Introduction: Mechanisms of size reduction are essential in pharmaceutical engineering for preparing powders with controlled particle size, crucial for drug dissolution, bioavailability, and formulation stability. This concise guide focuses on comminution principles — impact, compression, attrition, cutting and abrasion — and common equipment like ball mills, hammer mills, jet mills and crushers. It covers important concepts such as particle size distribution, sieve analysis, work index, energy efficiency, and wet vs dry grinding. Tailored for B.Pharm students, these MCQs reinforce theory and practical considerations in milling, screening, and scale-up. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary purpose of size reduction in pharmaceutical processing?
- To increase the melting point of a drug
- To improve solubility, dissolution rate and uniformity in formulations
- To change the chemical structure of the active ingredient
- To sterilize the powder
Correct Answer: To improve solubility, dissolution rate and uniformity in formulations
Q2. Which mechanism of size reduction involves particles being crushed between two surfaces?
- Impact
- Compression
- Cutting
- Attrition
Correct Answer: Compression
Q3. Which law relates the work required for size reduction to the change in particle size and is most accurate for coarse crushing?
- Rittinger’s law
- Kick’s law
- Bond’s law
- Haber’s law
Correct Answer: Kick’s law
Q4. Rittinger’s law assumes energy required is proportional to:
- Mass of material only
- New surface area created
- Change in volume
- Temperature rise
Correct Answer: New surface area created
Q5. Bond’s law is most applicable for which size reduction range?
- Very fine powders (submicron)
- Coarse crushing (>50 mm)
- Intermediate grinding
- Drying processes
Correct Answer: Intermediate grinding
Q6. Which apparatus uses high-speed rotating hammers to pulverize material?
- Ball mill
- Hammer mill
- Jet mill
- Roll crusher
Correct Answer: Hammer mill
Q7. Which mechanism predominates in a ball mill?
- Cutting
- Impact and attrition
- Compression between rolls
- Fluid shear only
Correct Answer: Impact and attrition
Q8. A fluid energy (jet) mill primarily uses which force for comminution?
- Mechanical compression
- High-velocity gas impact and collision
- Static pressure
- Thermal degradation
Correct Answer: High-velocity gas impact and collision
Q9. Which parameter describes the diameter below which 50% of particles by mass lie?
- D10
- D50
- D90
- Span
Correct Answer: D50
Q10. Sieve analysis is most appropriate for measuring particle sizes in which range?
- Submicron to 1 µm
- 1 µm to 10 µm
- Approximately 38 µm to several centimeters
- Only nanoparticle range
Correct Answer: Approximately 38 µm to several centimeters
Q11. Which crusher is typically used for primary size reduction of hard, brittle materials?
- Jaw crusher
- Colloid mill
- Fluidized bed mill
- Roller compactor
Correct Answer: Jaw crusher
Q12. Attrition as a mechanism involves:
- Particles hitting a solid surface at high velocity
- Particles rubbing and abrading each other
- Particles being sliced by blades
- Chemical degradation
Correct Answer: Particles rubbing and abrading each other
Q13. Which factor does NOT significantly affect grinding efficiency?
- Feed particle size distribution
- Mill temperature
- Gravitational constant
- Grinding media size
Correct Answer: Gravitational constant
Q14. Specific surface area increases when particle size:
- Increases
- Decreases
- Remains constant
- Changes to spherical shape only
Correct Answer: Decreases
Q15. In which milling operation is wet grinding preferred for heat-sensitive drugs?
- Hammer milling
- Colloid or bead milling with liquid medium
- Jaw crushing
- Dry jet milling
Correct Answer: Colloid or bead milling with liquid medium
Q16. Kick’s law is best suited when the size reduction involves:
- Large size changes (coarse crushing)
- Making nanoparticles
- Breaking chemical bonds
- Only wet grinding
Correct Answer: Large size changes (coarse crushing)
Q17. The Bond Work Index is a measure of:
- Thermal stability
- Energy required to reduce material to a given size
- Chemical reactivity
- Moisture content
Correct Answer: Energy required to reduce material to a given size
Q18. Critical speed of a ball mill refers to:
- The speed at which balls centrifuge and stick to wall
- The speed at which material melts
- The speed where no grinding occurs due to no abrasion
- The maximum recommended rpm for safety
Correct Answer: The speed at which balls centrifuge and stick to wall
Q19. Which mill is suitable for producing submicron particles using high shear between rotor and stator?
