Question

Why Aqua-Regia is a mixture of only HCL and HNO₃ and not of other acids?

Answer 1

Aqua regia is an extremely corrosive mixture of nitric and hydrochloric acid, used as an etchant, for some analytical chemistry procedures, and to refine gold. Aqua regia dissolves gold, platinum, and palladium, but not the other noble metals.
Here is what happens when nitric acid and hydrochloric acid are mixed:

HNO3 (aq) + 3HCl (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)

Over time, nitrosyl chloride (NOCl) will decompose into chlorine gas and nitric oxide (NO). Nitric acid auto-oxidizes into nitrogen dioxide (NO2):

2NOCl (g) → 2NO (g) + Cl2 (g)

2NO (g) + O2 (g) → 2NO2(g)

Nitric acid (HNO3), hydrochloric acid (HCl), and aqua regia are strong acids. Chlorine (Cl2), nitric oxide (NO), and nitrogen dioxide (NO2) are toxic.