Q.Choose the correct option.
What happens when crystalline Na2CO3 is heated ?
a. releases CO2 b. loses H2O c. decomposes into NaHCO3 d. colour changes.

Sodium carbonate, washing soda, Na2CO3.10H2O, is manufactured by the Solvay process. First, CO2 is passed into a concentrated NaCl solution saturated with NH3, forming ammonium bicarbonate in solution, which then undergoes a double-decomposition reaction with the dissolved sodium chloride to precipitate the comparatively less-soluble sodium bicarbonate: NH4HCO3(aq) + NaCl(aq) → NH4Cl(aq) + NaHCO3(s). Heating these NaHCO3 crystals gives sodium carbonate, 2NaHCO3 →(Δ) Na2CO3 + H2O + CO2. The ammonia used is recovered and reused by treating the leftover NH4Cl with slaked lime, Ca(OH)2, which regenerates NH3 and produces CaCl2 as a byproduct — this ammonia recycling is what makes the process economical. (Potassium carbonate cannot be made the same way, because potassium bicarbonate is too soluble to precipitate out.) Sodium carbonate itself is a highly water-soluble white solid that loses its water of crystallisation on heating (first to the monohydrate, then above 373 K to an …