Introduction: The mechanism of size separation is a core topic for B. Pharm students studying particle technology and manufacturing. This guide covers key concepts such as particle size separation, sieving, sedimentation, centrifugation, air classification, and the role of Stokes’ law and d50 in design and analysis. Clear explanations of sieving efficiency, cut-size, cyclone behavior, and cascade impactors help link theory to pharmaceutical unit operations like granulation, capsule filling, and inhalation product development. Emphasis on practical factors — particle shape, density, viscosity, and cohesion — prepares students for exams and lab work. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary mechanism in sieving for size separation?
- Particle settling by gravity through a fluid
- Passing particles through openings based on physical dimensions
- Separation by electrical charge
- Separation by density differences in a liquid media
Correct Answer: Passing particles through openings based on physical dimensions
Q2. Which law describes sedimentation velocity for small spherical particles in laminar flow?
- Newton’s law
- Stokes’ law
- Fick’s law
- Hagen-Poiseuille equation
Correct Answer: Stokes’ law
Q3. In Stokes’ law, terminal settling velocity is directly proportional to which property of the particle?
- Particle diameter squared
- Particle diameter
- Particle volume to the power of three
- Inverse of particle density
Correct Answer: Particle diameter squared
Q4. What does d50 (cut size) represent in size separation?
- Diameter at which 50% of particles are retained
- Diameter at which 50% of particles by number or mass pass the separator
- Smallest detectable particle diameter
- Diameter at which 100% of particles pass
Correct Answer: Diameter at which 50% of particles by number or mass pass the separator
Q5. Which factor does NOT affect sedimentation rate of a particle in a liquid?
- Particle density
- Fluid viscosity
- Ambient temperature
- Magnetic susceptibility of the particle
Correct Answer: Magnetic susceptibility of the particle
Q6. In centrifugation, which parameter increases the effective gravitational force on particles?
- Rotor radius and angular velocity
- Ambient pressure
- Solution pH
- Particle color
Correct Answer: Rotor radius and angular velocity
Q7. Hindered settling occurs when:
- Particles settle independently at Stokes velocity
- Particle concentration is high, reducing individual settling rates
- Particles are perfectly spherical
- Fluid viscosity is zero
Correct Answer: Particle concentration is high, reducing individual settling rates
Q8. Which device uses inertia and centrifugal forces to separate particles from an airstream?
- Sieve shaker
- Centrifugal classifier or cyclone
- Electrophoresis cell
- Membrane filter
Correct Answer: Centrifugal classifier or cyclone
Q9. What role does particle shape play in size separation?
- No role; only diameter matters
- Shape affects drag and effective settling behavior
- Shape only influences color
- Shape determines solubility but not separation
Correct Answer: Shape affects drag and effective settling behavior
Q10. Which correction accounts for non-continuum effects for very small particles in gas-phase sedimentation?
- Hagen correction
- Cunningham slip correction
- Stokes subtraction
- Reynolds amplification
Correct Answer: Cunningham slip correction
Q11. In air classification, what parameter primarily controls cut size?
- Airflow velocity and classifier geometry
- Solution ionic strength
- Temperature alone
- Type of binder used
Correct Answer: Airflow velocity and classifier geometry
Q12. Which size analysis method provides a particle size distribution by light scattering?
- Laser diffraction
- Sieve analysis
- Wet sedimentation by pipette
- Sieving with mesh screens
Correct Answer: Laser diffraction
Q13. What is the primary separation mechanism in filtration?
- Inertial impaction by air
- Retention by porous medium and sieving at pore scale
- Electrostatic attraction exclusively
- Centrifugal force in a liquid column
Correct Answer: Retention by porous medium and sieving at pore scale
Q14. Which dimensionless number indicates whether particle motion is laminar or turbulent during settling?
- Prandtl number
- Reynolds number
- Péclet number
- Schmidt number
Correct Answer: Reynolds number
Q15. For spherical particles in Stokes regime, drag force is proportional to:
- Velocity squared
- Velocity
- Particle volume squared
- Inverse of velocity
Correct Answer: Velocity
Q16. What is the effect of increasing fluid viscosity on terminal settling velocity?
- Terminal velocity increases
- Terminal velocity decreases
- Terminal velocity remains unchanged
- Terminal velocity oscillates
Correct Answer: Terminal velocity decreases
Q17. In cascade impactors used for inhalation aerosols, separation is primarily by:
- Electrostatic attraction
- Inertial impaction based on aerodynamic diameter
- Buoyancy in liquid medium
- Membrane diffusion
Correct Answer: Inertial impaction based on aerodynamic diameter
Q18. Which parameter defines aerodynamic diameter?
- Geometric diameter only
- Diameter of a unit-density sphere having the same settling velocity
- Radius of gyration
- Diameter measured by microscopy only
Correct Answer: Diameter of a unit-density sphere having the same settling velocity
Q19. What does a steep Tromp curve indicate about a classifier’s performance?
- Poor separation between sizes
- Sharp separation and good selectivity
- Uniform particle composition
- High cohesion among particles
Correct Answer: Sharp separation and good selectivity
Q20. Which process enhances separation of fine particles by forming larger aggregates?
- Dispersion
- Flocculation or agglomeration
- Sieving at high frequency
- Electrophoresis
Correct Answer: Flocculation or agglomeration
Q21. In wet sieving, why is water used during sieving of powders?
- To dissolve all particles
- To reduce electrostatic and cohesive forces and prevent blinding
- To increase particle density
- To change particle chemical composition
Correct Answer: To reduce electrostatic and cohesive forces and prevent blinding
Q22. Which instrument measures aerodynamic particle size distribution of aerosols in pharma?
- Optical microscope only
- Cascade impactor
- Sieving tower
- Mass spectrometer
Correct Answer: Cascade impactor
Q23. What determines the mesh number of a sieve?
- Number of openings per linear inch
- Thickness of the wire only
- Weight of the sieve
- Color of the frame
Correct Answer: Number of openings per linear inch
Q24. Which separation method exploits differences in electrical charge of particles?
- Electrostatic separation
- Sizing by laser diffraction
- Wet sedimentation
- Sieving by mesh
Correct Answer: Electrostatic separation
Q25. The term ‘cut point’ in a classifier most closely relates to:
- Particle color at the exit
- Particle size where probability of reporting to either product is 50%
- Maximum particle size present
- Minimum airflow used
Correct Answer: Particle size where probability of reporting to either product is 50%
Q26. In a cyclone separator, increasing inlet velocity will generally:
- Decrease centrifugal forces and lower efficiency
- Increase centrifugal forces and may improve collection of coarse particles
- Have no effect on separation
- Only change pressure without affecting particles
Correct Answer: Increase centrifugal forces and may improve collection of coarse particles
Q27. Which property must be known to convert geometric particle size to aerodynamic size?
- Particle color
- Particle density
- Surface charge
- Melting point
Correct Answer: Particle density
Q28. What is the meaning of span in particle size distribution?
- Span measures the width of the distribution (D90-D10)/D50
- Span equals D50 only
- Span is the number of particles counted
- Span relates to particle color range
Correct Answer: Span measures the width of the distribution (D90-D10)/D50
Q29. For non-spherical particles, which diameter is often used to describe aerodynamic behavior?
- Geometric mean diameter only
- Equivalent spherical aerodynamic diameter
- Optical diameter without density correction
- Actual longest dimension only
Correct Answer: Equivalent spherical aerodynamic diameter
Q30. Which phenomenon reduces sieving efficiency when fine particles clog mesh openings?
- Elastic rebound
- Blinding
- Flotation
- Electrophoresis
Correct Answer: Blinding
Q31. What is the main advantage of air classification over sieving for fine powders?
- Air classification handles cohesive, fine particles better and can separate by aerodynamic size
- Air classification requires larger sample sizes
- Air classification is slower than sieving
- Air classification cannot handle moisture
Correct Answer: Air classification handles cohesive, fine particles better and can separate by aerodynamic size
Q32. Which separation mechanism uses bubble attachment to lift particles to the surface?
- Flotation
- Centrifugation
- Sedimentation in a settling column
- Electrostatic precipitation
Correct Answer: Flotation
Q33. What is the effect of increasing particle density on settling velocity in a liquid?
- Settling velocity decreases
- Settling velocity increases
- No change in settling velocity
- Settling velocity becomes zero
Correct Answer: Settling velocity increases
Q34. Which term describes the percentage of particles that pass through a sieve size?
- Retention percentage
- Cumulative undersize
- Porosity value
- Viscosity index
Correct Answer: Cumulative undersize
Q35. In centrifugal sedimentation (disk-stack or bench centrifuge), separation is accelerated because centrifugal acceleration is:
- Less than g
- Equal to g
- Many times g depending on RPM and radius
- Independent of RPM
Correct Answer: Many times g depending on RPM and radius
Q36. Which practice improves sieving accuracy for a wide particle size range?
- Using a single very coarse sieve
- Stacking a series of sieves with decreasing mesh sizes
- Increasing humidity abruptly
- Changing the sieve material color
Correct Answer: Stacking a series of sieves with decreasing mesh sizes
Q37. What is a primary limitation of Stokes’ law when applied to many pharmaceutical particles?
- It assumes turbulent flow always
- It assumes small, rigid, spherical particles at low Reynolds number
- It accounts for electrostatic interactions perfectly
- It predicts color changes
Correct Answer: It assumes small, rigid, spherical particles at low Reynolds number
Q38. Which measurement gives D10, D50, and D90 values?
- Sieve analysis or particle size distribution analysis
- pH titration
- Thermogravimetric analysis
- UV-Vis spectroscopy
Correct Answer: Sieve analysis or particle size distribution analysis
Q39. In elutriation, separation is based on:
- Different electrical conductivities
- Particle settling velocity in an upward fluid flow
- Magnetic susceptibility
- Particle solubility in organic solvents
Correct Answer: Particle settling velocity in an upward fluid flow
Q40. Which parameter is improved by using vibration in sieving?
- Sieving efficiency by reducing interparticle friction and aiding particle passage
- Particle chemical stability
- Color uniformity
- Particle solubility
Correct Answer: Sieving efficiency by reducing interparticle friction and aiding particle passage
Q41. What is the effect of electrostatic charges on fine particle separation?
- They always improve separation
- They can cause agglomeration or repulsion, affecting separation efficiency
- They convert particles to liquids
- They have no effect in dry processes
Correct Answer: They can cause agglomeration or repulsion, affecting separation efficiency
Q42. Which technique separates particles by size using a fluidized bed of air?
- Gravity settling
- Air classification or elutriation
- Membrane filtration
- Freeze drying
Correct Answer: Air classification or elutriation
Q43. What does the term ‘oversize’ refer to in sieve analysis?
- Particles smaller than the sieve opening
- Particles retained on the sieve larger than the opening
- Particles dissolved during testing
- Particles that evaporate during analysis
Correct Answer: Particles retained on the sieve larger than the opening
Q44. Which process can be used to separate nanoparticles in a suspension by centrifugation?
- Sedimentation at low g only
- Ultracentrifugation to reach very high relative centrifugal forces
- Simple decantation without force
- Standard sieving
Correct Answer: Ultracentrifugation to reach very high relative centrifugal forces
Q45. Which of these increases particle separation by inertia in an airstream?
- Reducing particle density
- Increasing particle mass and air velocity
- Using lower airflow velocities
- Making particles perfectly porous
Correct Answer: Increasing particle mass and air velocity
Q46. What is a common reason to classify particles in pharmaceutical processing?
- To change the chemical identity
- To obtain a desired size fraction for uniform dosing and process control
- To make particles magnetic
- To alter melting points
Correct Answer: To obtain a desired size fraction for uniform dosing and process control
Q47. Which factor is critical when designing a cyclone for a specific cut-size?
- Material color and taste
- Cyclone geometry, air velocity, and inlet dimensions
- Ambient humidity only
- Type of sieve cloth
Correct Answer: Cyclone geometry, air velocity, and inlet dimensions
Q48. How does cohesion between fine particles influence separation?
- Cohesion enhances individual particle settling
- Cohesion leads to agglomeration, affecting apparent size and separation
- Cohesion reduces particle density always
- Cohesion converts powders into gases
Correct Answer: Cohesion leads to agglomeration, affecting apparent size and separation
Q49. In pharmaceutical inhaler testing, which metric is most relevant to deposition in the deep lung?
- Geometric diameter only
- Mass median aerodynamic diameter (MMAD)
- Particle color distribution
- Electrical conductivity
Correct Answer: Mass median aerodynamic diameter (MMAD)
Q50. Which analytical approach can distinguish between primary particle size and agglomerate size?
- Single-method sieving always
- Combination of microscopy (SEM/TEM) and laser diffraction or dynamic light scattering
- Only odor analysis
- Simple visual inspection
Correct Answer: Combination of microscopy (SEM/TEM) and laser diffraction or dynamic light scattering
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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