RAS question
Assertion (A): The iron pillar near the Qutb Minar in Delhi, dating to the 4th–5th century CE, is still largely rust-free after 1,600 years. Reason (R): The pillar's rust-resistance is due to the high phosphorus content resulting from the ancient Indian smelting process, which forms a protective passive layer.
Correct answer: (A) Both A and R are correct, and R is the correct explanation of A.
The Iron Pillar near the Qutb Minar is largely rust-free because phosphorus in its iron helps form a thin protective iron hydrogen phosphate hydrate layer on the surface.
Explanation
The assertion and the reason are both correct, and the reason explains the assertion. The pillar stands in the Qutb Minar complex in Delhi and is famous for remaining relatively rust-free more than 1,600 years after its creation around 400 CE. Its corrosion resistance is not treated as a miracle in modern materials science: the accepted explanation is chemical. The ancient iron contains phosphorus from the smelting process. As the pillar ages, phosphate-related reactions produce a thin iron hydrogen phosphate hydrate coating on the surface. This passive layer slows further oxidation, which is why the pillar could survive for centuries with so little rust.
Why the other options are wrong
- (B) This option misses the key link: the phosphorus-driven passive layer is precisely the scientific explanation for the pillar's limited rusting.
- (C) The reason is not wrong, because the corrosion resistance is explained by phosphorus-related chemical changes that form a protective surface layer.
- (D) The assertion is not wrong, because the pillar in the Qutb Minar complex is documented as relatively rust-free after more than 1,600 years.
Concept
This tests ancient Indian science and technology, especially Gupta-period metallurgy. RAS repeats such examples because they connect cultural history with material evidence rather than relying only on dynastic chronology.