Unit Operation: Distillation – azeotropic and steam distillation MCQs With Answer

Unit Operation: Distillation – azeotropic and steam distillation MCQs With Answer

This set of MCQs focuses on azeotropic and steam distillation as covered in MPC 204T Pharmaceutical Process Chemistry. The questions are designed for M.Pharm students who need a clear, applied understanding of why azeotropes form, how they affect separation by rectification, and practical routes to overcome them (entrainers, pressure-swing, extractive methods, molecular sieves). Steam distillation questions emphasize the thermodynamic basis, its suitability for heat-sensitive and immiscible systems, and typical pharmaceutical applications (essential oils, volatile actives). Expect conceptual, mechanism-level and applied design questions to prepare for exams and real-world process decisions.

Q1. What defines an azeotrope in a binary liquid mixture?

• A mixture that separates completely on simple distillation
• A liquid mixture whose vapor has a different composition than the liquid at boiling
• A constant-boiling mixture whose vapor and liquid compositions are identical
• A mixture that cannot vaporize

Correct Answer: A constant-boiling mixture whose vapor and liquid compositions are identical

Q2. A minimum-boiling azeotrope occurs when:

• the azeotrope boils at a temperature higher than both pure components
• positive deviation from Raoult’s law causes the mixture to boil at lower temperature than either pure component
• negative deviation from Raoult’s law causes strong association in liquid
• components are completely immiscible

Correct Answer: positive deviation from Raoult’s law causes the mixture to boil at lower temperature than either pure component

Q3. Which of the following is a classic example of a minimum-boiling azeotrope important in pharmacy?

• Water–glycerol
• Ethanol–water (~95.6% ethanol)
• Methanol–benzene
• Acetic acid–water (maximum boiling)

Correct Answer: Ethanol–water (~95.6% ethanol)

Q4. Which phenomenon most directly causes azeotrope formation?

• Complete miscibility of components at all compositions
• Non-ideal vapor–liquid behavior (deviation from Raoult’s law) leading to constant boiling composition
• Very high vapor pressures of both components
• Low surface tension differences

Correct Answer: Non-ideal vapor–liquid behavior (deviation from Raoult’s law) leading to constant boiling composition

Q5. A maximum-boiling azeotrope is characterized by:

• A boiling point lower than both components
• A boiling point equal to the lighter component only
• A boiling point higher than both pure components due to strong negative deviation
• Complete immiscibility of components

Correct Answer: A boiling point higher than both pure components due to strong negative deviation

Q6. Which technique is commonly used to break an ethanol–water azeotrope to obtain absolute ethanol?

• Simple batch distillation at atmospheric pressure
• Addition of a suitable entrainer and azeotropic distillation (or use of molecular sieves)
• Cooling crystallization
• Direct condensation without fractionation

Correct Answer: Addition of a suitable entrainer and azeotropic distillation (or use of molecular sieves)

Q7. In azeotropic distillation using an entrainer, the entrainer functions primarily to:

• Raise the boiling point of the mixture
• Form a new azeotrope with one component that separates preferentially in the vapor phase
• React chemically with impurities
• Make the mixture nonvolatile

Correct Answer: Form a new azeotrope with one component that separates preferentially in the vapor phase

Q8. Pressure-swing distillation can separate an azeotrope when:

• The azeotrope composition and boiling point are independent of pressure
• Changing pressure alters the azeotropic composition or causes the azeotrope to disappear
• The compounds are chemically reactive under pressure
• The system is fully ideal and follows Raoult’s law

Correct Answer: Changing pressure alters the azeotropic composition or causes the azeotrope to disappear

Q9. Which of the following correctly describes steam distillation?

• Distillation where the total pressure equals the vapor pressure of pure solute only
• Distillation of immiscible or partially miscible high-boiling compounds by introducing steam to reduce the boiling temperature
• Distillation under vacuum only for azeotropic mixtures
• Single-component fractional distillation using inert gases

Correct Answer: Distillation of immiscible or partially miscible high-boiling compounds by introducing steam to reduce the boiling temperature

Q10. The fundamental requirement for successful steam distillation of an organic compound is:

• The compound must be fully miscible with water at all compositions
• The compound must have a significant vapor pressure at the steam temperature and be immiscible or form a separate phase with water
• The compound must polymerize in steam
• Both compound and water must have identical vapor pressures

Correct Answer: The compound must have a significant vapor pressure at the steam temperature and be immiscible or form a separate phase with water

