Answer: Oxygen and water (moist air).
- A Oxygen and water (moist air)
- B Carbon dioxide gas dissolved in dry air
- C Nitrogen gas present in the surrounding atmosphere
- D Dilute hydrochloric acid splashed onto the surface
Correct answer: A. Oxygen and water (moist air)
Explanation: Rusting: 4Fe + 3O2 + 6H2O → 4Fe(OH)3; this dehydrates to Fe2O3·xH2O (rust) over time.
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.