Particle size and distribution MCQs With Answer

Particle size and distribution MCQs With Answer

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Introduction

Understanding particle size and particle size distribution is essential for B.Pharm students because these factors strongly affect drug dissolution, bioavailability, stability, filtration and formulation performance. This focused MCQ set covers core and advanced topics: definitions, D10/D50/D90, span, polydispersity index, specific surface area (BET), zeta potential, sieving, laser diffraction, dynamic light scattering, Coulter counter, sedimentation and milling methods. Questions emphasize analytical principles, interpretation of distributions (number/volume/surface), sample preparation, and practical formulation consequences. Clear explanations build exam readiness and laboratory competence in particle characterization and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the commonly used definition of particle size in pharmaceutical characterization?

  • Largest linear dimension of a particle
  • Diameter of a sphere having equivalent volume to the particle
  • Particle surface area per unit mass
  • Volume fraction of particles in a suspension

Correct Answer: Diameter of a sphere having equivalent volume to the particle

Q2. Particle size distribution (PSD) describes which characteristic of a powder or suspension?

  • The chemical composition of each particle
  • The distribution of particle sizes across defined size classes
  • The color variation among particles
  • The total mass of the sample only

Correct Answer: The distribution of particle sizes across defined size classes

Q3. What does D50 (median diameter) represent in a cumulative particle size distribution?

  • The diameter below which 10% of particles lie
  • The average diameter weighted by surface area
  • The diameter below which 50% of the sample lies
  • The largest particle diameter in the sample

Correct Answer: The diameter below which 50% of the sample lies

Q4. Which technique is most suitable for measuring nanoparticles in the submicron range (typically <1 µm)?

  • Sieve analysis
  • Dynamic light scattering (DLS)
  • Tap density testing
  • Standard optical sieving

Correct Answer: Dynamic light scattering (DLS)

Q5. Laser diffraction determines particle size primarily by measuring which physical phenomenon?

  • Electrical resistance change as particles pass an aperture
  • Intensity and angle distribution of scattered light
  • Direct imaging of particle contours
  • Gas adsorption on particle surfaces

Correct Answer: Intensity and angle distribution of scattered light

Q6. Sieving is most appropriate for particle sizing in which approximate size range?

  • Less than 0.1 µm
  • 0.1–1 µm
  • Greater than about 45 µm
  • Only for nanoparticles below 10 nm

Correct Answer: Greater than about 45 µm

Q7. Which method reports hydrodynamic diameter and is sensitive to the Brownian motion of particles?

  • BET gas adsorption
  • Dynamic light scattering (DLS)
  • Sieve analysis
  • Cascade impaction

Correct Answer: Dynamic light scattering (DLS)

Q8. What does a high polydispersity index (PDI) indicate about a sample measured by DLS?

  • The sample is highly monodisperse with uniform particle sizes
  • The sample is optically clear with no scattering
  • The sample has a broad size distribution and heterogeneity
  • The sample contains only plate-like particles

Correct Answer: The sample has a broad size distribution and heterogeneity

Q9. D90 in a particle size report means:

  • 90% of particles are larger than this diameter
  • The particle diameter at which 90% of the cumulative sample is smaller
  • The mean diameter of the smallest 90 particles
  • The diameter that represents the top 10% by number

Correct Answer: The particle diameter at which 90% of the cumulative sample is smaller

Q10. Which expression gives the span of a particle size distribution?

  • Span = (D90 − D10) / D50
  • Span = D50 − D10
  • Span = (D50 + D10) / D90
  • Span = D90 × D10 × D50

Correct Answer: Span = (D90 − D10) / D50

Q11. The Coulter counter measures particle size by which principle?

  • Optical scattering at multiple angles
  • Changes in electrical resistance as particles pass through an aperture
  • Gas adsorption profile at liquid nitrogen temperature
  • Sedimentation time in a gravitational field

Correct Answer: Changes in electrical resistance as particles pass through an aperture

Q12. BET analysis is used to measure which particle characteristic?

  • Particle size by light scattering
  • Specific surface area via gas adsorption
  • Hydrodynamic diameter in suspension
  • Particle zeta potential

Correct Answer: Specific surface area via gas adsorption

Q13. According to the Noyes–Whitney relationship, decreasing particle size generally causes what effect on dissolution rate?

  • Decreases dissolution rate
  • No change in dissolution rate
  • Increases dissolution rate due to larger surface area
  • Makes the drug insoluble

Correct Answer: Increases dissolution rate due to larger surface area

Q14. Which milling technique is commonly used to produce fine or submicron particles in pharmaceutical formulations?

