Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Identify the pollutant gases largely responsible for the discoloured and lustreless nature of marble of the Taj Mahal. (a) \(\mathrm{O}_{3}\) and \(\mathrm{CO}_{2}\) (b) \(\mathrm{CO}_{2}\) and \(\mathrm{NO}_{2}\) (c) \(\mathrm{SO}_{2}\) and \(\mathrm{NO}_{2}\) (d) \(\mathrm{SO}_{2}\) and \(\mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The gases are SO_2 and NO_2 (option c).

Step 1: Understanding the Problem

We need to determine which gases from the options provided are responsible for the discoloration and damage to the marble of the Taj Mahal. Marble, which is primarily calcium carbonate, reacts with certain pollutants that can lead to such damage.

Step 2: Identifying the Pollutant-Reactions

The main reactions causing discoloration of marble are sulfur dioxide ( SO_2 ) turning into sulfuric acid in the presence of water and nitrogen dioxide ( NO_2 ) forming nitric acid. Both acids can lead to the deterioration of marble.

Step 3: Evaluating Option (a) (O_3 and (CO_2 )

Ozone ( O_3 ) does not react significantly with marble, and carbon dioxide ( CO_2 ), while it reacts with marble, forms mild carbonic acid which is not primarily responsible for major discoloration.

Step 4: Evaluating Option (b) (CO_2 and (NO_2 )

Carbon dioxide, as stated, forms mild carbonic acid. However, nitrogen dioxide ( NO_2 ) can convert to nitric acid, contributing some damage but not primarily discoloration.

Step 5: Evaluating Option (c) (SO_2 and (NO_2 )

Sulfur dioxide ( SO_2 ) turns into sulfuric acid when moist and nitrogen dioxide ( NO_2 ) can produce nitric acid. Both acids are known to be responsible for marble discoloration and damage.

Step 6: Evaluating Option (d) (SO_2 and (O_3 )

While sulfur dioxide ( SO_2 ) can damage marble, ozone does not inherently lead to discoloration of marble since it doesn’t lead to acid formation impacting marble.

Step 7: Conclusion

Sulfur dioxide ( SO_2 ) and nitrogen dioxide ( NO_2 ) are responsible for creating discoloration and a lustreless nature due to their reactions leading to sulfuric and nitric acid. Therefore, option (c) is the correct answer.

Key Concepts

Sulfur Dioxide (SO2)

Sulfur dioxide, commonly referred to as \(\mathrm{SO}_2\), is a colorless gas with a pungent odor. It is primarily produced through the burning of fossil fuels and the smelting of mineral ores that contain sulfur. This gas plays a significant role in air pollution and is a key contributor to acid rain.

When \(\mathrm{SO}_2\) is emitted into the atmosphere, it can react with water vapor to form sulfuric acid \((\mathrm{H}_2\mathrm{SO}_4)\). This transformation is catalyzed by atmospheric processes and can occur relatively quickly. The resultant sulfuric acid can then precipitate onto surfaces, causing material corrosion and degradation.

In the context of marble, which is primarily composed of calcium carbonate \((\mathrm{CaCO}_3)\), sulfuric acid reacts with the calcium carbonate to produce calcium sulfate \((\mathrm{CaSO}_4)\), water, and carbon dioxide:

• \(\mathrm{CaCO}_3 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}\)

This reaction leads to the deterioration of marble, resulting in a loss of sheen and discoloration. The calcium sulfate formed is more soluble and can be washed away by rain, accelerating the erosion process.

Nitrogen Dioxide (NO2)

Nitrogen dioxide, or \(\mathrm{NO}_2\), is another significant pollutant. It is a reddish-brown gas with a sharp, biting odor and is a major contributor to smog and poor air quality.

\(\mathrm{NO}_2\) is chiefly produced during high-temperature combustion processes, such as those found in vehicle engines and power plants. Once in the atmosphere, \(\mathrm{NO}_2\) undergoes complex reactions that can lead to the formation of nitric acid \((\mathrm{HNO}_3)\).

Similar to sulfuric acid, nitric acid plays a crucial role in the process of marble deterioration. It reacts with calcium carbonate to form calcium nitrate \((\mathrm{Ca(NO}_3)_2)\), water, and carbon dioxide:

• \(\mathrm{CaCO}_3 + 2\mathrm{HNO}_3 \rightarrow \mathrm{Ca(NO}_3)_2 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}\)

This chemical reaction results in the weakening of the marble structure. The new compound formed, calcium nitrate, is much more soluble than calcium carbonate and readily dissolves in water, leading to the loss of material and a lustreless surface.

Marble Deterioration

Marble, a luxurious and attractive stone, holds high historical and architectural value. Its primary component, calcium carbonate \((\mathrm{CaCO}_3)\), is what gives marble its beautiful sheen and brilliance. However, marble is also susceptible to the effects of air pollution, particularly from acidic gases like \(\mathrm{SO}_2\) and \(\mathrm{NO}_2\).

The process of marble deterioration involves these acidic gases reacting with the marble's surface. As discussed, sulfur dioxide and nitrogen dioxide can transform into sulfuric and nitric acids, which subsequently react with the marble's calcium carbonate.

The results of these reactions include:

• Discoloration due to the formation of new, differently colored byproducts.
• A loss of sheen and luster as the surface becomes etched and roughened.
• Material erosion, where the surface slowly erodes due to the solubility of the reaction products.

The cumulative effect of these processes results in significant damage to marble structures, such as the iconic Taj Mahal. This deterioration is a prime example of the adverse effects of air pollution on cultural heritage sites. Effective pollution control and conservation efforts are crucial to preserving these valuable monuments.