CBSE Class 10 Science 2020 · Paper 31/5/1 · Question 16(b)

(a) Chemical name: sodium carbonate decahydrate; common name: washing soda; chemical formula: ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (b) $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3⟶{\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$; then $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$; then ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}⟶{\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (c) It removes the permanent hardness of the water: the calcium and magnesium salts are converted into insoluble calcium carbonate and magnesium carbonate, which separate out, for example ${\text{CaCl}}_2+{\text{Na}}_2{\text{CO}}_3⟶{\text{CaCO}}_3+2\text{NaCl}$.