Case Study:

The Taj Mahal, located in Agra, is one of India’s most iconic monuments and is built mainly from white marble (calcium carbonate). Over the years, its surface has shown signs of yellowing and corrosion. Environmental studies have revealed that industries in nearby areas and vehicular emissions release sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere.

During rainfall, these gases react with water vapour to form sulphuric acid (H₂SO₄) and nitric acid (HNO₃), leading to acid rain. The pH of normal rainwater is about 5.6 due to dissolved carbon dioxide. However, in polluted regions near Agra, rainwater pH has been recorded as low as 4.2.

Marble (CaCO₃) reacts with sulphuric acid as follows:

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Calcium sulphate (CaSO₄) formed is more soluble and can flake off, causing surface damage. To protect the monument, the Government of India established the Taj Trapezium Zone (TTZ), restricting polluting industries within a 10,400 sq km area around the monument.

If acid rain continues unchecked, structural weakening and loss of artistic carvings may occur. This situation highlights the importance of controlling emissions and adopting cleaner fuels such as CNG in nearby regions.

Questions:

Section A – MCQs

The yellowing and corrosion of the Taj Mahal marble mainly occur due to:
A. Reaction of marble with oxygen
B. Reaction of marble with acid rain
C. Reaction of marble with sunlight
D. Reaction of marble with carbon dioxide

2. If rainwater pH decreases from 5.6 to 4.2, it indicates:
A. Decrease in acidity
B. Increase in basic nature
C. Increase in acidity
D. No chemical change

3. The gas primarily responsible for forming sulphuric acid in acid rain is:
A. Oxygen
B. Nitrogen
C. Sulphur dioxide
D. Hydrogen

4. Formation of calcium sulphate on marble surface leads to:
A. Increased shine
B. No effect
C. Surface flaking and damage
D. Strengthening of marble

Section B – Short Answer Questions

1. Explain why marble reacts faster with acid rain than with normal rainwater.

2. Write the balanced chemical equation showing the reaction of sulphuric acid with calcium carbonate. State one observable change.

3. Suggest two measures that can reduce acid rain formation in industrial areas near heritage monuments.

Section C – Long Answer Question

Suppose the average pH of rainwater near the Taj Mahal remains at 4.0 for the next 10 years. Analyze the long-term chemical and structural impact on the monument. Suggest a scientific action plan combining chemical knowledge and environmental policy to prevent further damage.

Answer Key:

MCQ Answers:

1. B

2. C

3. C

4. C

Section B Answers:

1. Acid rain contains strong acids (H₂SO₄ and HNO₃), making it more acidic (lower pH) than normal rainwater (pH 5.6). Stronger acidity increases reaction rate with CaCO₃, leading to faster corrosion.

2. CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
Observable change: Effervescence due to CO₂ gas release and surface roughness of marble.

3. a)  Use cleaner fuels like CNG instead of coal/diesel.

b) Install scrubbers in industries to reduce SO₂ emissions.

Section C Answer:

1. a) Continuous exposure to pH 4.0 rain increases rate of CaCO₃ reaction.

b) Gradual formation of CaSO₄ causes flaking and surface erosion.

c)  Loss of intricate carvings and reduction in structural strength.

d) Action plan: Strict emission control under TTZ, promotion of electric vehicles, regular chemical cleaning using mild base solutions, monitoring pH levels.

e) Combined scientific and policy approach ensures long-term preservation.