Question

Hydrogen chloride is a colorless gas, yet when a bottle of concentrated hydrochloric acid \([\mathrm{HCl}(\mathrm{conc} \text { aq) }]\) is opened, mist-like fumes are often seen to escape from the bottle. How do you account for this?

Short answer

Mist-like fumes are visible when a bottle of concentrated hydrochloric acid is opened because the hydrochloric acid vaporizing from the liquid comes into contact with the moisture in the air. This causes a reaction, forming hydronium and chloride ions. The mist is the resulting tiny droplets of this solution, formed by the condensation of the solution in the air.

Step-by-step solution

Identify the state of Hydrochloric Acid

Hydrochloric acid in a concentrated aqueous solution \([\mathrm{HCl(conc\:aq)}]\) is a liquid. When a bottle of it is opened, it comes into contact with the surrounding air. Because hydrochloric acid is a strong acid, it is also volatile in nature, which means it tends to vaporize.

Understand the interaction with air

Air contains moisture, or water vapor, in it. When the hydrochloric acid gas comes into contact with the moisture in the air, an acid-base reaction takes place.

Discuss the reaction with moisture

In a specific reaction, hydrochloric acid \([\mathrm{HCl}]\) gas reacts with water \([H_2O]\) present in the atmosphere and produces hydronium ions \([H_3O^{+}]\) and chloride ions \([Cl^{-}]\). This reaction can be represented as: \(\mathrm{HCl(g)} + \mathrm{H_2O(g)} \rightarrow \mathrm{H_3O^{+}(aq)} + \mathrm{Cl^{-}(aq)}\). Consequently, a visible mist forms due to the condensation of tiny droplets of the solution formed by this reaction.

Key concepts

Volatility of acids

Volatility refers to how easily a substance changes from a liquid to a gas. Hydrochloric acid (HCl) is a particularly volatile acid. When you open a bottle of concentrated hydrochloric acid, even though it is a liquid inside the container, it quickly turns into gas upon contact with the air.
This nature means that HCl molecules can escape rapidly into the atmosphere. The result is the noticeable misty fumes that you see, which is a common characteristic of volatile acids like hydrochloric acid.
In general, many strong acids tend to display volatility. It’s important to handle them with caution to avoid inhalation or other exposure risks.

Acid-base reaction

An acid-base reaction is a chemical process that occurs when an acid reacts with a base. In the case of hydrogen chloride, an acid-base reaction occurs when the gaseous HCl meets the moisture present in the air.
This interaction happens because water, which is made of H₂O, acts as a base in this scenario. It reacts with the acid (HCl) to form a new type of compound.
The process results in the creation of hydronium ions and chloride ions. This particular reaction is an example of how substances in gaseous form can interact with those in liquid form.

Hydronium ions

Hydronium ions are important in chemistry, especially involving acids and their interactions. When hydrochloric acid in its gaseous form mixes with water, it generates hydronium ions, denoted as \(\mathrm{H_3O^{+}}\).
This species comes into existence when a hydrogen ion \(\mathrm{H^{+}}\) from the acid attaches to a water molecule.
Through this alteration, hydronium ions become prevalent, playing a major role in defining the acidic nature of the solution. The ability to create hydronium ions is what makes substances like hydrochloric acid behave strongly as acids.

Chloride ions

Chloride ions, represented as \(\mathrm{Cl^{-}}\), are another product of the reaction between hydrochloric acid and water. When the \(\mathrm{HCl}\) gas dissociates in the presence of moisture, it not only forms hydronium ions but also releases chloride ions.
These ions are particularly stable and form part of the overall ionic composition of the resulting solution. Chloride ions are integral in maintaining the charge balance within solutions and participate in various chemical processes.
Understanding the role of chloride ions helps to explain the electrical conductivity and other properties of solutions resulting from such reactions.