Question

If there is condensation of carrier steam in the steam distillation of high-boiling organic materials, then:

• A. Both temperature and pressure must be fixed
• B. Either the temperature or the pressure may be fixed
• C. The temperature is always more than \( 100^\circ\text{C} \) at 1 atm
• D. The temperature is always equal to \( 100^\circ\text{C} \) at 1 atm

Hint:

Gibbs Phase Rule Application: For steam distillation with a condensed water phase present: - 2 immiscible components + 3 phases (2 liquids + 1 vapor) \( \rightarrow F = 2 - 3 + 2 = 1 \). - Since \( F = 1 \), specifying either temperature or pressure uniquely determines the state of the system.

Answer 1

The Correct Option is B

Concept: Steam distillation is a separation technique used to purify high-boiling, water-immiscible organic compounds at temperatures well below their normal boiling points. This prevents thermal decomposition. To determine the degrees of freedom and system constraints during this process, we apply the Gibbs Phase Rule: \[ F = C - P + 2 \] Where \( F \) is the variance or degrees of freedom, \( C \) is the number of independent chemical components, and \( P \) is the number of coexisting phases in equilibrium.

Step 1: Identifying components and phases when carrier steam condenses.
Let us analyze the system variables when carrier steam condenses during distillation:
• Components (\( C \)): There are 2 distinct chemical species present: the immiscible organic compound (Component A) and water (Component B). Thus, \( C = 2 \).
• Phases (\( P \)): If the carrier steam undergoes partial condensation inside the distillation vessel, the following phases coexist in equilibrium:
• An organic liquid phase (Liquid A).
• A condensed aqueous liquid phase (Liquid B).
• A shared vapor phase containing both organic vapor and water vapor. Thus, there are 3 distinct phases coexisting simultaneously, so \( P = 3 \).

Step 2: Calculating system degrees of freedom via the Phase Rule.
Substitute these component and phase counts into the Gibbs phase rule equation: \[ F = 2 - 3 + 2 = 1 \] The calculation shows that the system has exactly one degree of freedom (\( F = 1 \)), meaning it is univariant. A univariant system means that specifying exactly one intensive variable (such as either the operating temperature or the total system pressure) completely fixes the remaining state variables of the system at equilibrium. Therefore, either the temperature or the pressure may be fixed independently, but not both simultaneously. Specifying one variable uniquely determines the other.