Mechanism of solid mixing MCQs With Answer

Mechanism of solid mixing MCQs With Answer is a focused guide for B. Pharm students covering fundamental and advanced concepts of powder blending, mixing mechanisms, segregation, and equipment selection. This introduction explores convective, diffusive, and shear mixing, factors affecting blend uniformity such as particle size, density, cohesion, humidity, and operational parameters for common mixers like V-blenders, ribbon blenders, and high-shear granulators. The content emphasizes practical troubleshooting, sampling, evaluation of homogeneity (coefficient of variation), and scale-up considerations, providing clear, exam-oriented practice. Ideal for preparing for university exams and practicals, these targeted MCQs reinforce core principles in pharmaceutical powder technology. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary objective of solid mixing in pharmaceutical formulations?

• To produce a chemical reaction between powders
• To achieve a uniform distribution of components throughout the batch
• To change the particle size distribution
• To sterilize the powder blend

Correct Answer: To achieve a uniform distribution of components throughout the batch

Q2. Which three fundamental mechanisms describe solid mixing?

• Convection, diffusion, and shear
• Evaporation, condensation, and crystallization
• Osmosis, filtration, and adsorption
• Oxidation, reduction, and polymerization

Correct Answer: Convection, diffusion, and shear

Q3. What is percolation (sifting) segregation?

• The rise of large particles to the top during vibration
• The downward movement of smaller particles through voids between larger particles
• The mixing due to random Brownian motion
• The charging of particles causing adhesion to surfaces

Correct Answer: The downward movement of smaller particles through voids between larger particles

Q4. Which mechanism is dominant in tumbling blenders like V‑blenders?

• Electrostatic attraction
• Convective bulk movement of particle masses
• Chemical bonding of particles
• Magnetic alignment of particles

Correct Answer: Convective bulk movement of particle masses

Q5. Which metric is commonly used to quantify blend uniformity?

• Bulk density
• Angle of repose
• Coefficient of variation (CV) of assay results
• pH of the blend

Correct Answer: Coefficient of variation (CV) of assay results

Q6. Which mixer typically imparts higher shear suitable for more intensive dry mixing?

• V‑blender
• Ribbon blender
• Static mixer
• Gentle tumbling drum

Correct Answer: Ribbon blender

Q7. Which factor most commonly causes percolation segregation?

• Differences in particle color
• Significant differences in particle size
• Identical particle densities
• Uniform particle shape

Correct Answer: Significant differences in particle size

Q8. Increasing particle cohesion in a powder blend typically has what effect on mixing time?

• Decreases mixing time
• Has no effect on mixing time
• Increases mixing time
• Immediately causes chemical reactions

Correct Answer: Increases mixing time

Q9. Which mixer is preferred for heat-sensitive, friable powders due to gentle action?

• High-shear granulator
• V‑blender (tumble blender)
• Fluidized bed with high temperature
• Hammer mill

Correct Answer: V‑blender (tumble blender)

Q10. What are “dead zones” in a blender?

• Regions where material is overworked and overheated
• Areas within the blender where little or no mixing occurs
• Points where electrical grounding is effective
• Zones used for sampling only

Correct Answer: Areas within the blender where little or no mixing occurs

Q11. How can electrostatic segregation be mitigated during solid mixing?

• By reducing humidity to very low levels
• By adding antistatic agents, grounding equipment, or increasing humidity
• By increasing particle fragility
• By painting the blender walls

Correct Answer: By adding antistatic agents, grounding equipment, or increasing humidity

Q12. What is the standard laboratory approach to evaluate blend uniformity?

• Single-point visual inspection
• Assaying multiple spatially distributed samples and calculating CV
• Measuring only bulk density
• Recording blender motor current

Correct Answer: Assaying multiple spatially distributed samples and calculating CV

Q13. Why is mixing order important when adding minor potent APIs?

• It changes the chemical structure of the API
• To minimize segregation and ensure uniform distribution by geometric dilution
• It allows for microbial growth
• It increases the melting point of excipients

Correct Answer: To minimize segregation and ensure uniform distribution by geometric dilution

Q14. What is the effect of a large difference in particle size between components?

• Improved homogeneity without intervention
• Increased risk of segregation and poor content uniformity
• Immediate chemical bonding
• Elimination of electrostatic charges

Correct Answer: Increased risk of segregation and poor content uniformity

Q15. Which theoretical model often describes diffusive mixing in powders?

• Fick’s diffusion model
• Newton’s cooling law
• Raoult’s law
• Avogadro’s hypothesis

Correct Answer: Fick’s diffusion model

Q16. Residence time distribution (RTD) analysis is most useful for which type of mixing?

• Batch tumbling only
• Continuous mixing processes
• Only for liquid mixing
• Static layered storage

Correct Answer: Continuous mixing processes

Q17. Which mixer is known to promote strong axial flow and good axial mixing?

