Animated Solution for Chemistry - Environmental Chemistry: Taj Mahal is being slowly disfigured and discoloured. This is primarily due to

The Taj Mahal, one of the Seven Wonders of the World, is renowned for its pristine white marble architecture. However, over the years, this magnificent monument has been slowly losing its shine, turning yellowish, and suffering from surface disfigurement. But what is causing this slow destruction? The answer lies in the chemistry of our atmosphere.

Acid Rain

The primary cause of the Taj Mahal's discolouration is a phenomenon known as acid rain. Normal rainwater is slightly acidic due to dissolved carbon dioxide, but acid rain has a much lower pH, typically ranging between $3.5$ and $5.6$. This high acidity is a direct result of severe air pollution in the surrounding areas.

When fossil fuels are burned in industries and vehicles, they release large amounts of sulphur dioxide (${\text{SO}}_2$) and nitrogen oxides (${\text{NO}}_x$) into the atmosphere. These gases react with atmospheric moisture and oxygen to form strong acids. Specifically, they form sulphuric acid (${\text{H}}_2{\text{SO}}_4$), sulphurous acid (${\text{H}}_2{\text{SO}}_3$), and nitric acid (${\text{HNO}}_3$).

When it rains, these strong acids fall to the ground, bathing everything in their path—including the Taj Mahal—in a highly corrosive acidic solution.

The Taj Mahal is built entirely of white marble, which is chemically composed of calcium carbonate (${\text{CaCO}}_3$). Calcium carbonate is highly susceptible to attack by strong acids. When acid rain falls on the monument, the hydrogen ions (${\text{H}}^{+}$) from the acids react with the solid marble.

The chemical reaction can be represented as:

In this reaction, the solid calcium carbonate is dissolved into aqueous calcium ions, producing water and releasing carbon dioxide gas. This continuous dissolution of the marble surface leads to pitting, structural disfigurement, and the yellowish tint that plagues the monument today. This phenomenon is often referred to as "Marble Cancer."

Understanding this chemical process highlights the urgent need to control air pollution. By reducing the emissions of sulphur and nitrogen oxides through the use of cleaner fuels like CNG and implementing stricter emission norms for industries, we can mitigate the effects of acid rain and preserve our invaluable cultural heritage for future generations.