Question

Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation?

Short answer

In steam distillation, the total pressure is the sum of the pressures of steam and the compound, lowering the temperature needed to boil the organic liquid.

Step-by-step solution

Understanding Steam Distillation

Steam distillation is a separation process used to purify or isolate temperature-sensitive substances. It involves passing steam through a mixture to vaporize the desired organic compound.

Explain How Vaporization Occurs

During steam distillation, the mixture is heated such that the steam and the organic compound form a mixture of vapors. As a result, the total pressure in the distillation system is the sum of the partial pressures of both the steam and the organic compound.

Total Pressure and Boiling Point Reduction

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure. In steam distillation, the combined vapor pressures of the steam and the organic compound need to reach the atmospheric pressure. This allows the organic compound to vaporize at a temperature lower than its usual boiling point.

Implications of Vapor Pressure Contributions

Since the steam contributes to reaching the atmospheric pressure, the required vapor pressure for the organic liquid alone to reach boiling is reduced. This results in the organic liquid vaporizing at a lower temperature than its standard boiling point.

Key concepts

Vaporization Process

The vaporization process is a fundamental concept in steam distillation, helping to explain how an organic liquid can transform into vapor at lower temperatures. Vaporization refers to the phase change from liquid to vapor and occurs when molecules in a liquid gain enough energy to break free from the liquid phase and enter the gaseous phase.
During steam distillation, heat is applied to the mixture, and the molecules of the liquid receive energy, increasing their movement. This energy allows the molecules to overcome intermolecular forces binding them in the liquid phase, causing them to escape into the air as vapor.
It's important to note that the vaporization process is influenced by temperature and pressure. The key is that steam distillation modifies these conditions, helping liquids to vaporize at lower temperatures than normal.

Boiling Point Reduction

Steam distillation is a clever technique that utilizes boiling point reduction to achieve the vaporization of certain substances at lower temperatures. Normally, a liquid boils when its vapor pressure equals the atmospheric pressure.
By introducing steam, we effectively alter the vaporization dynamics. In a steam distillation setup, both water vapor (steam) and the organic compound contribute to the total vapor pressure. This additional pressure from steam reduces the required vapor pressure that the organic compound must achieve on its own to equal atmospheric pressure and start boiling.
As a result, the organic compound doesn't have to reach its high boiling point. Instead, it can vaporize at a much lower temperature thanks to the collective pressure exerted by both the steam and itself. This is particularly useful for purifying heat-sensitive compounds which might degrade at their standard boiling point.

Partial Pressure

Partial pressure is a crucial concept in understanding why steam distillation allows a liquid to vaporize at reduced temperatures. It refers to the pressure contributed by a single type of gas in a mixture of gases. In the context of steam distillation, both the steam and the organic compound exert their own partial pressures.
When you introduce steam into the system, the combined pressures from the steam and the organic compound's vapors sum up to meet or exceed atmospheric pressure earlier than they would separately. Each component of the mixture—the steam and the organic liquid—contributes its partial pressure, and together they lower the temperature at which the liquid can start vaporizing.
This concept not only explains how the total vapor pressure reaches atmospheric levels more quickly, but also how it allows for the distillation of sensitive compounds that could decompose if subjected to standard boiling temperatures. Through the management of partial pressures, steam distillation can be efficiently tailored to handle various compounds gently and effectively.