Solvay process : It is an industrial method for obtaining sodium carbonate from sodium chloride (NaCl) and limestone ($\mathrm{CaCO_3}$). The raw materials required in this process are common salt, ammonia and limestone. The equations for the complete process may be written as : $2\mathrm{NH_3} + \mathrm{H_2O} + \mathrm{CO_2}$ → $\mathrm{(NH_4)_2CO_3}$ $\mathrm{(NH_4)_2CO_3} + \mathrm{H_2O} + \mathrm{CO_2}$ → $2\mathrm{NH_4HCO_3}$ $\mathrm{NH_4HCO_3}$ + NaCl → $\mathrm{NH_4Cl} + \mathrm{NaHCO_3}$ Sodium hydrogen carbonate crystal separates out and is heated to give sodium carbonate. $2\mathrm{NaHCO_3}$ $\xrightarrow[\Delta]{}$ $\mathrm{Na_2CO_3}+\mathrm{CO_2}+\mathrm{H_2O}$ $\mathrm{CO_2}$ required in this process is produced by heating $\mathrm{CaCO_3}$. $\mathrm{CaCO_3}\xrightarrow[\Delta]{}\mathrm{CaO}+\mathrm{CO_2}$ CaO + $\mathrm{H_2O}$ → $\mathrm{Ca(OH)_2}$ In this process, $\mathrm{NH_3}$ is recovered when the solution containing $\mathrm{NH_4Cl}$ is treated with $\mathrm{Ca(OH)_2}. \mathrm{CaCl_2}$ is obtained as a by product. $2\mathrm{NH_4Cl} + \mathrm{Ca(OH)_2}$ → $2\mathrm{NH_3} + \mathrm{CaCl_2} + \mathrm{H_2O}$