Separation method for particle size MCQs With Answer

Separation method for particle size MCQs With Answer

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Introduction

Separation methods for particle size are essential in pharmaceutical sciences, underpinning formulation, processing, quality control, and drug delivery. B.Pharm students must understand mechanical and physical techniques—sieving, sedimentation, centrifugation, classification, filtration, air classification, cyclones, elutriation, and electrostatic separation—to control particle size distributions, blend uniformity, dissolution rate, and stability. Practical concepts include cut-point (d50), Stokes’ law, cascade sieving, classifier performance, and effect of particle shape, density, and agglomeration. Analytical methods like sieve analysis, laser diffraction, and microscopy complement separation processes. Mastery of these principles helps in granulation, milling, and powder handling during formulation development and scale-up. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which principle primarily governs gravitational sedimentation of particles in a fluid?

  • Fick’s law of diffusion
  • Stokes’ law
  • Henry’s law
  • Raoult’s law

Correct Answer: Stokes’ law

Q2. In sieve analysis, the term d50 refers to:

  • The largest particle size present
  • The diameter at which 50% of the sample by mass is finer
  • The average particle density
  • The porosity of the powder bed

Correct Answer: The diameter at which 50% of the sample by mass is finer

Q3. Which factor does NOT directly affect the terminal settling velocity of a particle in a liquid according to Stokes’ law?

  • Particle density
  • Fluid viscosity
  • Particle color
  • Particle radius

Correct Answer: Particle color

Q4. A cyclone separator primarily separates particles from a gas stream using:

  • Electrostatic attraction
  • Centrifugal forces from a vortex
  • Magnetic fields
  • Membrane filtration

Correct Answer: Centrifugal forces from a vortex

Q5. What is the main purpose of elutriation in particle separation?

  • To separate particles by magnetic susceptibility
  • To separate particles by their terminal velocity using an upward fluid stream
  • To chemically dissolve the fines
  • To compact particles into granules

Correct Answer: To separate particles by their terminal velocity using an upward fluid stream

Q6. In centrifugal separation, ‘RCF’ stands for:

  • Rotational centrifugal force
  • Relative centrifugal force
  • Radial centrifugal friction
  • Rate of centrifugal fractionation

Correct Answer: Relative centrifugal force

Q7. Which separation technique is most suitable for removing coarse particles from a powder blend prior to compression?

  • High-pressure liquid chromatography
  • Vibratory sieving
  • Dialysis
  • Freeze drying

Correct Answer: Vibratory sieving

Q8. Hindered settling occurs when:

  • Particles settle independently at Stokes’ velocity
  • Particle concentration is high so that interactions slow settling
  • Temperature falls below freezing
  • Particles are all perfectly spherical

Correct Answer: Particle concentration is high so that interactions slow settling

Q9. Which property of particles most strongly affects their behavior in an air classifier?

  • Electrical conductivity
  • Particle density and aerodynamic diameter
  • Color and odor
  • Magnetic susceptibility

Correct Answer: Particle density and aerodynamic diameter

Q10. Wet sieving is preferred over dry sieving when:

  • Particles are hydrophobic and repel water
  • Fines form electrostatic aggregates and cake during dry sieving
  • The sample is completely insoluble in water
  • Particle size is larger than 1 mm only

Correct Answer: Fines form electrostatic aggregates and cake during dry sieving

Q11. Cascade sieving helps to:

  • Remove moisture from hygroscopic powders
  • Obtain a reproducible particle size distribution by sequentially using multiple sieves
  • Increase particle density
  • Sterilize powder samples

Correct Answer: Obtain a reproducible particle size distribution by sequentially using multiple sieves

Q12. Which dimensionless number helps predict whether flow around a particle is laminar or turbulent during settling?

  • Reynolds number
  • Péclet number
  • Prandtl number
  • Biot number

Correct Answer: Reynolds number

Q13. In a particle classifier, the ‘cut-point’ refers to:

  • The maximum feed rate allowable
  • The particle size at which 50% is separated to each fraction
  • The temperature at which classification occurs
  • The electrical charge threshold for separation

Correct Answer: The particle size at which 50% is separated to each fraction

Q14. Electrostatic separation is most effective for:

  • Separating metallic particles by magnetism
  • Separating particles with different electrical charging behaviors
  • Separating particles solely by density in water
  • Sterilizing particles using charge

Correct Answer: Separating particles with different electrical charging behaviors

Q15. Which statement about laser diffraction as a complementary technique to physical separation is correct?

  • It physically removes fines from a sample
  • It provides particle size distribution rapidly based on scattering patterns
  • It only measures particle density
  • It is unreliable for spherical particles

Correct Answer: It provides particle size distribution rapidly based on scattering patterns

Q16. In centrifugal sedimentation, increasing rotor radius at constant RPM will:

  • Decrease relative centrifugal force experienced by particles
  • Increase relative centrifugal force and accelerate sedimentation
  • Have no effect on separation
  • Invert particle size distribution

Correct Answer: Increase relative centrifugal force and accelerate sedimentation

Q17. Particle agglomeration affects separation efficiency by:

  • Reducing effective particle size and potentially behaving as larger units
  • Always making separation easier
  • Changing chemical composition only
  • Eliminating density differences

Correct Answer: Reducing effective particle size and potentially behaving as larger units

Q18. A classifier that relies on differential settling in a liquid medium is known as:

  • Electrostatic precipitator
  • Hydrocyclone or elutriator depending on flow
  • Membrane ultrafilter
  • Gas chromatograph

Correct Answer: Hydrocyclone or elutriator depending on flow

Q19. Which of the following improves sieving efficiency for fine powders?

  • Decreasing amplitude and frequency of vibration
  • Using a sieve with smaller mesh openings appropriate to target size and applying sonication or tapping
  • Using a completely static sieve and long time only
  • Heating the sieve above 200°C

Correct Answer: Using a sieve with smaller mesh openings appropriate to target size and applying sonication or tapping

Q20. In an industrial cyclone, where do the heaviest particles typically collect?

  • In the vortex finder
  • At the outer wall and fall to the collection hopper
  • They remain suspended indefinitely
  • They exit through the gas outlet

Correct Answer: At the outer wall and fall to the collection hopper

Q21. Which measurement describes the spread of particle sizes in a sample?

  • Polydispersity index or span of distribution
  • pH of powder slurry
  • Melting point range
  • Dielectric constant

Correct Answer: Polydispersity index or span of distribution

Q22. Why is particle shape (sphericity) important in separation by air classification?

  • Shape does not affect aerodynamic behavior
  • Less spherical particles have different drag and fall rates, changing cut-points
  • Only density matters in air classifiers
  • Shape only affects color

Correct Answer: Less spherical particles have different drag and fall rates, changing cut-points

Q23. Which separation method is most suitable to separate sub-micron particles from suspension at laboratory scale?

  • Gravitational settling in a beaker
  • Ultracentrifugation
  • Standard mesh sieving
  • Manual decantation only

Correct Answer: Ultracentrifugation

Q24. In a liquid-solid cyclone (hydrocyclone), particle separation is influenced mainly by:

  • Electrical conductivity of the liquid
  • Relative density difference and centrifugal forces created by swirling flow
  • Surface tension alone
  • Ambient light intensity

Correct Answer: Relative density difference and centrifugal forces created by swirling flow

Q25. What is the primary limitation of using sieves for very fine pharmaceutical powders (<20 μm)?

  • Sieve mesh numbering becomes impractical and blinding/agglomeration occurs
  • Sieves dissolve in the powder
  • Sieves chemically react with API
  • Sieving generates radioactivity

Correct Answer: Sieve mesh numbering becomes impractical and blinding/agglomeration occurs

Q26. Which technique can separate particles based on both size and density simultaneously?

  • Size-exclusion chromatography
  • Density-gradient centrifugation
  • Simple mesh sieving
  • Optical microscopy only

Correct Answer: Density-gradient centrifugation

Q27. The term ‘cascade impactor’ is associated with which application in pharmaceutics?

  • Measuring aerosol particle size distribution for inhalation products
  • Separating tablets by hardness
  • Drying powders by impact
  • Measuring tablet disintegration time

Correct Answer: Measuring aerosol particle size distribution for inhalation products

Q28. Which of the following improves separation in a hydrocyclone?

  • Increasing feed solids concentration to very high levels always
  • Optimizing feed velocity and vortex finder dimensions
  • Removing all centrifugal forces
  • Adding strong magnets to the hydrocyclone shell

Correct Answer: Optimizing feed velocity and vortex finder dimensions

Q29. For fragile particles that should not be damaged, the preferred size-separation method is:

  • High-energy milling followed by sieving
  • Gentle air classification or low-shear sedimentation
  • Aggressive ultrasonic dispersion
  • Exposing to high temperatures during sieving

Correct Answer: Gentle air classification or low-shear sedimentation

Q30. Which practice helps ensure reproducible sieve analysis results?

  • Using inconsistent sieving times and random tap intensity
  • Standardizing sieve stack, sieve time, amplitude, and sample pretreatment
  • Only using visual estimation of retained fractions
  • Ignoring calibration and maintenance of sieves

Correct Answer: Standardizing sieve stack, sieve time, amplitude, and sample pretreatment

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