Question

In steam distillation, the vapour pressure of the volatile organic compound is: (a) More than atmospheric pressure (b) Equal to atmospheric pressure (c) Less than atmospheric pressure (d) None of these

Short answer

(c) Less than atmospheric pressure

Step-by-step solution

Understanding Steam Distillation

In steam distillation, a mixture of water and organic material is heated. Both components have their own vapour pressures at a given temperature which contribute to the total pressure.

Concept of Vapour Pressure

The vapour pressure of each component in steam distillation contributes to the total vapour pressure. When the sum of these vapour pressures equals atmospheric pressure, boiling occurs, allowing the mixture to volatilize at a lower temperature.

Analyze Vapour Pressure Relationship

The volatile organic compound's vapour pressure, in combination with the vapour pressure of water, allows the system to reach atmospheric pressure. Therefore, individually, the vapour pressures are less than atmospheric pressure.

Key concepts

Vapour Pressure

When we discuss vapour pressure in the context of steam distillation, we are referring to the pressure exerted by a vapor in equilibrium with its liquid or solid phase. This is crucial for understanding why steam distillation allows for the distillation of compounds at temperatures lower than their boiling points. In steam distillation, the mixture of water and another liquid (often a volatile organic compound) is heated. Both substances contribute separately to the total vapour pressure within the distillation setup. The vapour pressure of each substance is lower than the atmospheric pressure individually, but together their combined vapour pressures reach this threshold. By adding their vapour pressures within the system, the mixture boils when the sum equals the atmospheric pressure, all at a temperature lower than the normal boiling points of the components involved. Hence, this method is particularly useful for isolating substances that could be damaged or decomposed by higher heat.

Volatile Organic Compound

Volatile organic compounds, often abbreviated as VOCs, are a group of organic chemicals that have high vapor pressures. This characteristic allows them to evaporate or vaporize readily under atmospheric conditions. The volatile nature of these compounds makes them ideal for steam distillation. In the process of steam distillation, the compounds don't have to reach their normal boiling points to be vaporized and collected. Instead, they can evaporate along with the steam, due to their considerable vapour pressures. VOCs are often used in perfumes, essential oils, and industrial processes for their distinctive properties. By lowering the overall boiling requirements, steam distillation preserves such qualities of VOCs, making it efficient for both industrial applications and laboratory-scale extractions.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the earth's atmosphere. It is an important factor in determining the conditions under which steam distillation occurs. At sea level, standard atmospheric pressure is approximately 101.3 kPa. When performing steam distillation, the goal is to reach this pressure through the combined vapour pressures of the liquid mixture in the distillation apparatus. This allows boiling to occur below the standard boiling point of the individual components. Understanding atmospheric pressure is crucial during distillation, as it helps determine when a compound will start to boil. In a steam distillation system, when the mixture's total vapour pressure matches atmospheric pressure, the liquid mixture boils, causing the vapor to move through the system and allowing for separation of the components.