Particle size, shape and surface area analysis MCQs With Answer

Introduction

This quiz collection on Particle size, shape and surface area analysis is tailored for M.Pharm students preparing for Product Development and Technology Transfer examinations. It covers core principles, measurement techniques, interpretation of particle size distributions, surface area determination (including BET), porosity, and shape descriptors relevant to pharmaceutical solids. Questions emphasize practical aspects such as sample preparation, limitations of methods (laser diffraction, DLS, microscopy, mercury porosimetry), and implications for formulation performance (dissolution, flow, stability). Use these MCQs to test conceptual understanding, analytical choices, and critical thinking needed when selecting characterization methods for drug product development and scale-up.

Q1. Which definition most precisely describes “particle size” commonly used in pharmaceutical characterization?

• Length of the longest axis of an irregular particle
• Equivalent spherical diameter of a particle having the same physical property (e.g., volume)
• Mean projected area diameter measured by 2D microscopy
• Estimated size based on powder bulk density

Correct Answer: Equivalent spherical diameter of a particle having the same physical property (e.g., volume)

Q2. What does D50 (or median diameter) represent in a particle size distribution?

• The largest particle diameter in the sample
• The diameter at which 50% of the particles by number are larger
• The diameter at which 50% of the sample mass/volume is composed of particles smaller than this value
• The diameter corresponding to the mode of the distribution

Correct Answer: The diameter at which 50% of the sample mass/volume is composed of particles smaller than this value

Q3. Laser diffraction particle sizing primarily provides which type of size distribution?

• Number-based size distribution measured by individual particle counting
• Volume/area-based size distribution derived from light scattering intensity
• Surface area distribution directly equivalent to BET measurement
• Mass-based distribution measured by sedimentation rates

Correct Answer: Volume/area-based size distribution derived from light scattering intensity

Q4. Dynamic light scattering (DLS) reports which characteristic diameter for particles in suspension?

• Projected geometric diameter measured by microscopy
• Aerodynamic diameter used for inhalation products
• Hydrodynamic diameter that includes solvation layer and Brownian motion effects
• Volume equivalent diameter assuming rigid spheres

Correct Answer: Hydrodynamic diameter that includes solvation layer and Brownian motion effects

Q5. The BET method for specific surface area determination is based on which principle?

• Single-layer chemisorption of oxygen on metal surfaces
• Multilayer physical adsorption of an inert gas (e.g., nitrogen) and application of the BET equation
• Measurement of particle size by electron microscopy and geometrical calculation
• Mercury intrusion into porous networks under high pressure

Correct Answer: Multilayer physical adsorption of an inert gas (e.g., nitrogen) and application of the BET equation

Q6. Which technique is most appropriate for quantifying micropore surface area and pore size in fine powders?

• Sieving analysis
• Mercury intrusion porosimetry for pores <2 nm
• Gas adsorption (BET and BJH analysis) for micropores and mesopores
• Laser diffraction for internal pore structure

Correct Answer: Gas adsorption (BET and BJH analysis) for micropores and mesopores

Q7. Mercury intrusion porosimetry measures pore size distribution by:

• Adsorbing nitrogen and applying the BET theory
• Forcing a non-wetting fluid (mercury) into pores under applied pressure and using Washburn equation
• Measuring mass loss upon heating
• Imaging pores with scanning electron microscopy and calculating diameters

Correct Answer: Forcing a non-wetting fluid (mercury) into pores under applied pressure and using Washburn equation

Q8. Zeta potential measurement is primarily used to assess which property of colloidal suspensions?

• Average particle diameter by number
• Surface charge and electrostatic stability against aggregation
• Specific surface area in m2/g
• Porosity and pore volume

Correct Answer: Surface charge and electrostatic stability against aggregation

Q9. Stokes’ law is applicable for calculating sedimentation velocity under which conditions?

