CBSE Class 10 Science 2020 · Paper 31(B) · Question 25(b)

(a) Common salt is formed by the combination of hydrochloric acid ($\text{HCl}$) and sodium hydroxide ($\text{NaOH}$): $\text{HCl}+\text{NaOH}⟶\text{NaCl}+{\text{H}}_2\text{O}$. Formula: $\text{NaCl}$; chemical name: sodium chloride. Source: sea water (it is also mined as rock salt). (b) Rock salt is common salt (sodium chloride) found as deposits of solid salt in the form of large crystals, formed when seas of bygone ages dried up; it is mined like coal. Its colour is often brown, and the colour is due to impurities. (c) The brine decomposes: sodium hydroxide is formed, chlorine gas is given off at the anode and hydrogen gas at the cathode (the chlor-alkali process). $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$