Laws governing size reduction MCQs With Answer provide B.Pharm students a focused review of comminution principles critical to pharmaceutical processing. This introduction covers key concepts like Rittinger’s law, Kick’s law, Bond’s law, work index, specific energy, and common milling equipment used for tablets, suspensions and inhalation powders. Understanding how particle size reduction affects surface area, dissolution, bioavailability, heat generation and contamination helps optimize formulation and scale-up. These Student-friendly keywords — laws governing size reduction, Rittinger, Kick, Bond, work index, comminution, particle size, milling — make this resource ideal for exam prep. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which law states that the energy required for size reduction is proportional to the new surface area produced?
- Rittinger’s law — energy proportional to new surface area
- Kick’s law — energy proportional to volume change
- Bond’s law — energy proportional to inverse square root of size
- Stoke’s law — energy proportional to settling velocity
Correct Answer: Rittinger’s law — energy proportional to new surface area
Q2. Which law is most appropriate for estimating energy required in coarse crushing operations?
- Rittinger’s law — best for fine grinding
- Kick’s law — best for coarse crushing
- Bond’s law — best for ultra-fine milling
- Fick’s law — best for diffusion-controlled breakdown
Correct Answer: Kick’s law — best for coarse crushing
Q3. Bond’s law relates specific energy to particle sizes using which mathematical dependence?
- Energy ∝ (1/d2 – 1/d1)
- Energy ∝ ln(d1/d2)
- Energy ∝ (1/√d2 – 1/√d1)
- Energy ∝ (d1 – d2)
Correct Answer: Energy ∝ (1/√d2 – 1/√d1)
Q4. The Bond Work Index (Wi) is typically expressed in which units?
- kJ/kg
- kWh/ton
- J/m2
- kg/m3
Correct Answer: kWh/ton
Q5. Which law uses a logarithmic relation between energy and particle size ratio?
- Rittinger’s law
- Kick’s law
- Bond’s law
- Newton’s law
Correct Answer: Kick’s law
Q6. For very fine grinding (particle size < 100 µm), which law gives the most accurate energy prediction?
- Kick’s law
- Rittinger’s law
- Bond’s law
- Archimedes’ principle
Correct Answer: Rittinger’s law
Q7. Which milling equipment is commonly used for primary coarse size reduction of granules in pharmaceutical manufacturing?
- Fluid energy mill
- Ball mill
- Jaw crusher
- Bead mill
Correct Answer: Jaw crusher
Q8. Which mill type is most suitable for micronization to submicron sizes without introducing metal contamination?
- Hammer mill
- Fluid energy (jet) mill
- Jaw crusher
- Roller crusher
Correct Answer: Fluid energy (jet) mill
Q9. In Rittinger’s formulation, which size parameter is directly used to estimate new surface area?
- Particle volume
- Particle diameter (inverse relationship)
- Particle mass only
- Feed moisture content
Correct Answer: Particle diameter (inverse relationship)
Q10. Which factor increases the energy required for size reduction of a pharmaceutical powder?
- Lower hardness of material
- Higher feed moisture causing agglomeration
- Reduced feed size
- Use of appropriate milling media
Correct Answer: Higher feed moisture causing agglomeration
Q11. What is the primary physical quantity increased by comminution that enhances drug dissolution?
- Particle density
- Specific surface area
- Bulk volume
- Viscosity
Correct Answer: Specific surface area
Q12. Which law is often considered empirical and uses a material-specific constant (Work Index) for calculations?
- Rittinger’s law
- Kick’s law
- Bond’s law
- Pascal’s law
Correct Answer: Bond’s law
Q13. In practical pharmaceutical milling, what is the main advantage of wet milling over dry milling?
- Increases mill contamination by media wear
- Reduces heat generation and minimizes dust
- Always produces larger particles
- Is only effective for brittle materials
Correct Answer: Reduces heat generation and minimizes dust
Q14. Which measurement technique is most suitable for characterizing submicron particle size distributions?
- Sieve analysis
- Laser diffraction or dynamic light scattering
- Tap density measurement
- Bulk volume measurement
Correct Answer: Laser diffraction or dynamic light scattering
Q15. What is the expected effect of reducing particle size on powder flowability, generally speaking?
- Flowability always improves with smaller size
- Flowability may decrease due to increased cohesiveness of fine particles
- Flowability becomes independent of particle size
- Flowability improves because surface energy decreases
Correct Answer: Flowability may decrease due to increased cohesiveness of fine particles
Q16. Which milling method is most likely to generate the highest temperature rise and possible thermal degradation?
