 content of 85% is: CaC2 85.17%, CaO 10.60%, SiO2 2.34%, (Fe2O3+Al2O3) 2.55%, MgO 0.15%. Its density decreases with the increase of CaC2 content. Its melting point varies with the content of CaC2. The melting point of pure CaC2 is 2300℃. Industrial calcium carbide generally contains 80% CaC2, and its melting point is about 2000℃. The melting point of calcium carbide containing 69% CaC2 is the lowest, 1750℃. When the CaC2 content decreases further, the melting point rises again, and then drops to 1800℃ (equivalent to 35.6% CaC2). There is a highest temperature point of 1980℃ (equivalent to 52.5% CaC2) between 1750℃ and 1800℃. When CaC2 is lower than 35.6%, its melting point rises again.
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