UPSC NDA II 2023 General Ability Solved Paper

Concept:Goldsmiths use nitric acid for cleaning gold and silver because it dissolves impurities (like copper) without attacking the precious metals.Explanation:Nitric acid ($\mathrm{HNO}_3$) is a strong mineral acid that reacts with base metals like copper present in alloys.When gold or silver articles are dipped in dilute nitric acid, the acid dissolves the surface impurities, leaving the gold or silver clean.Pure gold does not react with nitric acid, so the acid test is also used to verify gold purity.For cleaning silver, nitric acid removes tarnish and base metal alloys effectively.However, to dissolve gold itself, a mixture of nitric acid and hydrochloric acid (called aqua regia in 3:1 ratio) is required.Other options:• Sulphuric acid ($\mathrm{H}_2\mathrm{SO}_4$) is too corrosive and not used for cleaning precious metals.• Hydrochloric acid ($\mathrm{HCl}$) alone does not dissolve base metals effectively for cleaning purposes.• Phosphoric acid ($\mathrm{H}_3\mathrm{PO}_4$) is weaker and used for rust removal, not for gold/silver cleaning.Thus, nitric acid is the correct choice for routine cleaning by goldsmiths.Answer:Nitric acid