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Difficulty: Very hardComposition and Pollution of Air

In an industrial plant, dry air is cooled to −200∘C-200^\circ\text{C} to form liquid air after removing carbon dioxide and water vapor. When this liquid air undergoes fractional distillation in a fractionating column as its temperature is slowly raised, which statement correctly identifies the constituent that boils off first and the rationale behind its separation?

  1. Nitrogen boils off first at −196∘C-196^\circ\text{C} because it has a lower boiling point than argon and oxygen.Answer
  2. B
    Oxygen boils off first at −183∘C-183^\circ\text{C} because −183∘C-183^\circ\text{C} represents a higher numerical boiling point than −196∘C-196^\circ\text{C}.
  3. C
    Carbon dioxide boils off first as a gas because it is the lightest component remaining in the liquid mixture.
  4. D
    Argon boils off first at −186∘C-186^\circ\text{C} because unreactive noble gases always distill before diatomic gases.

Answer

Nitrogen boils off first at −196∘C-196^\circ\text{C} because it has the lowest boiling point among the main liquefied atmospheric constituents.
During the fractional distillation of liquid air, components separate according to their boiling points. As the column temperature warms up from −200∘C-200^\circ\text{C}, nitrogen (boiling point −196∘C-196^\circ\text{C}) reaches its boiling point first, vaporizes, and is collected at the top of the fractionating column.

Step-by-Step Solution

1
Identify the main liquefied components of air after pre-purification
Purified liquid air consists primarily of liquid nitrogen, argon, and liquid oxygen. Water vapor and carbon dioxide are removed prior to liquefaction to prevent clogging the equipment.
Water freezes at 0°C and carbon dioxide sublimes at -78.5°C, making them solids well above the liquefaction temperature of air (-200°C).
2
Compare the boiling points of the liquefied components
Nitrogen boils at −196∘C-196^\circ\text{C} (77 K), Argon boils at −186∘C-186^\circ\text{C} (87 K), and Oxygen boils at −183∘C-183^\circ\text{C} (90 K).
Lower boiling point values on the Celsius scale correspond to colder temperatures (e.g., -196°C is colder than -183°C).
3
Determine the distillation sequence as temperature rises from −200∘C-200^\circ\text{C}
Nitrogen reaches its boiling point first at −196∘C-196^\circ\text{C} and turns into gas, leaving argon and oxygen behind until further warming.
In fractional distillation, the liquid with the lowest boiling point boils off first at the top of the column.

Key Concept

Fractional Distillation of Liquid Air
Estimated Time:1m 30s
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Topics

Composition and Pollution of Air

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