- Cryogenic milling
- High-energy ball milling without cooling
- Fluid energy milling with cold gas
- Wet bead milling with coolant
Correct Answer: High-energy ball milling without cooling
Q17. Cryogenic milling is primarily used for:
- Increasing humidity in powders
- Reducing heat-related degradation of heat-sensitive drugs
- Coarse crushing of tablets
- Drying powders prior to formulation
Correct Answer: Reducing heat-related degradation of heat-sensitive drugs
Q18. Which statement correctly describes Kick’s law scope?
- Kick’s law is best for ultra-fine grinding (<10 µm)
- Kick’s law is best for coarse size reduction and describes proportionality to the logarithm of size ratio
- Kick’s law directly relates energy to created surface area
- Kick’s law is used only for wet milling
Correct Answer: Kick’s law is best for coarse size reduction and describes proportionality to the logarithm of size ratio
Q19. What parameter is defined as the ratio of feed particle diameter to product particle diameter?
- Reduction ratio (size reduction ratio)
- Specific surface
- Work index
- Bulk density
Correct Answer: Reduction ratio (size reduction ratio)
Q20. Which of the following best explains why Rittinger’s law works well for fine grinding?
- Fine grinding creates negligible new surface area
- Energy input is primarily used to create new surface area, which increases strongly at small sizes
- Fine grinding follows linear volume reduction with energy
- Rittinger’s law ignores material hardness
Correct Answer: Energy input is primarily used to create new surface area, which increases strongly at small sizes
Q21. Which instrument is commonly used to measure specific surface area of powders?
- Mercury intrusion porosimetry
- BET nitrogen adsorption surface area analyser
- pH meter
- Optical microscope without analysis
Correct Answer: BET nitrogen adsorption surface area analyser
Q22. The critical speed of a ball mill (in rpm) depends mainly on which parameter?
- Mill length only
- Mill diameter
- Feed particle hardness only
- Ambient humidity
Correct Answer: Mill diameter
Q23. Which factor does NOT directly affect the energy efficiency of a size reduction operation?
- Material hardness and toughness
- Feed moisture content
- Ambient lighting in the milling room
- Mill design and operating speed
Correct Answer: Ambient lighting in the milling room
Q24. In a fluid energy mill, the predominant mechanism of particle breakage is:
- Compression between rollers
- Particle–particle and particle–jet collisions (impact and attrition)
- Shear from static blades only
- Dissolution followed by recrystallization
Correct Answer: Particle–particle and particle–jet collisions (impact and attrition)
Q25. Which law would most likely underestimate energy for fine grinding when compared to experimental data?
- Rittinger’s law
- Kick’s law
- Bond’s law
- Coulomb’s law
Correct Answer: Kick’s law
Q26. Which practical concern is critical when milling highly potent pharmaceutical APIs?
- Use of open mills to speed processing
- Containment to prevent operator exposure and cross-contamination
- Eliminating all cooling to save costs
- Using large media only to avoid fines
Correct Answer: Containment to prevent operator exposure and cross-contamination
Q27. What is the main reason for using dispersing agents during wet milling?
- To increase agglomeration
- To reduce particle–particle adhesion and prevent re-agglomeration
- To increase milling temperature
- To change the crystal structure permanently
Correct Answer: To reduce particle–particle adhesion and prevent re-agglomeration
Q28. The term ‘comminution’ in pharmaceutical processing refers to:
- Liquid filtration
- Size reduction of solid particles by crushing, grinding or cutting
- Chemical degradation of APIs
- Compression of powders into tablets
Correct Answer: Size reduction of solid particles by crushing, grinding or cutting
Q29. Which mill type typically uses rotating hammers to impact material and is common for granule size reduction?
- Hammer mill
- Jet mill
- Fluidized bed dryer
- Roll compactor
Correct Answer: Hammer mill
Q30. Which law is defined by E = Kk ln(d1/d2) where E is energy per unit mass?
- Rittinger’s law
- Kick’s law
- Bond’s law
- Avogadro’s law
Correct Answer: Kick’s law
Q31. In Bond’s equation, the exponent of particle diameter is:
- −1 (inverse)
- −0.5 (inverse square root)
- 0 (independent)
- +1 (direct)
Correct Answer: −0.5 (inverse square root)
Q32. What is the likely effect of increased mill speed (above optimal) in a ball mill?
- Reduced impact and therefore finer grinding
- Increased centrifugal force causing balls to stick to the wall and reduce impact
- No change in grinding efficiency regardless of speed
- Immediate conversion of material to vapour
Correct Answer: Increased centrifugal force causing balls to stick to the wall and reduce impact
Q33. Which parameter is commonly used to compare energy consumption of different mills on a per-mass basis?
- Specific energy (kJ/kg or kWh/ton)
- Bulk density (g/mL)
- pH value
- Optical rotation
Correct Answer: Specific energy (kJ/kg or kWh/ton)
Q34. Which mill is best suited for wet dispersive grinding of suspensions using grinding beads?
