Distillation under reduced pressure MCQs With Answer

Distillation under reduced pressure MCQs With Answer

Distillation under reduced pressure, also known as vacuum distillation, is an essential technique in pharmaceutical chemistry for purifying heat-sensitive drugs and solvents. This introduction covers core concepts B.Pharm students must master: reduced boiling points, Clausius–Clapeyron relationships, vacuum equipment (rotary evaporator, vacuum pump, cold trap), azeotrope behavior, and process safety. Understanding operational parameters—pressure control, trap cooling, and bump prevention—improves yield and material integrity during solvent removal and active pharmaceutical ingredient (API) purification. These MCQs emphasize theory, calculations, apparatus selection, and common troubleshooting to prepare you for practical lab work and exams. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary reason for performing distillation under reduced pressure in pharmaceutical labs?

• To increase solvent polarity
• To raise the boiling point of solvents
• To lower the boiling point of heat-sensitive compounds
• To completely prevent vaporization

Correct Answer: To lower the boiling point of heat-sensitive compounds

Q2. Which law relates vapor pressure and composition for ideal liquid mixtures commonly used in distillation theory?

• Henry’s law
• Raoult’s law
• Pascal’s law
• Boyle’s law

Correct Answer: Raoult’s law

Q3. Which equation is used to estimate the change in boiling point with pressure for a pure substance?

• Arrhenius equation
• Van ‘t Hoff equation
• Clausius–Clapeyron equation
• Henderson–Hasselbalch equation

Correct Answer: Clausius–Clapeyron equation

Q4. In vacuum distillation, what is the typical unit of pressure often reported in labs?

• Pascal only
• Millimeters of mercury (mmHg or Torr)
• Liters
• Degrees Celsius

Correct Answer: Millimeters of mercury (mmHg or Torr)

Q5. Which component is essential to protect the vacuum pump from solvent vapors and condensable vapors?

• Heating mantle
• Cold trap
• Magnetic stirrer
• Reflux condenser

Correct Answer: Cold trap

Q6. What is the function of a rotary evaporator in reduced-pressure distillation?

• To heat the receiver only
• To rotate the condenser for mixing
• To increase surface area and efficient solvent removal under vacuum
• To pressurize the distillation flask

Correct Answer: To increase surface area and efficient solvent removal under vacuum

Q7. Which type of vacuum pump is commonly used for routine solvent removal in a B.Pharm lab?

• Rotary vane pump
• Thermal diffusion pump
• Diffusion oil pump
• Turbo molecular pump

Correct Answer: Rotary vane pump

Q8. What problem is indicated by sudden violent boiling (bumping) during vacuum distillation?

• Excessive cooling of the receiver
• Insufficient vapor pressure
• Superheating and lack of nucleation sites
• Too low flask rotation speed

Correct Answer: Superheating and lack of nucleation sites

Q9. Which of the following is a common precaution to prevent bumping?

• Use of boiling chips or gentle rotation
• Increasing tray height
• Adding more solvent
• Raising ambient temperature

Correct Answer: Use of boiling chips or gentle rotation

Q10. How does lowering pressure affect the relative volatility of two components in a mixture?

• Relative volatility always increases with lower pressure
• Relative volatility is independent of pressure for all mixtures
• Relative volatility can change, potentially affecting separation efficiency
• Relative volatility is converted to azeotropy at low pressure

Correct Answer: Relative volatility can change, potentially affecting separation efficiency

Q11. What is molecular distillation?

• Distillation at atmospheric pressure with reflux
• Short-path distillation under very low pressure to separate high-boiling, thermally sensitive molecules
• Steam distillation used for oils only
• Fractional distillation with many theoretical plates

Correct Answer: Short-path distillation under very low pressure to separate high-boiling, thermally sensitive molecules

Q12. Which measurement device is used to monitor vacuum in small-scale labs?

• Thermocouple manometer
• Barometer only
• Pressure cooker gauge
• Hydrometer

Correct Answer: Thermocouple manometer

Q13. When distilling a solvent with a known boiling point of 150°C at 760 mmHg, which general effect will reducing pressure to 100 mmHg have?

• Boiling point will significantly increase
• Boiling point will decrease substantially
• Boiling point will remain the same
• Solvent will polymerize

Correct Answer: Boiling point will decrease substantially

Q14. Which of the following best describes an azeotrope’s behavior under reduced pressure?

• Azeotropes always disappear under vacuum
• Azeotrope composition and boiling point can shift and may persist or break depending on conditions
• Azeotropes become ideal mixtures under vacuum
• Azeotropes convert to gases instantly

Correct Answer: Azeotrope composition and boiling point can shift and may persist or break depending on conditions

Q15. In a vacuum distillation setup, what is the role of a trap (between flask and pump)?

• To add more solvent to the system
• To prevent backflow of oil and collect condensable vapors
• To measure boiling point directly
• To increase flask pressure

Correct Answer: To prevent backflow of oil and collect condensable vapors

Q16. Which solvent is most likely to require vacuum distillation due to high boiling point and thermal sensitivity?

• Diethyl ether
• Water
• Glycerol
• Methanol

Correct Answer: Glycerol

Q17. Which property of a solvent is directly reduced by applying vacuum, facilitating distillation?

• Density
• Vapor pressure
• Boiling point
• Surface tension

Correct Answer: Boiling point

Q18. What is a potential disadvantage of using an improperly sized vacuum pump?

• It may create too strong cold trap cooling
• It may cause insufficient vacuum or solvent flooding and bumping
• It always improves separation efficiency
• It eliminates the need for boiling chips

Correct Answer: It may cause insufficient vacuum or solvent flooding and bumping

Q19. During reduced-pressure distillation, which parameter is most critical to control for reproducible boiling points?

• Ambient humidity
• System pressure
• Color of the flask
• Volume of the receiving flask

Correct Answer: System pressure

Q20. Which safety precaution is essential when distilling flammable solvents under vacuum?

• Use an open flame for heating to speed distillation
• Ensure proper grounding, use explosion-proof pumps, and avoid spark sources
• Never use a cold trap
• Seal the system without a pressure relief

Correct Answer: Ensure proper grounding, use explosion-proof pumps, and avoid spark sources

Q21. Which parameter determines the number of theoretical stages in fractional distillation, relevant when using reduced pressure?

• Relative volatility and reflux ratio
• Weight of the flask
• Ambient pressure only
• Type of cold trap used

Correct Answer: Relative volatility and reflux ratio

Q22. In a rotovap, why is rotating the flask beneficial?

• It reduces surface area for evaporation
• It creates a uniform thin film, increasing evaporation rate
• It increases flask temperature dramatically
• It prevents the vacuum from forming

Correct Answer: It creates a uniform thin film, increasing evaporation rate

Q23. How does adding an inert gas (e.g., nitrogen) into a vacuum distillation system affect the process?

• Enhances vacuum level
• Prevents oxidation and can be used to backfill, but reduces vacuum and slows distillation
• Removes nonvolatile residues
• Increases solvent polarity

Correct Answer: Prevents oxidation and can be used to backfill, but reduces vacuum and slows distillation

Q24. What is the effect of non-condensable gases in a vacuum distillation system?

• They improve condensation efficiency
• They increase achievable vacuum and speed up distillation
• They reduce vacuum quality and hinder vapor flow to the pump
• They convert solvents to solids

Correct Answer: They reduce vacuum quality and hinder vapor flow to the pump

Q25. Which gauge type is best for measuring high vacuums typical of molecular distillation?

• U-tube mercury manometer
• Thermocouple or Pirani gauge for rough vacuum
• Ionization gauge for high and ultra-high vacuum
• Liquid-in-glass thermometer

Correct Answer: Ionization gauge for high and ultra-high vacuum

Q26. During vacuum distillation, why might a distillate show azeotropic-like behavior even when no azeotrope is expected?

• Due to measurement errors only
• Because pressure changes alter relative volatility and can create composition plateaus
• Because solvents always form azeotropes under vacuum
• Because condensation temperature is irrelevant

Correct Answer: Because pressure changes alter relative volatility and can create composition plateaus

Q27. What does a short-path distillation apparatus minimize compared to standard distillation?

• Path length between vaporization and condensation, reducing residence time at elevated temperature
• The need for a cold trap
• The vacuum level required
• The efficiency of separation

Correct Answer: Path length between vaporization and condensation, reducing residence time at elevated temperature

Q28. Which factor does NOT generally affect the boiling point at reduced pressure?

• Ambient barometric pressure changes outside the system
• System leak allowing air ingress
• Composition of the liquid mixture
• Color of the liquid

Correct Answer: Color of the liquid

Q29. When scaling up vacuum distillation, what operational change is commonly required?

• Using a weaker vacuum pump than at small scale
• Proportional increase in condenser and trap capacity and pump throughput
• Decreasing cooling capacity
• Using smaller flasks only

Correct Answer: Proportional increase in condenser and trap capacity and pump throughput

Q30. Which of the following analytical approaches helps predict boiling point at reduced pressure?

• Using the Antoine equation or Clausius–Clapeyron relation
• Estimating by color change
• Measuring viscosity only
• Performing only a refractive index test

Correct Answer: Using the Antoine equation or Clausius–Clapeyron relation

Q31. What is the typical sign that a vacuum system has a leak during distillation?

• Pressure remains stable at setpoint
• Pressure rises when pump is on and cannot reach target vacuum
• Temperature of the water bath drops
• Rotation speed increases automatically

Correct Answer: Pressure rises when pump is on and cannot reach target vacuum

Q32. Which solvent would be safest to distill under reduced pressure with common lab pumps and cold traps?

• Strong oxidizer like nitric acid
• Low-boiling flammable solvent without explosion controls
• Moderate boiling, non-explosive solvent like dichloromethane with proper precautions
• Peroxides without testing

Correct Answer: Moderate boiling, non-explosive solvent like dichloromethane with proper precautions

Q33. In vacuum distillation, what does a rising temperature trend in the distillation flask typically indicate when pressure is constant?

• The liquid composition is becoming enriched in higher-boiling components
• The vacuum pump is failing
• The condenser is freezing
• The flask is empty

Correct Answer: The liquid composition is becoming enriched in higher-boiling components

Q34. Which glassware joint component is recommended for vacuum-tight connections?

• Loose cotton plug
• Greased ground glass joints or PTFE sleeves where appropriate
• Open rubber bands
• Tacky tape only

Correct Answer: Greased ground glass joints or PTFE sleeves where appropriate

Q35. For quantitative solvent recovery, which practice improves yield during vacuum distillation?

• Using minimal cooling on the condenser
• Gradually reducing pressure and controlling bath temperature to collect fractions
• Distilling at the highest achievable vacuum only
• Allowing rapid pressure changes to speed process

Correct Answer: Gradually reducing pressure and controlling bath temperature to collect fractions

Q36. What is the effect of using a higher boiling liquid as the distillation medium on viscosity and mass transfer under vacuum?

• Viscosity decreases and mass transfer improves
• Viscosity increases, reducing mass transfer and potentially slowing distillation
• No effect on viscosity
• It always forms an azeotrope

Correct Answer: Viscosity increases, reducing mass transfer and potentially slowing distillation

Q37. Which of the following best describes flash distillation relevant under reduced pressure?

• Continuous vaporization of a small portion of liquid due to sudden pressure drop
• Slow batch heating at atmospheric pressure
• Only used for nonvolatile solids
• Distillation with reflux only

Correct Answer: Continuous vaporization of a small portion of liquid due to sudden pressure drop

Q38. Why is it important to degas solvents before vacuum distillation in some cases?

• Dissolved gases can cause bubble formation and inconsistent boiling
• Degassing increases solvent polarity
• Dissolved gases enhance condensation efficiency
• Degassing prevents any evaporation

Correct Answer: Dissolved gases can cause bubble formation and inconsistent boiling

Q39. How does condenser temperature affect efficiency during vacuum distillation?