• High Reynolds number turbulent flow and irregular particles
• Laminar flow around spherical particles where inertia is negligible
• Particles in a porous matrix during mercury intrusion
• Gas-phase aerosols dominated by diffusive deposition

Correct Answer: Laminar flow around spherical particles where inertia is negligible

Q10. The aerodynamic diameter of a particle is defined as:

• The physical diameter measured by microscopy irrespective of density
• The diameter of a unit-density sphere that settles at the same velocity as the particle
• The hydrodynamic diameter measured by DLS in liquid media
• The maximum Feret diameter obtained from image analysis

Correct Answer: The diameter of a unit-density sphere that settles at the same velocity as the particle

Q11. In DLS, the polydispersity index (PDI) indicates:

• The absolute mean particle size in nanometers
• The breadth or heterogeneity of the particle size distribution
• The zeta potential stability threshold
• The specific surface area derived from diffusion coefficients

Correct Answer: The breadth or heterogeneity of the particle size distribution

Q12. How does decreasing mean particle size (for roughly spherical particles) affect specific surface area?

• Specific surface area decreases as particle size decreases
• Specific surface area increases as particle size decreases
• Specific surface area remains constant regardless of size
• Specific surface area is only controlled by porosity and not by particle size

Correct Answer: Specific surface area increases as particle size decreases

Q13. Which shape descriptor represents the ratio of particle length to width as determined by image analysis?

• Circularity
• Aspect ratio
• Roundness factor
• Fractal dimension

Correct Answer: Aspect ratio

Q14. Circularity is a shape metric calculated as 4π·Area/Perimeter²; what value corresponds to a perfect circle?

• 0
• 0.5
• 1
• π

Correct Answer: 1

Q15. In particle science, how do “aggregates” differ from “agglomerates”?

• Aggregates are weakly bound and reversible; agglomerates are strongly bound and irreversible
• Agglomerates are weakly bound clusters usually reversible; aggregates are strongly bonded primary particles often irreversible
• Both terms are interchangeable with no practical difference
• Agglomerates refer only to dry powders and aggregates only to suspensions

Correct Answer: Agglomerates are weakly bound clusters usually reversible; aggregates are strongly bonded primary particles often irreversible

Q16. Which method is most suitable for routine sizing of coarse pharmaceutical powders (>50 µm)?

• Dynamic light scattering (DLS)
• Sieving (sieve analysis)
• BET nitrogen adsorption
• Small-angle X-ray scattering (SAXS)

Correct Answer: Sieving (sieve analysis)

Q17. A major limitation of laser diffraction for non-spherical particles is:

• It directly measures chemical composition instead of size
• It assumes spherical particles, so shape anisotropy can bias size interpretation
• It cannot measure particles larger than 1 µm
• It provides number-based distributions only

Correct Answer: It assumes spherical particles, so shape anisotropy can bias size interpretation

Q18. Why is BET surface area often preferred over geometric calculations for porous pharmaceutical powders?

• BET ignores internal porosity and measures only external surface
• BET directly measures adsorbent-accessible surface area including internal pores
• Geometric calculations are always more accurate than BET
• BET cannot detect surface heterogeneity or micropores

Correct Answer: BET directly measures adsorbent-accessible surface area including internal pores

Q19. How does particle shape affect bulk powder flowability?

• More spherical particles generally improve flowability compared to elongated or irregular shapes
• Irregular, plate-like particles always flow better than spheres
• Shape has no impact; only particle size matters
• Higher aspect ratio particles improve flow by reducing interparticle friction

Correct Answer: More spherical particles generally improve flowability compared to elongated or irregular shapes

Q20. Proper sample dispersion prior to laser diffraction or DLS measurement is necessary to:

• Ensure formation of larger agglomerates to reflect real powder behavior
• Prevent multiple scattering, reduce agglomeration, and provide representative particle size data
• Remove surface moisture permanently by chemical reaction
• Convert all particles to perfect spheres for accurate reading

Correct Answer: Prevent multiple scattering, reduce agglomeration, and provide representative particle size data

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