- Bead mill (milli or nano mill)
- Jaw crusher
- Hammer mill
- Roll crusher
Correct Answer: Bead mill (milli or nano mill)
Q35. Which of the following is a common consequence of over-milling an active pharmaceutical ingredient (API)?
- Improved chemical stability
- Changes in crystal form or amorphization and possible loss of potency
- Guaranteed better dissolution for all APIs
- Decrease in surface area
Correct Answer: Changes in crystal form or amorphization and possible loss of potency
Q36. Scale-up of milling processes is most reliably based on which parameter?
- Geometric similarity only
- Constant specific energy input per unit mass
- Keeping batch time identical regardless of mill size
- Preserving ambient room temperature exactly
Correct Answer: Constant specific energy input per unit mass
Q37. Which factor increases generation of fine explosive dust and requires special precautions?
- Large particle sizes (>1 mm)
- Production of very fine powders (<100 µm) from organic materials
- Use of wet milling exclusively
- Operating under inert atmosphere always
Correct Answer: Production of very fine powders (<100 µm) from organic materials
Q38. Which analysis method separates particles by size using a stack of sieves with decreasing mesh sizes?
- Laser diffraction
- Sieve analysis (sieving)
- DLS (dynamic light scattering)
- Thermogravimetric analysis
Correct Answer: Sieve analysis (sieving)
Q39. Which law implies energy required is proportional to increase in total surface area and thus favors surface-sensitive breakage?
- Kick’s law
- Rittinger’s law
- Bond’s law
- Henry’s law
Correct Answer: Rittinger’s law
Q40. Which characteristic of feed material typically decreases milling efficiency?
- Low moisture and friable material
- High hardness and high toughness
- Uniform small particle size
- Appropriate pre-size reduction
Correct Answer: High hardness and high toughness
Q41. What is the principal reason to control particle size distribution (PSD) in tablet formulations?
- To ensure uniformity of colour only
- To optimize flow, content uniformity and dissolution rate
- To eliminate the need for binders
- To increase tablet porosity exclusively
Correct Answer: To optimize flow, content uniformity and dissolution rate
Q42. Which law would be least appropriate for predicting energy in a transition from 1 mm to 1 µm particles?
- Rittinger’s law
- Kick’s law
- Bond’s law
- None; all are equally accurate
Correct Answer: Kick’s law
Q43. During milling, wear of milling media can lead to which pharmaceutical concern?
- Improved API solubility
- Contamination of product with metal or ceramic fragments
- Automatic reduction of microbial load
- Guaranteed narrower PSD
Correct Answer: Contamination of product with metal or ceramic fragments
Q44. Which approach reduces heat generation for heat-sensitive APIs during milling?
- Increasing mill speed without cooling
- Using cryogenic milling with liquid nitrogen
- Operating in direct sunlight
- Removing all grinding media
Correct Answer: Using cryogenic milling with liquid nitrogen
Q45. Which law suggests energy requirements scale with inverse particle size (1/d) for spherical particles when surface area is proportional to 1/d?
- Kick’s law
- Rittinger’s law
- Bond’s law
- Hagen-Poiseuille law
Correct Answer: Rittinger’s law
Q46. What is the main breakage mechanism in an attrition mill?
- Compression by two rolls
- Friction and shear between particles and surfaces
- Thermal decomposition
- Water-induced swelling
Correct Answer: Friction and shear between particles and surfaces
Q47. Which characteristic of particle size distribution is commonly reported as D50?
- Diameter at which 50% of the particles are shorter than a given length
- Median particle diameter where 50% of the mass or volume is finer
- Diameter corresponding to the smallest particle
- The largest sieve opening used in analysis
Correct Answer: Median particle diameter where 50% of the mass or volume is finer
Q48. Which statement correctly describes Bond’s work index test?
- It is purely theoretical with no laboratory test method
- It uses a standard laboratory ball mill test to determine a material-specific work index
- It measures flow properties of powders in a hopper
- It determines solubility of an API
Correct Answer: It uses a standard laboratory ball mill test to determine a material-specific work index
Q49. Which of the following will most likely decrease agglomeration during fine wet milling?
- Omitting surfactants and dispersants
- Adding suitable dispersing agents and controlling pH
- Increasing solids concentration to maximum
- Using larger grinding beads only
Correct Answer: Adding suitable dispersing agents and controlling pH
Q50. When selecting a mill for pharmaceutical API size reduction, which combined considerations are most important?
- Only the mill cost
- Target particle size, heat sensitivity, contamination risk and energy efficiency
- Colour of the mill housing and supplier logo
- Only the manufacturer’s brand name
Correct Answer: Target particle size, heat sensitivity, contamination risk and energy efficiency
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