  • High-energy ball milling (wet milling)
  • Cold pressing
  • Simple hand grinding with mortar and pestle only
  • Lyophilization without milling

Correct Answer: High-energy ball milling (wet milling)

Q15. Ostwald ripening in suspensions refers to:

  • Aggregation of equal-sized particles only
  • Smaller particles dissolving and re-depositing onto larger particles
  • Immediate crystallization of amorphous drug into uniform crystals
  • Mechanical breakage of particles during milling

Correct Answer: Smaller particles dissolving and re-depositing onto larger particles

Q16. Zeta potential is primarily an indicator of which property of a colloidal dispersion?

  • Thermal conductivity
  • Electrostatic stability against aggregation
  • True geometric particle diameter
  • Specific surface area measured by gas adsorption

Correct Answer: Electrostatic stability against aggregation

Q17. A PDI value below 0.1 obtained from DLS typically indicates:

  • A highly polydisperse and unstable sample
  • A very narrow, nearly monodisperse particle size distribution
  • That the measurement failed and must be repeated
  • That particles are larger than 1 mm

Correct Answer: A very narrow, nearly monodisperse particle size distribution

Q18. Which particle size distribution representation gives the greatest relative weight to the smallest particles?

  • Volume-weighted distribution
  • Number-weighted distribution
  • Mass-weighted distribution
  • Laser-intensity-weighted distribution

Correct Answer: Number-weighted distribution

Q19. Which parameter does NOT influence the dissolution rate in the Noyes–Whitney equation?

  • Surface area of the solid
  • Diffusion coefficient of the drug in the medium
  • Concentration gradient (saturation vs bulk)
  • Particle color

Correct Answer: Particle color

Q20. Which of the following is NOT a light scattering technique for particle sizing?

  • Laser diffraction
  • Dynamic light scattering (DLS)
  • Nephelometry
  • Gas adsorption (BET)

Correct Answer: Gas adsorption (BET)

Q21. A limitation of dynamic light scattering (DLS) is:

  • It directly measures true geometric dimensions for non-spherical particles
  • It is insensitive to aggregates and gives only number distributions
  • It is intensity-weighted and biased toward larger particles and aggregates
  • It provides specific surface area via gas adsorption

Correct Answer: It is intensity-weighted and biased toward larger particles and aggregates

Q22. For sizing inhalation aerosols and determining aerodynamic diameter, which instrument is typically used?

  • Sieve shaker
  • Cascade impactor
  • Coulter counter designed for dry powders only
  • BET surface area analyzer

Correct Answer: Cascade impactor

Q23. Increasing the specific surface area (SSA) of a drug powder typically results in:

  • Lower dissolution rate and reduced bioavailability
  • Higher dissolution rate and potentially improved bioavailability
  • No change to dissolution but reduced stability always
  • Conversion to a different crystal polymorph instantly

Correct Answer: Higher dissolution rate and potentially improved bioavailability

Q24. Particle shape most directly affects which bulk property of powders?

  • Color only
  • Flowability and packing behavior
  • Chemical potency
  • Melting point of the drug substance

Correct Answer: Flowability and packing behavior

Q25. Sedimentation techniques for particle sizing rely on which theoretical law to relate settling velocity to particle diameter?

  • Henry’s law
  • Stokes’ law
  • Raoult’s law
  • Beer’s law

Correct Answer: Stokes’ law

Q26. Which routine method is frequently referenced in pharmacopeial tests for coarse powder particle-size distribution?

  • Transmission electron microscopy for all powders
  • Sieve analysis (sieving)
  • Dynamic light scattering for tablets
  • BET gas adsorption for particle size

Correct Answer: Sieve analysis (sieving)

Q27. A multimodal particle size distribution indicates:

  • The sample contains particles of only one uniform size
  • The presence of two or more distinct size populations or peaks
  • That the sample is free of aggregates and impurities
  • That the distribution cannot be measured by any technique

Correct Answer: The presence of two or more distinct size populations or peaks

Q28. How does particle aggregation affect measured particle size distribution?

  • It reduces apparent particle size and narrows distribution
  • It increases apparent particle size and often broadens the distribution
  • It has no effect on any sizing technique
  • It decreases specific surface area measured by BET but not size

Correct Answer: It increases apparent particle size and often broadens the distribution

Q29. In sieve analysis, the term “percent retained” refers to:

  • The cumulative percent of particles passing through the sieve
  • The percentage of the sample mass remaining on a given sieve
  • The average particle diameter measured on that sieve
  • The time taken for sieving to be completed

Correct Answer: The percentage of the sample mass remaining on a given sieve

Q30. Why is controlling particle size important for parenteral formulations and injectables?

  • Because particle size affects color only
  • Because particle size influences stability, injectability and risk of embolism or phlebitis
  • Particle size is irrelevant for parenterals
  • Only tablet formulations require particle size control

Correct Answer: Because particle size influences stability, injectability and risk of embolism or phlebitis

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