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 \(\mathrm{HCl}\) 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 \(\mathrm{HCl}\) 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

Option (a) is correct; HCl gas forms droplets with moisture, creating a cloud.

Step 1: Analyze the Problem

We begin by analyzing the behavior of concentrated hydrochloric acid (HCl) when it's exposed to open air. This problem involves understanding how HCl interacts with its environment.

Step 2: Evaluate Options

We consider each option and determine its correctness. Option (a) states that HCl gas strongly attracts moisture, forming droplets. Option (b) suggests that HCl reacts with oxygen, forming chlorine gas. Option (c) implies that HCl pulls moisture from the air and forms water droplets. Option (d) mentions that HCl emits a strong smell, but it doesn't address cloud formation.

Step 3: Eliminate Incorrect Options

Option (b) is incorrect because HCl does not react with oxygen to form chlorine gas. Option (d) is also incorrect as it does not explain cloud formation through physical changes.

Step 4: Evaluate Correct Options

Both options (a) and (c) involve the strong affinity of HCl for moisture. Option (a) links this directly to the appearance of a cloud due to liquid droplets, while option (c) emphasizes moisture pulling more directly.

Step 5: Select the Best Explanation

The formation of white fumes or cloud is best explained by option (a). When HCl gas in the air attracts water vapor, it condenses into small droplets of dilute HCl solution, appearing as a mist or cloud.

Key Concepts

Hydrochloric Acid

Hydrochloric acid is a strong mineral acid composed of hydrogen and chlorine in the form of \(\text{HCl}\). In its pure form, it is a highly corrosive liquid that can release invisible vapors into the air. These vapors are heavier than air and can linger close to the ground if not dispersed by wind or ventilation.
Hydrochloric acid is known for its strong pungent smell. This odor is due to its high vapor pressure, which means it easily evaporates even at room temperature. When concentrated hydrochloric acid is exposed to the air, the released \(\text{HCl}\) gas quickly interacts with the environment, leading to various reactions.

• The strong acidity of hydrochloric acid makes it a powerful reagent in chemical processes.
• It is widely used in industries for metal processing, pH control, and as a cleaning agent.

Gas Reactions

Understanding the reactions of gases is essential when studying behaviors like those of hydrochloric acid in the air. When HCl gas is released, it doesn't just vanish but instead actively interacts with other gases and environmental factors present in the atmosphere.
This particular gas has a strong tendency to engage in reactions because it is an empirically powerful acid. For example, when it is in an open environment:

• It can potentially react with ammonia, another common atmospheric gas, to form ammonium chloride, a white solid.
• HCl does not, however, naturally react with oxygen from the atmosphere to produce chlorine gas. Hence, option (b) from the exercise is incorrect.

The reactivity of hydrochloric acid gas makes it crucial to handle with caution, as unintended reactions can produce harmful by-products.

Moisture Interaction

One of the most defining characteristics of hydrochloric acid gas is its strong affinity for moisture. This interaction with water vapor is a key reason why concentrated hydrochloric acid produces visible fumes when left exposed.
When water vapor in the air comes into contact with HCl gas, several things happen:

• The gas dissolves in the water, immediately forming tiny droplets of dilute hydrochloric acid.
• This process creates a mist or cloud, often appearing as white fumes, due to the small size and dispersion of the liquid droplets in the air.

The exercise correctly identifies this process under options (a) and (c), noting the acid’s ability to pull moisture from the air and form droplets, leading to the characteristic cloud of fumes.

Chemical Fume Formation

Chemical fumes often form when gases or vapors from volatile or reactive substances combine with elements in the air. With hydrochloric acid, the visible fumes occur when the gas encounters and reacts with water vapor.
The formation of chemical fumes is not exclusive to \(\text{HCl}\) but is common among many volatile acids and bases, often seen as:

• Fumes appearing as a result of an exothermic release of gases combining with moisture.
• Mists formed when gases condense into liquid droplets in the air due to temperature or pressure changes.

Understanding fume formation is crucial for safely handling chemicals, particularly in open or poorly ventilated spaces. Ensuring proper ventilation and using protective equipment can prevent the inhalation of hazardous fumes and exposure to potentially corrosive mists.