Acetylene reacts with ammonical solution of copper(I)chloride (Cu2Cl2or CuCl) to form red precipitate of copper(I) acetylide
C2H2 + 2CuCl + 2NH4OH → Cu2C2+ 2NH4Cl + H2O
copper(I) acetylide is reacted with HCl to get back acetylene
Cu2C2+ 2HCl → C2H2+ 2CuCl
Ethylene does not react
Thus, ethylene can be separated from acetylene by passing the mixture through ammoonical copper(I)chloride