Question

# Aqua Regia is ……………... part conc.HCl and…………….part $conc. HN{O_2}$.A. One, threeB. One, oneC. Three, threeD. Three, one

Hint: Aqua Regia is an extremely corrosive mixture of nitric acid hydrochloric acid, used as an etchant, for some analytical chemistry procedures and to refine gold.

Aqua Regia is three part conc.HCl and one part $conc.HN{O_3}$. When mixing the acids, it is important to add the nitric acid to the hydrochloric acid and not the other way around. It is used to dissolve gold, platinum, and palladium but no other noble metals.
The acid mixture is unstable, so it is usually prepared in small amounts and used immediately.
The following reaction takes place when nitric acid is mixed with Hydrochloric acid :

$HN{O_3}\left( {aq} \right) + 3HCl\left( {aq} \right) \to NOCl\left( g \right) + 2{H_2}O\left( l \right) + C{l_2}\left( g \right)$

Nitrosyl chloride $\left( {NOCl} \right)$ will decompose into chlorine gas and nitric oxide $\left( {NO} \right)$. Nitric acid autoxidizes into nitrogen dioxide $\left( {N{O_2}} \right)$:

$2NOCl\left( g \right)\xrightarrow{{}}2NO\left( g \right) + C{l_2}\left( g \right)$

$2NO\left( g \right) + {O_2}\left( g \right)\xrightarrow{{}}2N{O_2}\left( g \right)$

Nitric acid, hydrochloric acid and aqua regia are strong acids. Chlorine $\left( {C{l_2}} \right)$, nitric oxide $\left( {NO} \right)$ and nitrogen dioxide $\left( {N{O_2}} \right)$ are toxic.

Therefore, the correct answer is (D).

Note: Aqua Regia preparation involves mixing of strong acids. The reaction produces heat and evolves poisonous vapours. So, it is important to follow safety protocols when making and using this solution.
Prepare the minimum volume needed for your application. Make and use aqua regia solution inside a fume hood with the sash down as much as it is practical to contain the vapour and protect against injury in case of splashing or glassware breakage.