What are ''fuming" acids'?

Feb 12, 2016

The most common fuming acids are fuming nitric acid and fuming sulfuric acid.

Explanation:

Fuming nitric acid

Red fuming nitric acid consists of ${\text{84 %HNO"_3, "13% N"_2"O}}_{4}$, and $\text{1–2 % H"_2"O}$.

The color is due to the ${\text{N"_2"O}}_{4}$, which breaks down partially to form nitrogen dioxide.

$\underbrace{\text{N"_2"O"_4)_color(red)("colourless") ⇌ underbrace(2"NO"_2)_color(red)("reddish-brown}}$

When you open a bottle of fuming nitric acid, some of the dissolved ${\text{NO}}_{2}$ escapes into the atmosphere as a reddish-brown gas with a suffocating odour.

Fuming sulfuric acid

Fuming sulfuric is made by dissolving ${\text{SO}}_{3}$ in ${\text{H"_2"SO}}_{4}$.

When you open a bottle of fuming sulfuric acid, some of the dissolved ${\text{SO}}_{3}$ escapes into the atmosphere.

The colourless gas reacts with moisture in the air, creating a white cloud of tiny sulfuric acid droplets.

${\text{SO"_3 +"H"_2"O" → "H"_2"SO}}_{4}$

The white cloud is the "fume" in fuming sulfuric acid.