Salt A is sodium hydrogen carbonate (baking soda). When it is heated, it turns into sodium carbonate; Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia.  This is known as the Solvay process. $$mathrm{NaCl}+mathrm{CO}_{2}+mathrm{NH}_{3}+mathrm{H}_{2} mathrm{O} \rightarrow mathrm{NH}_{4} mathrm{Cl}+mathrm{NaHCO}_{3}$$ In this process, calcium carbonate is used as the source of $$CO_{2}$$, and the resultant calcium oxide is used to recover ammonia from ammonium chloride.which is salt B. In this reaction, carbon dioxide gas is also produced; which is the gas C. $$2 mathrm{NaHCO}_{3}+mathrm{Heat} \rightarrow mathrm{Na}_{2} mathrm{CO}_{3}+mathrm{CO}_{2}+mathrm{H}_{2} mathrm{O}$$ Sodium hydrogen carbonate is used in bakery products. Sodium carbonate is used for removing the hardness of water.