Fractional Distillation of Liquid Air
Separate components of air based on their boiling points
Drag to rotate • Scroll to zoom • Column base stays centered
Column Temperature
-200.0°C
Collected: 0/3
How to read the diagram
The 2D and 3D views show the same outlet heights and collection order.
1. Purify the air
Filter dust, then remove H₂O and CO₂ because water would freeze and carbon dioxide would solidify, blocking the apparatus at low temperature.
2. Liquefy the air
Air is first compressed and cooled, then allowed to expand until it reaches about −200°C, so the gaseous air becomes liquid air.
3. Separate in the column
Liquid air warms slowly in the column; the top stays coldest and the lower part is warmer, so nitrogen is collected first at the top, argon in the middle, and oxygen last near the bottom.
Temperature Profile
N₂ Ar O₂
How should you read the blue line?
The left side represents the top of the tower and the right side represents the lower, warmer part. As the blue line rises, the temperature increases, so higher-boiling components are collected lower down while lower-boiling components can rise higher before they condense.
Top of column
-196°C
N₂Nitrogen • -196°C
Fraction 1Coldest region, so nitrogen stays gaseous for the longest time and reaches the highest outlet.
The top is coldest, so nitrogen is collected first from the highest outlet.
Middle
-186°C
ArArgon • -186°C
Fraction 2Intermediate temperature, so argon condenses and is collected in the middle of the tower.
Argon is collected from the middle outlet between nitrogen and oxygen.
Lower column
-183°C
O₂Oxygen • -183°C
Fraction 3Warmer region, so oxygen with the highest boiling point is collected last from the lower outlet.
Oxygen has the highest boiling point, so it is collected only from the warmer lower outlet.
Component Monitor
N₂
WaitingB.P.-196°C
In Air78%
OutletTop
Ar
WaitingB.P.-186°C
In Air0.9%
OutletMiddle
O₂
WaitingB.P.-183°C
In Air21%
OutletLower
HKDSE Tip
- Physical change:Fractional distillation of liquid air is a physical change because the process only separates the components of air and does not form any new substance.
- Air purification:Before liquefaction, the air must be purified to remove dust, H₂O and CO₂; otherwise water would freeze and carbon dioxide would solidify, blocking the cold apparatus and pipes.
- Liquefaction:Air is compressed and cooled, then expanded to about -200°C so it changes from gas to liquid air before entering the fractionating column.
- Temperature gradient:The fractionating tower is colder at the top and warmer at the bottom, creating a temperature gradient so components with different boiling points separate at different heights.