Aqua regia used by alchemists to separate silver and gold is a mixture of ........

Explanation:

hydrochloric acid (concentrated) and nitric acid (concentrated)

"regal water" or "Royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a yellow-orange (sometimes red) fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals.