Introduction: Size separation is a core topic in pharmaceutics, covering methods like sieving, filtration, centrifugation, sedimentation and air classification to separate particles by particle size and density. For B.Pharm students, understanding principles such as Stokes’ law, pore size selection, cut-off and cut-size, and equipment design is crucial for formulation, granulation, powder flow, and drug delivery. This guide offers focused, keyword-rich revision on size separation techniques, applications in pharmaceutical processing, and common troubleshooting. Clear MCQs with answers reinforce learning and exam preparedness. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What principle does gravimetric sieving primarily rely on?
- Magnetic attraction between particles
- Gravitational force and particle size differences
- Electrostatic repulsion of charged particles
- Centrifugal force only
Correct Answer: Gravitational force and particle size differences
Q2. Which law describes the settling velocity of a small spherical particle in a viscous fluid under laminar flow?
- Raoult’s law
- Stokes’ law
- Fick’s law
- Newton’s law of cooling
Correct Answer: Stokes’ law
Q3. In membrane filtration for aseptic processing, which parameter is most critical for retaining bacteria?
- Pore size of the membrane
- Membrane color
- Thickness of the filtration unit casing
- Flow meter brand
Correct Answer: Pore size of the membrane
Q4. What does the term “cut-off” or “cut size” in size separation mean?
- The maximum temperature for separation
- The particle size at which 50% passes through the separation device
- The voltage applied to an electrostatic classifier
- The total mass of sample used
Correct Answer: The particle size at which 50% passes through the separation device
Q5. Which equipment uses centrifugal force to separate particles based on size and density?
- Vibratory sieve shaker
- Centrifuge
- Air classifier
- Sedimentation column without rotation
Correct Answer: Centrifuge
Q6. Which factor does NOT significantly affect sedimentation rate of particles in a liquid?
- Particle density
- Fluid viscosity
- Particle color
- Particle radius
Correct Answer: Particle color
Q7. What is the primary use of an air classifier in pharmaceutical powder processing?
- To dissolve powders in solvent
- To separate particles by aerodynamic diameter
- To sterilize powders with heat
- To compress powders into tablets
Correct Answer: To separate particles by aerodynamic diameter
Q8. Which parameter increases the centrifugal sedimentation velocity in a centrifuge?
- Decrease in rotor speed
- Increase in rotor radius
- Decrease in particle density
- Use of a less viscous rotor material
Correct Answer: Increase in rotor radius
Q9. In wet sieving, why is ultrasonic vibration sometimes applied?
- To change the color of particles
- To break agglomerates and improve separation
- To heat the solvent to boiling
- To magnetize the particles
Correct Answer: To break agglomerates and improve separation
Q10. Which size separation technique is most appropriate for nanoparticles (<100 nm)?
- Standard laboratory sieves
- Ultracentrifugation or membrane ultrafiltration
- Air jet sieving
- Gravity settling in beaker
Correct Answer: Ultracentrifugation or membrane ultrafiltration
Q11. What does the term “span” indicate in particle size distribution analysis?
- The optical density of a suspension
- Width of distribution, indicating polydispersity
- The pH range of a suspension
- The total mass fraction retained on a sieve
Correct Answer: Width of distribution, indicating polydispersity
Q12. Which separation method is generally used to remove oversized granules after wet granulation?
- High-speed centrifugation
- Sieving or screening
- Column chromatography
- Lyophilization
Correct Answer: Sieving or screening
Q13. In cyclone separators, what property primarily determines particle collection efficiency?
- Particle temperature
- Particle aerodynamic diameter and density
- Color of inlet gas
- Humidity of surrounding air only
Correct Answer: Particle aerodynamic diameter and density
Q14. Which parameter is used to express particle size measured by sieving?
- Median pore pressure
- Mesh opening size (e.g., µm or mesh number)
- Surface tension of particles
- Electrical conductivity
Correct Answer: Mesh opening size (e.g., µm or mesh number)
Q15. Why is controlling particle size important for tablet dissolution?
- Smaller particles reduce surface area and slow dissolution
- Particle size affects surface area and hence dissolution rate
- Larger particles always dissolve faster
- Particle size only affects tablet color, not dissolution
Correct Answer: Particle size affects surface area and hence dissolution rate
Q16. Which statement about membrane filtration for sterile filtration is correct?
- 0.45 µm membranes are always used for sterilizing solutions
- 0.22 µm membranes are commonly used to retain bacteria for sterilizing aqueous solutions
- Membrane pore size is irrelevant for sterility assurance
- 0.22 µm membranes are only for gas filtration
Correct Answer: 0.22 µm membranes are commonly used to retain bacteria for sterilizing aqueous solutions
Q17. What is the effect of increasing fluid viscosity on particle settling under Stokes’ law?
- Settling velocity increases
- Settling velocity decreases
- No effect on settling velocity
- Particles gain negative buoyancy
Correct Answer: Settling velocity decreases
Q18. In elutriation, how are particles separated?
- By magnetic properties
- By balancing upward fluid velocity against settling velocity
- By electrical charge alone
- By chemical solubility differences
Correct Answer: By balancing upward fluid velocity against settling velocity
Q19. What is a common limitation of wet sieving compared to dry sieving?
- Does not remove fines
- Potential for particle dissolution or change in surface chemistry
- Cannot separate cohesive powders
- Always faster than dry sieving
Correct Answer: Potential for particle dissolution or change in surface chemistry
Q20. Which device provides both size classification and particle charging useful in some industrial separations?
- Hydraulic press
- Electrostatic classifier
- Simple gravity table
- Vacuum oven
Correct Answer: Electrostatic classifier
Q21. Which measurement technique gives number-based particle size distribution rather than volume-based?
- Laser diffraction with volume reporting only
- Dynamic light scattering (DLS)
- Sieving with mass fraction reporting only
- Powder X-ray diffraction
Correct Answer: Dynamic light scattering (DLS)
Q22. What is the primary reason to deagglomerate powder before size analysis?
- To increase moisture content
- To obtain true particle size distribution by breaking bonded clusters
- To change chemical composition
- To sterilize the sample
Correct Answer: To obtain true particle size distribution by breaking bonded clusters
Q23. In a pharma context, why is removal of oversized particles important in capsule filling?
- Oversized particles improve capsule color uniformity
- They can cause content uniformity and flow problems
- They enhance dissolution rate uniformly
- They reduce powder bulk density always
Correct Answer: They can cause content uniformity and flow problems
Q24. Which particle characteristic primarily affects aerosol deposition in the lungs?
- Optical reflectance
- Aerodynamic diameter
- Magnetic susceptibility
- Surface pH
Correct Answer: Aerodynamic diameter
Q25. What is the role of a classifier in powder processing?
- To change powder chemical structure
- To separate particles into different size fractions based on air flow
- To dissolve powders into solvent
- To compress powders into tablets
Correct Answer: To separate particles into different size fractions based on air flow
Q26. Which factor is critical when selecting a filter membrane for sterile filtration of protein solutions?
- Membrane color only
- Protein binding and fouling properties as well as pore size
- Thickness in millimeters only
- Whether it was factory sterilized only
Correct Answer: Protein binding and fouling properties as well as pore size
Q27. Which separation method is most suitable for separating particles with very small density differences but different sizes?
- Simple gravity settling only
- Air classification or high-speed centrifugation
- Magnetic separation
- Distillation
Correct Answer: Air classification or high-speed centrifugation
Q28. What does an increase in Sauter mean diameter (D[3,2]) indicate?
- Average particle surface area decreases
- Increase in surface-to-volume mean diameter, often larger droplet/particle size
- Particles have become more electrically charged
- Total mass has decreased
Correct Answer: Increase in surface-to-volume mean diameter, often larger droplet/particle size
Q29. Which of the following best describes wet centrifugal separation used in pharma labs?
- Using centrifugal force to enhance settling of particles in a liquid medium
- Using only air flow to sort particles
- Filtering solids through a membrane without rotation
- Heating powders to evaporate liquids
Correct Answer: Using centrifugal force to enhance settling of particles in a liquid medium
Q30. In centrifugation, what does the term “RCF” stand for?
- Rotational compound factor
- Relative centrifugal force
- Radioactive contaminant factor
- Rapid centrifuge frequency
Correct Answer: Relative centrifugal force
Q31. Which of the following increases particle capture in a filter cake during depth filtration?
- Decreasing feed concentration always
- Formation of a stable cake that blocks pores and traps particles
- Using a membrane with larger pores than particles
- Removing the cake during filtration
Correct Answer: Formation of a stable cake that blocks pores and traps particles
Q32. What is the main advantage of cascade impaction for aerosol particle sizing?
- It measures chemical composition only
- It provides aerodynamic cut-off sizes and mass fraction distribution by stage
- It measures zeta potential directly
- It is only used for powders, not aerosols
Correct Answer: It provides aerodynamic cut-off sizes and mass fraction distribution by stage
Q33. When performing wet milling to reduce particle size, what potential issue must be monitored?
- Increase in particle color saturation only
- Generation of heat and possible polymorphic changes or degradation
- Guaranteed improvement of product stability always
- Complete elimination of all fines
Correct Answer: Generation of heat and possible polymorphic changes or degradation
Q34. What is the common cut-off pore size used to remove mycoplasma from solutions?
- 10 µm membranes
- 0.1 µm or smaller membranes
- 5 mm sieves
- 2 µm membranes only
Correct Answer: 0.1 µm or smaller membranes
Q35. Which instrument measures particle size by analyzing scattered light intensity at multiple angles?
- Optical microscope only
- Laser diffraction particle size analyzer
- Sieve shaker
- pH meter
Correct Answer: Laser diffraction particle size analyzer
Q36. In pharmaceutical sieving, what practice improves reproducibility?
- Using inconsistent sample masses
- Standardizing sample mass, sieving time and amplitude
- Changing sieve stack order randomly each run
- Using sieves with damaged mesh
Correct Answer: Standardizing sample mass, sieving time and amplitude
Q37. What is the primary purpose of a fluidized bed classifier in powder processing?
- To melt the powder
- To separate fines and control particle size by creating a fluidized state
- To press powders into granules by compression
- To sterilize powders with UV light
Correct Answer: To separate fines and control particle size by creating a fluidized state
Q38. Which parameter is most relevant for aerosol targeting to alveolar region (~1–5 µm)?
- Color intensity
- Aerodynamic particle diameter around 1–5 µm
- Electrical conductivity only
- Magnetic susceptibility
Correct Answer: Aerodynamic particle diameter around 1–5 µm
Q39. What is dynamic classification in the context of particle separation?
- Separation by dissolving particles in dynamic solvent
- Continuous separation by balancing drag and centrifugal or gravitational forces in flowing media
- Static sieving without airflow
- Separation based on color matching
Correct Answer: Continuous separation by balancing drag and centrifugal or gravitational forces in flowing media
Q40. For colloidal suspensions, which separation technique is often required?
- Conventional sieving
- Ultracentrifugation or membrane ultrafiltration
- Basic gravity filtration through cotton wool
- Hand shaking until settling
Correct Answer: Ultracentrifugation or membrane ultrafiltration
Q41. How does electrostatic separation classify particles?
- By magnetic domains
- By differences in electrical charge and conductivity
- By pH of the powder
- By color under UV light
Correct Answer: By differences in electrical charge and conductivity
Q42. A pharmaceutical powder shows poor flow due to a high fraction of fines. Which size separation step helps most?
- Adding water without sieving
- Elutriation or air classification to remove fines
- Heating to sinter particles
- Storing at low temperature only
Correct Answer: Elutriation or air classification to remove fines
Q43. Which phrase best describes a “screen analysis”?
- Analyzing chemical impurities by HPLC
- Determining particle size distribution by passing powder through a stack of sieves
- Testing tablet hardness only
- Measuring viscosity of a suspension
Correct Answer: Determining particle size distribution by passing powder through a stack of sieves
Q44. What is a potential consequence of using a filter with too small a pore size for a viscous pharmaceutical solution?
- Improved flux with no drawbacks
- Severe fouling and very low filtration rate
- Instant sterilization without clogging
- Complete chemical degradation of solutes
Correct Answer: Severe fouling and very low filtration rate
Q45. In particle settling, when does the Stokes’ law approximation fail?
- When Reynolds number is very low (<<1)
- When particles are non-spherical, large, or flow is turbulent (higher Reynolds number)
- Only when particles are colored
- It never fails for any practical case
Correct Answer: When particles are non-spherical, large, or flow is turbulent (higher Reynolds number)
Q46. Which approach can improve separation efficiency in multilayer sieving?
- Clogging the upper sieve intentionally
- Proper stack sequence from coarse to fine and controlled vibration
- Randomly orienting sieves
- Using damaged meshes for faster throughput
Correct Answer: Proper stack sequence from coarse to fine and controlled vibration
Q47. What role does agglomeration play in pharmaceutical powder size separation?
- Agglomeration always makes separation unnecessary
- It alters apparent particle size and may require deagglomeration techniques before accurate separation or analysis
- Agglomeration only affects color, not size
- It reduces the need for sieving
Correct Answer: It alters apparent particle size and may require deagglomeration techniques before accurate separation or analysis
Q48. Which of the following best describes cyclone separator operation?
- Fluid enters tangentially creating a vortex; centrifugal forces push particles to walls for collection
- Particles are magnetically attracted to the center
- Particles dissolve into the gas phase
- It uses a membrane to filter particles
Correct Answer: Fluid enters tangentially creating a vortex; centrifugal forces push particles to walls for collection
Q49. In quality control, why is reporting both d10, d50, and d90 useful?
- They represent color gradations
- They summarize distribution breadth and performance-related percentiles of particle sizes
- They are obsolete metrics with no use
- They only indicate moisture content
Correct Answer: They summarize distribution breadth and performance-related percentiles of particle sizes
Q50. For controlled release microparticles, why is tight control of particle size distribution critical?
- Particle size has no effect on release kinetics
- Size distribution directly influences drug release rate, bioavailability and stability
- Only chemical composition matters, not size
- It affects only the color of the final product
Correct Answer: Size distribution directly influences drug release rate, bioavailability and stability
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.
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