Question

What physical property allows the separation of nitrogen, oxygen, and argon from each other in fractional distillation of air?

Short answer

Separation is based on differences in boiling points.

Step-by-step solution

Understanding Fractional Distillation

Fractional distillation is a process used to separate mixtures based on differences in their boiling points. During fractional distillation, a mixture is heated to a temperature at which one or more components will vaporize and then these vapors are condensed back to liquid and collected.

Boiling Points of Air Components

When considering the air's primary components—nitrogen, oxygen, and argon—each has a unique boiling point: nitrogen boils at approximately -196°C, argon at -186°C, and oxygen at -183°C. These distinct boiling points allow each component to vaporize at a different temperature during the distillation process.

Separation Based on Boiling Points

As the mixture of liquefied air is gradually heated, each gas will vaporize when its specific boiling point is reached. Nitrogen, having the lowest boiling point, will vaporize first, followed by argon and then oxygen. This sequential vaporization enables the separate collection of each gas.

Key concepts

Boiling Points

The concept of boiling points is fundamental in understanding how substances change from liquid to gas. Every element or compound has a specific boiling point, which is the temperature at which it transitions from a liquid to a gaseous state. Boiling points vary greatly among different substances and are influenced by molecular weight, intermolecular forces, and atmospheric pressure.
This is why boiling points are pivotal in fractional distillation, a method that leverages these differences to separate components of a mixture. In the context of air and its major components—nitrogen, argon, and oxygen—each possesses a distinct boiling point. Nitrogen boils at -196°C, argon at -186°C, and oxygen at -183°C.

• Nitrogen: Boils at -196°C
• Argon: Boils at -186°C
• Oxygen: Boils at -183°C

These temperatures allow for a clear sequence during the distillation process, where the mixture is heated, and each gas vaporizes at its respective boiling point.

Separation of Gases

The separation of gases through fractional distillation relies on exploiting differences in boiling points. This method begins with liquefied air, which is then gradually heated. As the temperature increases, each component gas reaches its boiling point sequentially, allowing it to vaporize and be collected separately. This process is efficient because:

• Lowest Boiling Point Vapors First: Gases with the lowest boiling points vaporize first. In the case of air, nitrogen vaporizes before argon and oxygen because it has the lowest boiling point.


• Stepwise Separation: As each gas boils off at its specific temperature, it is channeled away and condensed back into a liquid, effectively separating it from the other components.

Such a step-wise approach ensures that each component is isolated with high purity, making fractional distillation a powerful technique for air separation.

Physical Properties

One of the key physical properties that influences the separation of gases by fractional distillation is the boiling point itself. However, several other physical properties also play a role in the efficiency of this process.

• Intermolecular Forces: These forces affect the boiling points, with weaker forces typically leading to lower boiling points. For instance, the relatively weak intermolecular forces in nitrogen lead to its low boiling point of -196°C.


• Molecular Weight and Structure: These factors influence the heat capacity and volatility of the components, impacting how quickly they reach their boiling points.


• Density: The density of the gas affects how it rises and separates when vaporized.

By understanding these properties, scientists and engineers can optimize the fractional distillation process, ensuring that each component of a gas mixture is separated efficiently and with minimal energy input.