- Hammer mill
- Colloid mill
- Jaw crusher
- Roller crusher
Correct Answer: Colloid mill
Q20. Size reduction ratio is defined as:
- Final particle size / Initial particle size
- Initial particle size / Final particle size
- Volume change during milling
- Energy consumed per unit time
Correct Answer: Initial particle size / Final particle size
Q21. Which mechanism is most important in a cutting mill?
- Impact
- Compression
- Shear and cutting
- Fluidization
Correct Answer: Shear and cutting
Q22. Which phenomenon can lead to agglomeration during fine grinding?
- Electrostatic attraction and van der Waals forces
- Increase in crystal lattice energy
- Reduced surface area
- Complete solvation
Correct Answer: Electrostatic attraction and van der Waals forces
Q23. In wet milling, what is one common advantage over dry milling?
- Higher dust generation
- Better heat dissipation and reduced agglomeration
- Higher risk of oxidation
- Less control over particle size distribution
Correct Answer: Better heat dissipation and reduced agglomeration
Q24. Which factor affects the selection of grinding media in a ball mill?
- Color of the media
- Density, hardness and potential contamination
- Electrical conductivity only
- Ambient humidity only
Correct Answer: Density, hardness and potential contamination
Q25. What does PSD stand for in particle technology?
- Particle specific density
- Particle size distribution
- Process speed data
- Physical solubility descriptor
Correct Answer: Particle size distribution
Q26. Which milling method is typically used for heat-sensitive and coagulation-prone materials because it avoids high temperatures?
- Dry hammer milling
- Cryogenic milling using liquid nitrogen
- Jet milling with hot gas
- Roller compaction
Correct Answer: Cryogenic milling using liquid nitrogen
Q27. The term comminution refers to:
- Mixing two liquids
- Size reduction of solids
- Measuring viscosity
- Filtering suspensions
Correct Answer: Size reduction of solids
Q28. Which instrument measures particle size by laser light scattering?
- Sieve shaker
- Laser diffraction analyzer
- Viscometer
- Micrometer screw gauge
Correct Answer: Laser diffraction analyzer
Q29. Which of the following is a disadvantage of excessive size reduction?
- Improved dissolution
- Increased stability
- Possible increase in chemical reactivity or degradation
- Reduced surface area
Correct Answer: Possible increase in chemical reactivity or degradation
Q30. In a roller crusher, the predominant mechanism is:
- Impact from rotating hammers
- Compression between rotating rolls
- High-velocity gas collisions
- Shear by blades
Correct Answer: Compression between rotating rolls
Q31. Which parameter indicates the fineness achieved by milling expressed as the percentage retained on a sieve?
- Bulk density
- Sieve residue or retention
- pH value
- Thermal conductivity
Correct Answer: Sieve residue or retention
Q32. Which is true for a jet mill compared to a ball mill for fine particle production?
- Jet mill is suitable for heat-sensitive materials and produces narrower PSD
- Ball mill produces finer particles than jet mill always
- Jet mill cannot produce submicron particles
- Ball mill has no wear from grinding media
Correct Answer: Jet mill is suitable for heat-sensitive materials and produces narrower PSD
Q33. What does D90 mean in particle size terminology?
- 90% of particles are larger than this size
- 90% of particles are smaller than this size
- The average particle density at 90% humidity
- Diameter of the largest particle
Correct Answer: 90% of particles are smaller than this size
Q34. Which of the following reduces contamination from metal wear in milling?
- Using harder metallic media
- Using ceramic or polytetrafluoroethylene-lined mills
- Increasing mill speed drastically
- Adding more abrasive media
Correct Answer: Using ceramic or polytetrafluoroethylene-lined mills
Q35. Which test evaluates the tendency of powders to segregate after size reduction?
