A metal carbonate X on treatment with a mineral acid liberates a gas which when passed through aqueous solution of a substance Y gives back X. The substance Y on reaction with the gas obtained at anode during electrolysis of brine gives a compound Z which can decolorise coloured fabrics. The compounds X, Y and Z respectively are
CaCO3,Ca(OH)2,CaOCl2
CaCO3+HCl→CaCl2+H2O+CO−2 (x)Ca(OH)2+CO2→CaCO3+H2O (y)Ca(OH)2+Cl2→CaOCl2 (z)