NDA (II) 2023 General Ability Test · Question 89 of 146

Correct — B, Nitric acid. Nitric acid is an oxidising acid, and that single property is what makes it the goldsmith's acid. Silver and copper sit below hydrogen in the activity series, so ordinary acids leave them alone, but nitric acid oxidises and dissolves them anyway. Gold and the platinum-group metals, by contrast, do not react with nitric acid at all. So a warm dip in dilute nitric acid eats away the tarnish and the base metal from the surface of an ornament and leaves the gold behind untouched. The same behaviour is why jewellery shops use nitric acid as a cheap way to spot low-gold alloys and judge purity, and why the separation of gold from silver is called nitric acid parting.

• (a)Sulphuric acid — In the dilute form a workshop would use, sulphuric acid is a non-oxidising acid. Silver, copper and gold all lie below hydrogen in the activity series, so it cannot dissolve any of them and cannot lift a tarnished surface layer off an ornament.
• (c)Hydrochloric acid — Hydrochloric acid is also non-oxidising. It does not dissolve gold, and on silver it only leaves an insoluble film of silver chloride. It matters here only when it is mixed with concentrated nitric acid to make aqua regia, which does dissolve gold — and that is the opposite of what a cleaning bath is meant to do.
• (d)Phosphoric acid — Phosphoric acid is a moderately weak, non-oxidising acid used mainly for rust removal and in the food industry. It has no oxidising power to attack silver or copper, so it cannot clean the surface of gold or silver work.

Acids attack metals in two different ways. A non-oxidising acid such as dilute hydrochloric or dilute sulphuric acid can only displace hydrogen, so it reacts with metals above hydrogen in the activity series and stops there. An oxidising acid such as nitric acid does not depend on that displacement — its nitrate ion does the oxidising, so it dissolves copper, silver and mercury as well. Gold and the platinum-group metals resist even that and give way only to aqua regia, the three-to-one mixture of concentrated hydrochloric and nitric acid.

The stem asks which acid is used, so the reasoning is a process of elimination on one property. Three of the four options cannot touch silver or copper, which is exactly what a cleaning bath has to remove. Only nitric acid can. One honest caveat belongs with this: nitric acid dissolves silver as readily as it dissolves copper, so on silver work it is used as a brief, controlled treatment that strips the discoloured surface, not as a soak. On gold work the same bath is far safer, because the gold itself is inert to the acid. Students often want to answer aqua regia here out of habit; aqua regia is the reagent for dissolving gold, not for cleaning it, and in any case it is not on the option list.

• Nitric acid is used in jewellery shops as a cheap means of spotting low-gold alloys below about 14 carat and rapidly assessing gold purity.
• Silver and copper are dissolved by nitric acid because it is an oxidising acid, while pure gold and the platinum-group metals do not react with it.
• Gold dissolves only in aqua regia, a mixture of concentrated nitric acid and concentrated hydrochloric acid.
• Nitric acid is also used in gold parting, the process that separates gold from the silver alloyed with it.

• Answering aqua regia by reflex; it dissolves gold rather than cleans it, and it is not among the options.
• Thinking any strong acid will do — dilute hydrochloric and dilute sulphuric acid cannot attack silver or copper at all.
• Forgetting that nitric acid does dissolve silver, so its use on silver articles is a short controlled treatment, not a long soak.

Asked as a single-fact application item — match a workshop practice to the acid whose chemistry allows it.

• practice — not a real PYQ

  Aqua regia, used to dissolve gold, is a mixture of concentrated

  • (a)sulphuric acid and nitric acid
  • (b)hydrochloric acid and nitric acid
  • (c)hydrochloric acid and sulphuric acid
  • (d)phosphoric acid and nitric acid

  Answer(b) hydrochloric acid and nitric acid — mixed in a three-to-one ratio by volume, it attacks gold, which neither acid can dissolve alone.

• practice — not a real PYQ

  Copper does not react with dilute hydrochloric acid but does react with dilute nitric acid. This is because

  • (a)copper lies above hydrogen in the activity series
  • (b)hydrochloric acid is a weaker acid than nitric acid in water
  • (c)nitric acid acts as an oxidising agent
  • (d)copper forms an insoluble chloride

  Answer(c) nitric acid acts as an oxidising agent — the nitrate ion oxidises copper even though copper lies below hydrogen and so cannot displace it from an acid.