When a sample of air is passed through alkaline pyrogalol, potash and finally through U-tube containing fused calcium chloride, the components of air left u...

When air is passed through a series of reagents, each one absorbs a specific component:

1. Alkaline pyrogalol (alkaline solution of pyrogallol) absorbs oxygen (O₂). Pyrogallol in alkaline solution reacts readily with oxygen and turns dark brown.
2. Potash (concentrated potassium hydroxide solution, KOH) absorbs carbon dioxide (CO₂). The CO₂ reacts with KOH to form potassium carbonate: CO₂ + 2KOH → K₂CO₃ + H₂O.
3. Fused calcium chloride (CaCl₂) in a U-tube absorbs water vapour (H₂O). Anhydrous CaCl₂ is a powerful desiccant (drying agent).

The main components of air are nitrogen (~78%), oxygen (~21%), argon and other noble gases (~0.9%), carbon dioxide (~0.04%), and water vapour (variable). After removing oxygen, carbon dioxide, and water vapour, the components that remain unabsorbed are noble gases and nitrogen. These are chemically inert (noble gases) or unreactive with the reagents used (nitrogen), so none of the three reagents can remove them.