- Segregation index or angle of repose and sifting tests
- pH test
- UV spectroscopy
- Melting point determination
Correct Answer: Segregation index or angle of repose and sifting tests
Q36. Which energy term accounts for creating new surface area during milling?
- Surface energy
- Magnetic energy
- Chemical bond energy exclusively
- Thermal energy only
Correct Answer: Surface energy
Q37. In wet bead milling, the role of beads is to:
- Act as catalysts
- Provide mechanical impact and shear to break particles
- Absorb moisture from slurry
- Coagulate particles
Correct Answer: Provide mechanical impact and shear to break particles
Q38. Which property often increases as particle size decreases, affecting dissolution?
- Bulk porosity decreases
- Specific surface area increases
- Melting point increases drastically
- Crystal structure becomes identical to bulk
Correct Answer: Specific surface area increases
Q39. What is one reason to perform granulometry after milling?
- To measure electrical conductivity
- To characterize particle size distribution and ensure specification compliance
- To sterilize the product
- To change chemical potency
Correct Answer: To characterize particle size distribution and ensure specification compliance
Q40. Which instrument measures very fine particles by sedimentation rate?
- Analytical balance
- Sedimentation analyzer (e.g., Coulter or disk centrifuge)
- Sieve shaker
- pH meter
Correct Answer: Sedimentation analyzer (e.g., Coulter or disk centrifuge)
Q41. Which statement about wet vs dry milling is correct?
- Dry milling always gives narrower PSD than wet milling
- Wet milling can reduce dust and heat buildup but requires drying post-process
- Wet milling never causes contamination
- Dry milling eliminates the need for any downstream processing
Correct Answer: Wet milling can reduce dust and heat buildup but requires drying post-process
Q42. What is the main drawback of jet milling?
- Low capital cost
- High energy consumption for very fine particles and possible low throughput
- No control over particle size
- Inability to mill heat-sensitive materials
Correct Answer: High energy consumption for very fine particles and possible low throughput
Q43. Which parameter describes particle shape influence on flow and packing?
- Aspect ratio and roundness
- Boiling point
- Dielectric constant
- pKa value
Correct Answer: Aspect ratio and roundness
Q44. Screening after milling is performed to:
- Cool down the material
- Separate particles by size and recycle oversize for regrinding
- Change chemical composition
- Measure humidity
Correct Answer: Separate particles by size and recycle oversize for regrinding
Q45. Which milling technique is preferred for producing nanosuspensions of poorly soluble drugs?
- Roller compaction
- High-pressure homogenization or wet bead milling
- Sieving
- Jaw crushing
Correct Answer: High-pressure homogenization or wet bead milling
Q46. What effect does moisture content often have on milling?
- Always improves milling efficiency
- Can cause agglomeration and clogging, reducing efficiency
- Has no effect
- Sterilizes the material
Correct Answer: Can cause agglomeration and clogging, reducing efficiency
Q47. Which of the following is a measure of milling efficiency?
- Energy consumed per unit mass reduced
- Food grade certification
- Color of ground powder
- Smell intensity
Correct Answer: Energy consumed per unit mass reduced
Q48. What role does temperature control play during milling?
- No role at all
- Prevents thermal degradation and controls viscosity in wet milling
- Only affects color, not particle size
- Guarantees elimination of impurities
Correct Answer: Prevents thermal degradation and controls viscosity in wet milling
Q49. Which equipment is best for shearing and producing narrow particle-size distribution in suspensions?
- Ball mill without media
- High-shear homogenizer or rotor-stator mill
- Jaw crusher
- Simple mortar and pestle
Correct Answer: High-shear homogenizer or rotor-stator mill
Q50. For scale-up of milling processes, which parameter is critical to maintain product similarity?
- Feeding color
- Specific energy per unit mass and geometric similarity of the mill
- Operator height
- Packaging material
Correct Answer: Specific energy per unit mass and geometric similarity of the mill
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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