Question

Aqua regia is a mixture of:
(a). Three parts of concentrated nitric acid and one part of concentrated hydrochloric acid
(b). Three parts of concentrated hydrochloric acid and one part of concentrated nitric acid
(c). Three parts of diluted hydrochloric acid and one part of diluted nitric acid
(d). Three parts of diluted nitric acid and one part of diluted hydrochloric acid

Answer 1

- Hint: Aqua regia is a combination of strong acids which possess the ability to dissolve some noble metals. It is widely used in refining gold with greatest purity.

Complete step-by-step solution -

Aqua regia is a mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid.
When two very strong acids are combined, they react together to form a very acidic solution. Aqua regia has a tendency to dissolve noble metals. Hydrochloric acid and nitric acid cannot do this alone.
When 1:3 molar ratio of concentrated nitric acid and concentrated hydrochloric acid is mixed together, it gives the following reaction –
\[HN{{O}_{3}}(aq)+3HCl(aq)\to NOCl(g)+C{{l}_{2}}(g)+2{{H}_{2}}O\]
The solution is dark yellow/orange coloured fuming liquid. The fumes hence produced are composed of nitrosyl chloride - \[NOCl\] and chlorine - \[C{{l}_{2}}\].
Aqua regia is very widely used in the refining of gold. This process is known as Wohlwill process and is used to refine the highest quality of gold. (Aqua regia is used as an electrolyte here)

Therefore, the answer is option (b).

Additional Information:
Aqua regia cannot be stored for longer times because it decomposes quickly. Nitrosyl chloride formed from hydrochloric acid and nitric acid decomposes to form nitric oxide and chlorine.
\[2NOCl(g)\to 2NO(g)+C{{l}_{2}}(g)\]

Note: Aqua regia is also known as “royal water” or “king’s water” due to its ability to dissolve some noble metals such as platinum, gold and silver.
Noble metals include –Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Rhenium (Re), Osmium (Os), Iridium (Ir), Platinum (Pt), Gold (Au), Mercury (Hg).