A highly corrosive mixture of nitric and hydrochloric acids that can dissolve gold and platinum, historically called 'royal water.'
From Latin 'aqua' (water) plus 'regia' (royal, kingly), literally 'royal water.' Medieval alchemists named it this because it could dissolve the 'noble' metals, which no other liquid could touch.
Alchemists believed that a liquid capable of dissolving gold—the noblest metal—must contain some royal or divine property, so they called it 'royal water,' never suspecting it was just two common acids mixed together.
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