In alchemy, 'aqua fortis' was the classical name of:

"Aqua fortis" is a Latin phrase meaning "strong water". Alchemists in medieval times gave this name to nitric acid because it could dissolve and corrode most metals, such as silver and copper, though not gold.

Pure nitric acid is a colourless, fuming liquid with a sharp smell. It is a strong acid and a powerful oxidising agent. Old samples often turn yellow, because some of the acid breaks down into nitrogen dioxide in light. This is why nitric acid is stored in dark brown bottles. If it touches the skin, it causes painful burns and stains the skin yellow, because it reacts with proteins (this is the basis of the xanthoproteic test for proteins).

Nitric acid is made industrially by the Ostwald process, in which ammonia is oxidised with air over a platinum-rhodium catalyst to form nitric oxide, which is then converted into nitrogen dioxide and absorbed in water.

Uses of nitric acid:

• Making fertilisers, especially ammonium nitrate.
• Making explosives, such as TNT (trinitrotoluene), nitroglycerine and gun cotton.
• Making dyes, drugs and plastics.
• Etching and in the purification of gold and silver.

Aqua regia ("royal water") is a mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3:1. It can dissolve "noble" metals such as gold and platinum, which neither acid can dissolve alone. During the Second World War, the Hungarian chemist George de Hevesy dissolved the gold Nobel medals of two German scientists in aqua regia to keep them from the Nazis; the gold was later recovered and the medals recast.

Why not the other options?

• ✗Sulphuric acid was called "oil of vitriol".
• ✗Hydrochloric acid was called "muriatic acid" or "spirit of salt".
• ✗Aqua regia is a 3:1 mixture of HCl and HNO3.