• Pan pelletizer
• Ribbon blender
• Static mixer for liquids
• Mortar and pestle

Correct Answer: Ribbon blender

Q18. What is a notable downside of intense shear mixing for fragile particles?

• Improves flow without change
• Causes particle attrition and generation of fines
• Eliminates segregation completely
• Reduces electrostatic charge permanently

Correct Answer: Causes particle attrition and generation of fines

Q19. Typical optimal fill level for many tumbling blenders to ensure effective mixing is approximately:

• 5–10% of blender volume
• 40–60% of blender volume
• 100% (completely full)
• Overfilled to 120%

Correct Answer: 40–60% of blender volume

Q20. Which factors strongly influence power consumption during dry powder mixing?

• Ambient lighting and sound
• Particle size, cohesion, and fill level
• Color of powder
• Brand of the blender paint

Correct Answer: Particle size, cohesion, and fill level

Q21. What adverse effect can overmixing cause in powder blends?

• Complete chemical stability enhancement
• Attrition, generation of fines, and potential segregation changes
• Instant sterilization
• Reduction of particle density to zero

Correct Answer: Attrition, generation of fines, and potential segregation changes

Q22. The mixing index often used by formulators is based on which statistical measure?

• Median particle diameter only
• Coefficient of variation (standard deviation relative to mean)
• pKa of the drug molecule
• Absolute humidity of the lab

Correct Answer: Coefficient of variation (standard deviation relative to mean)

Q23. Which mechanism best describes particle exchange across adjacent layers during mixing?

• Convective bulk flow only
• Diffusive (random) particle exchange
• Magnetic realignment
• Electrolytic conduction

Correct Answer: Diffusive (random) particle exchange

Q24. For highly cohesive powders, which excipient addition commonly improves flow and mixing?

• High concentration of lubricant like magnesium stearate at 10% w/w
• Small amounts of glidants such as colloidal silicon dioxide
• Large crystals of sugar
• Colorants and perfumes

Correct Answer: Small amounts of glidants such as colloidal silicon dioxide

Q25. Which test is commonly used to assess powder flowability relevant to mixing behavior?

• Melting point determination
• Angle of repose measurement
• pH titration
• UV spectral scan

Correct Answer: Angle of repose measurement

Q26. The “Brazil nut effect” refers to which segregation behavior?

• Fines consolidating at the bottom uniformly
• Larger particles tending to rise to the top during vibration
• Magnetically aligned particles clustering
• Complete dissolution of small particles

Correct Answer: Larger particles tending to rise to the top during vibration

Q27. Which of the following is an example of a continuous blender used in industry?

• Nauta (conical) mixer operated in batch mode only
• Continuous paddle or screw-type ribbon blender
• Mortar and pestle
• Static liquid mixer only

Correct Answer: Continuous paddle or screw-type ribbon blender

Q28. In mixing terminology, what is a “shear plane”?

• A region where relative motion produces shear forces between particle layers
• A plane used to cut tablets
• The location of electrical grounding on the mixer
• A decorative marking on the blender

Correct Answer: A region where relative motion produces shear forces between particle layers

Q29. Which approach reduces segregation tendency in a powder blend?

• Increasing size disparities between ingredients
• Increasing cohesion via slight granulation or use of binders
• Decreasing humidity to absolute zero
• Leaving powders in a static layered configuration

Correct Answer: Increasing cohesion via slight granulation or use of binders

Q30. A practical scale-up rule for tumbling blenders often attempts to maintain which parameter?

• Identical blender paint color
• Comparable tip speed or dynamic similarity (e.g., Froude number) to keep similar flow regimes
• Same absolute blender volume regardless of batch
• Same ambient room temperature only

Correct Answer: Comparable tip speed or dynamic similarity (e.g., Froude number) to keep similar flow regimes

Q31. Which instrument is commonly used to measure particle size distribution for powders prior to mixing?

• Thermogravimetric analyzer
• Laser diffraction particle size analyzer
• pH meter
• Bomb calorimeter

Correct Answer: Laser diffraction particle size analyzer

Q32. How is diffusive mixing of powders conceptually similar to liquid diffusion?

• Both are governed by laminar flow only
• Both involve random movement down concentration gradients leading to homogenization
• Both require solvents to occur
• Both are driven solely by gravity

Correct Answer: Both involve random movement down concentration gradients leading to homogenization

Q33. What is elutriation in the context of powder handling?

• The process of particle dissolution in solvent
• The entrainment and removal of fine particles by air flow
• The chemical degradation of APIs on storage
• The deliberate coating of particles with polymers

Correct Answer: The entrainment and removal of fine particles by air flow

Q34. Which mixer is most suitable for very friable materials to minimize breakage?

