Question

Aqua regia is a mixture of _______ and _______ in the ratio ________.

Answer 1

- Hint: Aqua regia is the mixture of two strong acids and the molar volume of one of them is three times the other.

Complete step-by-step solution -

Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid in the ratio of 1:3. Aqua regia which means “King’s Water” or “Regal Water” is a fuming liquid that is orange-yellow in colour, sometimes appears red, and it is used for dissolving the noble metals such as gold and platinum.

Additional Information:
For the preparation of Aqua regia, concentrated nitric acid and concentrated hydrochloric acid are mixed. On mixing, they form very volatile products such as nitrosyl chloride and chlorine gas. The chemical equation for this reaction is given below.
$HN{ { O }_{ 3 } }_{ \left( aq \right) }+3HCl_{ \left( aq \right) }\longrightarrow NOCl_{ \left( g \right) }\uparrow +{ { Cl }_{ 2 } }_{ \left( g \right) }\uparrow +2{ { H }_{ 2 }O }_{ \left( l \right) }$
The nitrosyl chloride (NOCl) formed can easily decompose to form nitric oxide (NO) and chlorine gas (${ Cl }_{ 2 }$). The nitric oxide readily reacts with oxygen in the air and forms nitrogen dioxide (${ NO }_{ 2 }$). So, the aqua regia is a fuming liquid because of these gases that form in the solution and its typical yellow colour due to the formation of ${ Cl }_{ 2 }$ gas.
As we know, aqua regia is used for dissolving gold and platinum. Apart from that, we can use it in laboratories for cleaning glassware containing metal particles. It can be used in the process of refining the highest quality gold by producing the electrolyte chloroauric acid ($HAu{ Cl }_{ 4 }.{ \left( { H }_{ 2 }O \right) }_{ x }$) which is the main electrolyte in this process.

Note: Always remember that the ratio of $HN{O}_{3}$ to HCl in the aqua regia is 1:3. This means in 4 volumes of aqua regia, there is 1 volume of $HN{O}_{ 3 }$ and 3 volumes of HCl and not the other way round.