Problem 194 - A Complete Resource Book in Chemistry for JEE Main

Question

Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes. The explanation for this is that (a) strong affinity of HCl gas for moisture in air results in forming of droplets of liquid solution which appears like a cloudy smoke. (b) oxygen in air reacts with the emitted HCl gas to form a cloud of chlorine gas (c) due to strong affinity for water, concen-trated hydrochloric acid pulls moisture of air towards itself. This moisture forms droplets of water and hence the cloud. (d) concentrated hydrochloric acid emits strongly smelling HCl gas all the time.

Step-by-Step Solution

Answer

(a) and (c) explain the phenomenon, but option (a) is most accurate.

Step 1: Understanding the scenario

Concentrated hydrochloric acid (HCl) is exposed to the open air and produces a cloud of white fumes. We must identify the correct explanation for this phenomenon.

Step 2: Analyzing options

We will analyze each option to understand which one correctly explains the formation of white fumes in the presence of concentrated hydrochloric acid.

Step 3: Option (a) Analysis

Option (a) suggests that HCl gas has a strong affinity for moisture in the air, resulting in the formation of droplets of liquid solution resembling a cloud. This explanation emphasizes the interaction between HCl and the moisture in the air.

Step 4: Option (b) Analysis

Option (b) proposes that oxygen in the air reacts with emitted HCl gas to form a cloud of chlorine gas. This option is inaccurately describing chemical behavior since hydrochloric acid reacting directly with oxygen to form chlorine gas is not a plausible reaction under normal atmospheric conditions.

Step 5: Option (c) Analysis

Option (c) indicates that due to its strong affinity for water, concentrated hydrochloric acid pulls moisture from the air towards itself, forming water droplets and thus a cloud. This is similar to (a) but focuses on the attraction of moisture rather than the emission of a gas.

Step 6: Option (d) Analysis

Option (d) states that concentrated hydrochloric acid emits a strongly smelling HCl gas all the time. While HCl gas is indeed strong smelling, this does not explain the formation of white clouds.

Step 7: Conclusion

A white cloud forms due to the strong affinity of HCl gas for moisture in the air, which causes the formation of droplets (solutions of hydrochloric acid in water). Hence, the most plausible explanation is a combination of elements from options (a) and (c).

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into different substances, called products. This transformation involves breaking and forming chemical bonds.
For example, when hydrochloric acid (HCl) is exposed to air, it doesn't directly react with oxygen or other air components to produce any unusual compounds like chlorine gas, which is why option (b) in the exercise is inaccurate. In our exercise, the key chemical interaction involves the hydrogen chloride gas being attracted to water molecules suspended in the air, which is part of a process called hygroscopy. This interaction is not so much a traditional chemical reaction as a physical absorption process, which will be explored in other sections.
Understanding these basic principles of chemical reactions helps grasp why some scenarios in questions might be inaccurate, like the implied possibility of HCl directly producing chlorine with the oxygen in the air.

Atmospheric Chemistry

Atmospheric chemistry involves the study of chemical processes that occur in the atmosphere, which includes gas, liquid, and solid phase substances.
When it comes to hydrochloric acid's behavior in the atmosphere, the dominant process is its interaction with water vapor. In the case of our exercise, when HCl is exposed to the atmosphere, it does not primarily interact with oxygen or nitrogen in the air, as one might think. Instead, HCl has a strong affinity for water vapor.
This affinity leads to the formation of tiny droplets, making the air appear cloudy or misty.
Another interesting aspect of hydrochloric acid in the atmosphere is that it does not persist as a gas for long because it quickly absorbs moisture. This behavior exemplifies the union between atmospheric chemistry and hygroscopic properties, which create those mysterious white fumes seen over an open container of HCl.

Hygroscopic Substances

Hygroscopic substances are materials that can absorb moisture from their surroundings. Hydrochloric acid, particularly in its concentrated form, is a prime example.
The exercise highlights hydrochloric acid's strong hygroscopic nature as a reason for the formation of white fumes when the acid is left open in the air.
As its vapors rise, they collide with water vapor in the air, forming a dilute hydrochloric acid solution, which becomes visible as white clouds.
Understanding hygroscopicity is essential for those studying atmospheric chemistry since many substances in air (including pollutants) exhibit these properties. This means they can bond with moisture, leading to either beneficial or detrimental effects on environmental and human health.
Therefore, the concept of hygroscopic substances explains both the fascinating phenomena observed here as well as broader implications in environmental science.