Animated Solution for Chemistry - s and p-Block Elements: Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes. The explanation for it is that

Have you ever opened a bottle of concentrated hydrochloric acid in the lab and noticed a mysterious, ghostly white cloud escaping from the mouth of the bottle? It looks like smoke, but nothing is burning! This classic phenomenon is a beautiful demonstration of physical chemistry in action. Let's dive into the molecular world to understand exactly what creates these white fumes.

When we talk about "concentrated hydrochloric acid," we are not talking about a pure liquid. Pure hydrogen chloride ($\mathrm{HCl}$) is actually a gas at room temperature. Concentrated $\mathrm{HCl}$ is an aqueous solution where a massive amount of $\mathrm{HCl}$ gas is dissolved in water.

Because the gas is so concentrated, it is highly volatile. The moment you open the bottle or keep it in open air, the $\mathrm{HCl}$ molecules start escaping from the liquid phase into the surrounding air as $\mathrm{HCl}(\mathrm{g})$.

Now, the air around us is never perfectly dry. It contains a significant amount of water vapor, which we commonly refer to as humidity or moisture. So, as the invisible $\mathrm{HCl}$ gas escapes the bottle, it immediately encounters these invisible water vapor molecules (${\mathrm{H}}_2\mathrm{O}(\mathrm{g})$) floating in the air.

Here is where the magic happens. $\mathrm{HCl}$ gas has an incredibly strong affinity for water. It is highly soluble because it readily ionizes into ${\mathrm{H}}^{+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ ions when it meets water.

As the escaping $\mathrm{HCl}$ gas collides with the moisture in the air, it rapidly dissolves into the water vapor. This rapid dissolution causes the water vapor to condense around the $\mathrm{HCl}$ molecules, forming microscopic droplets of liquid hydrochloric acid ($\mathrm{HCl}(\mathrm{aq})$) suspended in the air.

These newly formed liquid droplets are tiny, but there are millions of them. When ambient light hits these suspended droplets, the light scatters in all directions (a phenomenon related to Mie scattering). To our eyes, this scattered light appears as a dense, cloudy white smoke or "fumes."

If we were to perform this exact same action in a completely dry environment (zero humidity), no white fumes would form because there would be no moisture for the $\mathrm{HCl}$ gas to dissolve into. The correct explanation for this phenomenon is that the strong affinity of $\mathrm{HCl}$ gas for moisture in the air results in the formation of droplets of liquid solution, which appear like a cloudy smoke.

This is a fundamental concept that also applies to other moisture-sensitive compounds like $\mathrm{PC}{\mathrm{l}}_5$ and $\mathrm{SiC}{\mathrm{l}}_4$, which also fume in moist air due to hydrolysis.