Calcium carbonate $CaCO3$ is heated to form calcium carbide $CaC2$, which reacts with water to produce acetylene gas $C2H2$. Acetylene gas is then used to synthesize ethylene $C2H4$ through a process called hydrochlorination, where acetylene is reacted with hydrogen chloride $HCl$ to form vinyl chloride $C2H3Cl$ which is then hydrolyzed to form ethylene $C2H4$. Ethylene can further be reacted with hydrogen chloride to form ethyl chloride $C2H5Cl$.