When air is passed through alkaline pyrogallol and then lime water, the gases left are

Analyze the question: Air is passed through two reagents in sequence. We need to find which gases are left after both have absorbed what they react with.

Concept:

• Air contains roughly 78% nitrogen, 21% oxygen, 0.03% carbon (IV) oxide, about 1% noble gases (mainly argon), plus water vapour.
• Alkaline pyrogallol absorbs oxygen. It also absorbs CO₂, because it is alkaline.
• Lime water (calcium hydroxide solution) absorbs carbon (IV) oxide, turning milky as CaCO₃ forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Step-by-step:

1. Start with air: N₂, O₂, CO₂, noble gases.
2. Pass through alkaline pyrogallol: O₂ is removed, and the alkaline medium also takes up CO₂. Left: N₂, noble gases (and CO₂ if any survives).
3. Pass through lime water: any remaining CO₂ is removed. Left: N₂ and noble gases.

Nitrogen is unreactive towards both reagents, and the noble gases are inert, so they pass through unchanged.

Checking the options:

• A: CO₂ is absorbed, so it is not left. Wrong.
• B: Nitrogen and noble gases is exactly what remains. Correct.
• C: Oxygen is absorbed by pyrogallol. Wrong.
• D: CO₂ is absorbed (and "carbon (VI) oxide" does not exist anyway). Wrong.
• E: Nitrogen is not absorbed, so it remains too. Wrong.

**Answer: B.

Exam tip: Remember the absorbers: alkaline pyrogallol → oxygen, lime water or KOH → CO₂, anhydrous CaCl₂ or conc. H₂SO₄ → water vapour. After removing O₂, CO₂ and water, what is left is nitrogen plus the noble gases.