• Lower condenser temperature improves condensation and collection efficiency
• Condenser temperature has no effect under vacuum
• Higher condenser temperature always improves yield
• Condenser temperature only affects color

Correct Answer: Lower condenser temperature improves condensation and collection efficiency

Q40. What is the purpose of using a pressure-control valve or bleed valve in a vacuum distillation system?

• To rapidly vent the system to atmosphere during distillation
• To maintain a stable target vacuum by admitting small amounts of gas
• To increase flask rotation speed
• To heat the cold trap

Correct Answer: To maintain a stable target vacuum by admitting small amounts of gas

Q41. Which parameter is directly determined from a distillation curve collected under reduced pressure?

• Melting point of the sample
• Boiling behavior and fraction composition versus temperature
• Optical activity of the mixture
• Glass transition temperature

Correct Answer: Boiling behavior and fraction composition versus temperature

Q42. In vacuum distillation, why is it important to avoid oil backstreaming from rotary vane pumps?

• Oil backstreaming cools the condenser excessively
• Oil contamination can contaminate sensitive distillates and create hazards
• Oil backstreaming increases vacuum quality
• Backstreaming reduces solvent polarity

Correct Answer: Oil contamination can contaminate sensitive distillates and create hazards

Q43. When separating a binary mixture under reduced pressure, what indicates a successful fraction collection?

• Constant distillate composition across the run
• Distinct temperature plateaus and changes in refractive index or composition between fractions
• No change in flask mass
• Immediate dryness of all residues

Correct Answer: Distinct temperature plateaus and changes in refractive index or composition between fractions

Q44. What is the main advantage of short-path over traditional long-path distillation for APIs?

• Longer residence time at high temperatures
• Shorter vapor path reduces thermal decomposition and product loss
• It requires no vacuum
• It increases the number of theoretical plates

Correct Answer: Shorter vapor path reduces thermal decomposition and product loss

Q45. Which maintenance practice prolongs rotary vane pump life in a lab performing frequent vacuum distillations?

• Never changing pump oil
• Using impure oil and no filters
• Regular oil change, proper cold trap use, and leak checks
• Operating pump without a trap at all times

Correct Answer: Regular oil change, proper cold trap use, and leak checks

Q46. How should one select solvent recovery temperatures when distilling azeotropic mixtures under reduced pressure?

• Use the highest possible bath temperature to speed recovery
• Use literature boiling data at the chosen pressure and monitor composition to avoid co-distillation of impurities
• Ignore pressure because azeotropes are unaffected
• Always distill at atmospheric pressure

Correct Answer: Use literature boiling data at the chosen pressure and monitor composition to avoid co-distillation of impurities

Q47. Which of the following best describes how a cold trap should be maintained during repeated distillations?

• Never emptied to keep continuous operation
• Regularly emptied and cleaned to prevent blockage and contamination
• Left at room temperature for convenience
• Replaced with a heater

Correct Answer: Regularly emptied and cleaned to prevent blockage and contamination

Q48. In pressure-dependent distillation, what role does the Antoine equation play for a B.Pharm student?

• It predicts refractive index only
• It provides an empirical relation to estimate vapor pressures and boiling points at different temperatures
• It measures viscosity changes
• It defines pump maintenance schedules

Correct Answer: It provides an empirical relation to estimate vapor pressures and boiling points at different temperatures

Q49. Why is it important to consider vapor molecular weight when designing traps and cold surfaces?

• Heavier vapors condense more easily and require less surface area
• Vapor molecular weight is irrelevant to condensation
• Lighter vapors are easier to trap without cooling
• All vapors condense at the same temperature

Correct Answer: Heavier vapors condense more easily and require less surface area

Q50. For regulatory compliance in pharmaceutical distillation, which documentation is most critical?

• Only operator names
• Batch records, vacuum and temperature logs, and validated procedures
• No documentation is required
• Only the color of the distillate

Correct Answer: Batch records, vacuum and temperature logs, and validated procedures

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com