Q.How is backing soda prepared by solvay-process. Give its principle only.

The Solvay (ammonia-soda) process manufactures sodium carbonate from brine, ammonia and limestone. Limestone is first heated to give quicklime and carbon dioxide, $CaCO_3 \rightarrow CaO + CO_2$; the $CO_2$ is passed into brine that has been saturated with ammonia, where the ammonia keeps the solution alkaline and reacts with $CO_2$ and water to form ammonium bicarbonate, $NH_3 + H_2O + CO_2 \rightarrow NH_4HCO_3$. This then reacts with the dissolved sodium chloride; since $NaHCO_3$ is the least soluble of the four ionic species present in solution, it precipitates out, $NH_4HCO_3 + NaCl \rightarrow NaHCO_3\downarrow + NH_4Cl$. The precipitated sodium bicarbonate is filtered off and calcined (heated) to give sodium carbonate, $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$, with the evolved $CO_2$ recycled back into the process. Ammonia is regenerated (and so is not consumed overall) by treating the ammonium chloride filtrate with the quicklime left over from the first step, $2NH_4Cl + C …