# Why does concentrated HCl fume?

It is a common observation that a concentrated solution of hydrochloric acid kept in the lab fumes. What is the chemistry behind this phenomenon? Does $\ce{HCl}$ somehow react with the moisture in the air due to its hygroscopic nature?

• Answer-looking strategies: Hmm, HCl fumes...that means atoms from the surface of HCl are escaping, why? the atoms from any solid doesn't seem to leave out atoms of it so quickly, but HCl is a liquid, but water is also liquid, it doesn't fume that much, then is it that atoms in HCl are held so loosely than any other general liquid? What might be the cause...? See if you get any idea... – Immortal Player Aug 4 '15 at 7:47

In the case of $\ce{HNO3}$, according the abstract of this paper$^\dagger$, nitric acid vapors act as nucleation sites for ambient water, causing the mists/fumes that are observed above concentrated nitric acid solutions:

The gas phase reaction [forming nitric acid from other reagents] supplies $\ce{HNO3}$ nuclei on which the water vapor readily condenses to form strong nitric acid mist droplets.

According to this book$^\ddagger$:

Hydrochloric acid has thermodynamic properties rather similar to those of nitric acid.

So, I would hypothesize that the physical chemistry phenomena of evaporation and hygroscopic nucleation are responsible for the fuming of $\ce{HCl}$.

$^\dagger$ Goyer, J Colloid Sci 18(7), 616 (1963)

$^\ddagger$ Kulmala and Wagner, "Nucleation and Atmospheric Aerosols," 1996, p591.

Hydrogen chloride ($\ce{H-Cl}$) is a gas at room temperature (boils already at −85.05 °C). The fumes you see is HCl escaping from the reaction mixture when the equilibrium is distorted due to e.g. heat. The more concentrated your HCl is, the more the equilibrium between the aqueous phase and the ambivalent air shifts towards the ambivalent air, creating fumes. These fumes are nasty. Rather not work too much with concentrated HCl (> 30%) outside of the hood!

• Also, when you get higher and higher concentrations of strong acids, you tend to get larger proportions of non-dissociated acid molecules. This is due to fewer water molecules available to form solvent shells around the constituent ions. – Dan Burden Aug 4 '15 at 16:58