• High-shear wet granulator
• V‑blender (tumble blender)
• Hammer mill
• Rotor-stator homogenizer

Correct Answer: V‑blender (tumble blender)

Q35. How is the endpoint of mixing commonly identified experimentally?

• When motor current drops to zero
• When coefficient of variation (CV) reaches a steady plateau with further mixing
• When visual color fades completely
• When temperature exceeds 100°C

Correct Answer: When coefficient of variation (CV) reaches a steady plateau with further mixing

Q36. Increasing humidity can reduce which mixing-related problem?

• Chemical incompatibility
• Electrostatic charging and some segregation by fines
• Particle compaction beyond use
• Melting of crystalline APIs

Correct Answer: Electrostatic charging and some segregation by fines

Q37. What property is measured by the angle of repose?

• Thermal conductivity
• Powder flowability
• Chemical reactivity
• Color uniformity

Correct Answer: Powder flowability

Q38. In the diffusion model for powder mixing, the mixing rate is proportional to what?

• The concentration gradient between regions
• The color contrast of components
• The ambient light intensity
• The taste of the powder

Correct Answer: The concentration gradient between regions

Q39. Excessive use of lubricants like magnesium stearate during mixing can cause what effect?

• Improved dissolution for all drugs
• Coating of particles leading to reduced wettability and poor compaction
• Complete elimination of segregation
• Increase in API potency

Correct Answer: Coating of particles leading to reduced wettability and poor compaction

Q40. What is the recommended technique for uniformly incorporating a very low-dose API into a bulk excipient?

• Directly adding API in one step at high blender speed
• Using geometric dilution by stepwise blending with portions of excipient
• Heating the API to melt and pour
• Spraying solvent into the blender without drying

Correct Answer: Using geometric dilution by stepwise blending with portions of excipient

Q41. Geometric dilution is used primarily to:

• Increase the particle size of the API
• Uniformly distribute a small amount of potent drug into a larger quantity of excipient
• Sterilize the powder blend
• Convert powders into granules without binders

Correct Answer: Uniformly distribute a small amount of potent drug into a larger quantity of excipient

Q42. Installing internal baffles in a tumbling blender generally does what?

• Prevents any mixing from occurring
• Promotes turbulence, increases shear and improves mixing efficiency
• Turns the blender into a liquid reactor
• Eliminates the need for sampling

Correct Answer: Promotes turbulence, increases shear and improves mixing efficiency

Q43. Increasing ribbon speed in a ribbon blender typically results in:

• Longer mixing time and lower shear
• Shorter mixing time but potentially higher energy input and shear
• No change in mixing behavior ever
• Instant chemical reactions

Correct Answer: Shorter mixing time but potentially higher energy input and shear

Q44. Which factor is least likely to influence powder mixing efficiency?

• Particle size distribution
• Surface energy and cohesion
• Filling level in the blender
• Color of the mixing container

Correct Answer: Color of the mixing container

Q45. Representative sampling for blend analysis requires:

• A single sample from the top of the blender
• Multiple samples from different spatial locations combined for analysis
• Only measuring the powder color
• Sampling only after the batch is compressed into tablets

Correct Answer: Multiple samples from different spatial locations combined for analysis

Q46. Which post-processing step can help reduce segregation in a powder blend?

• Drying at extremely high temperatures causing thermal degradation
• Slight granulation or agglomeration to make particle sizes and densities more uniform
• Adding free-flowing dyes
• Painting the particles

Correct Answer: Slight granulation or agglomeration to make particle sizes and densities more uniform

Q47. For dry powder inhaler (DPI) formulations, which properties are most critical to achieve uniform dosing?

• Particle size distribution and adhesive/cohesive forces between API and carrier
• Color and odor of the powder
• High concentration of liquid binder
• Magnetic susceptibility

Correct Answer: Particle size distribution and adhesive/cohesive forces between API and carrier

Q48. Which property of powders most reduces mixing efficiency if present in excess?

• Low cohesion and free flow
• High cohesion and strong inter-particle attractions
• Perfect spherical shape
• Identical densities and sizes

Correct Answer: High cohesion and strong inter-particle attractions

Q49. Which common lubricant is often used in small amounts during mixing and tableting but can adversely affect dissolution if overused?

• Colloidal silicon dioxide
• Magnesium stearate
• Microcrystalline cellulose
• Sucrose

Correct Answer: Magnesium stearate

Q50. Which practical strategy helps minimize segregation during discharge from a hopper or silo?

• Using rapid, uncontrolled discharge to move material quickly
• Controlling flow rate and using flow aids or mass flow design to prevent percolation and funnel flow
• Raising the hopper temperature above melt point
• Painting the hopper interior with oil-based paint

Correct Answer: Controlling flow rate and using flow aids or mass flow design to prevent percolation and funnel flow

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com