Q11. During steam distillation of an immiscible oil and water mixture at 1 atm, the boiling temperature is determined when:

• The vapor pressure of the oil alone equals 1 atm
• The sum of the vapor pressures of water and the oil at that temperature equals 1 atm
• The partial pressure of water equals the oil vapor pressure
• The oil is fully soluble in the vapor

Correct Answer: The sum of the vapor pressures of water and the oil at that temperature equals 1 atm

Q12. Steam distillation is especially useful in pharmaceutical processing for:

• Concentrating ionic salts from aqueous solutions
• Recovering volatile, heat-sensitive natural products such as essential oils at lower temperatures
• Separating azeotropes of fully miscible pairs like ethanol–water
• Sublimation of solids

Correct Answer: Recovering volatile, heat-sensitive natural products such as essential oils at lower temperatures

Q13. Which statement about the vapor composition in steam distillation of an immiscible binary (water + oil) is true?

• The vapor composition equals the liquid mole fractions directly
• The oil is always the major component in vapor regardless of its liquid fraction
• The vapor contains significant amounts of both components; their partial pressures determine vapor composition
• Only water vaporizes and transports the oil by entrainment without oil vapor pressure contribution

Correct Answer: The vapor contains significant amounts of both components; their partial pressures determine vapor composition

Q14. Which method is least appropriate to separate a close-boiling azeotropic mixture?

• Pressure-swing distillation when azeotrope composition shifts with pressure
• Extractive distillation using a high-boiling selective solvent
• Simple single-stage distillation at constant pressure
• Azeotropic distillation with a suitable entrainer

Correct Answer: Simple single-stage distillation at constant pressure

Q15. Why are molecular sieves used after distillation in ethanol production?

• To catalyze ethanol formation
• To adsorb residual water and produce anhydrous ethanol after azeotropic/rectification limits are reached
• To separate ethanol and heavier impurities by boiling point differences
• To polymerize trace impurities

Correct Answer: To adsorb residual water and produce anhydrous ethanol after azeotropic/rectification limits are reached

Q16. In azeotropic distillation employing benzene as an entrainer for ethanol–water, the role of benzene historically was to:

• Form a lower-boiling azeotrope with water allowing water to be removed as azeotrope vapor
• React with ethanol to form ethyl benzene
• Form a ternary azeotrope that carries water overhead with benzene and ethanol
• Increase the viscosity of the liquid phase

Correct Answer: Form a ternary azeotrope that carries water overhead with benzene and ethanol

Q17. In design of a steam distillation unit for essential oil recovery, which consideration is most critical?

• Maximizing residence time to polymerize the oil
• Controlling steam temperature and flow to avoid thermal degradation while achieving adequate partial pressures
• Using corrosive materials for construction to enhance extraction
• Operating at extremely high pressures to force solubility

Correct Answer: Controlling steam temperature and flow to avoid thermal degradation while achieving adequate partial pressures

Q18. Which of the following best describes a ternary azeotrope relevant to pharmaceutical separations?

• An azeotrope involving three components that distills at a constant composition and temperature
• An azeotrope that exists only for binary mixtures
• A mixture that can be separated by a single theoretical plate
• A situation where only two components interact non-ideally

Correct Answer: An azeotrope involving three components that distills at a constant composition and temperature

Q19. Which statement correctly compares extractive distillation and azeotropic distillation?

• Extractive distillation uses a low-boiling entrainer to form a new azeotrope; azeotropic distillation uses a high-boiling solvent to change relative volatilities
• Both methods always chemically react with feed components
• Extractive distillation uses a high-boiling non-volatile solvent to change relative volatilities without forming a new azeotrope, whereas azeotropic distillation adds an entrainer that forms a separable azeotrope
• They are identical techniques with different names

Correct Answer: Extractive distillation uses a high-boiling non-volatile solvent to change relative volatilities without forming a new azeotrope, whereas azeotropic distillation adds an entrainer that forms a separable azeotrope

Q20. Which advantage of steam distillation makes it particularly suitable for processing plant-derived actives?

• It requires higher temperatures than simple distillation
• It enables volatilization at temperatures below the decomposition point, preserving sensitive compounds
• It increases chemical reactivity of actives
• It guarantees complete removal of all non-volatiles

Correct Answer: It enables volatilization at temperatures below the decomposition point, preserving sensitive compounds

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