Disperse Systems – Mixers & Mills MCQs With Answer

Introduction:

This quiz collection on Disperse Systems – Mixers & Mills is designed for M.Pharm students preparing for MIP 203T – Pharmaceutical Production Technology. It covers core principles of dispersed systems, suspension and emulsion stability, mechanisms of particle size reduction, and selection and operation of mixers and milling equipment used in pharmaceutical manufacturing. Questions emphasize practical aspects such as energy input, scale-up considerations, process parameters, and equipment limitations. Use these MCQs to test understanding of shear, impact and cavitation mechanisms, device selection for wet and dry milling, and quality attributes influenced by mixing and milling. Answers are provided to help reinforce learning and identify areas for further study.

Q1. Which of the following best describes a disperse system?

• A system in which a continuous phase contains one or more dispersed phases of different state or size
• A homogeneous solution where all components are molecularly mixed
• A crystalline solid with uniform lattice structure
• A gas-phase mixture of ideal gases

Correct Answer: A system in which a continuous phase contains one or more dispersed phases of different state or size

Q2. In emulsification, which mechanism primarily governs droplet break-up in high-shear rotor–stator mixers?

• Diffusion driven coalescence
• Viscous heating only
• Intense shear and turbulent eddies causing interfacial disruption
• Gravity separation due to density difference

Correct Answer: Intense shear and turbulent eddies causing interfacial disruption

Q3. Which parameter is most directly related to the stability of charged colloidal dispersions?

• Polydispersity index
• Zeta potential
• Melting point
• Vapor pressure

Correct Answer: Zeta potential

Q4. Which milling technique is best suited for obtaining submicron particles with minimal thermal damage for heat-sensitive drugs?

• Hammer mill operating at high tip speed
• Ball mill with high media temperature
• Jet mill using fluid energy impact
• Mortar and pestle manual grinding

Correct Answer: Jet mill using fluid energy impact

Q5. What is the primary advantage of wet milling over dry milling for poorly soluble drugs?

• It always produces larger particles
• It reduces electrostatic charging but increases crystallinity
• It enables better control of particle size and prevents dust generation
• It eliminates the need for stabilizers entirely

Correct Answer: It enables better control of particle size and prevents dust generation

Q6. Which mill type uses impact and attrition between rotating pins to achieve size reduction?

• Colloid mill
• Pin mill
• Jet mill
• Fluidized bed mill

Correct Answer: Pin mill

Q7. In a high-shear mixer, increasing the rotor speed primarily affects which outcome?

• Reduces viscosity of the continuous phase independent of shear
• Increases energy dissipation and reduces droplet or particle size
• Eliminates the need for surfactants
• Increases residence time in the mixing vessel

Correct Answer: Increases energy dissipation and reduces droplet or particle size

Q8. Which property of the dispersed phase most strongly influences the selection of surfactant for an emulsion?

• Color of the dispersed phase
• Interfacial tension and polarity between phases
• Ambient room noise level
• Boiling point of the continuous phase only

Correct Answer: Interfacial tension and polarity between phases

Q9. For scaling up a mixing process, which dimensionless number is commonly used to maintain dynamic similarity for turbulent mixers?

• Reynolds number
• Prandtl number
• Biot number
• Froude number only

Correct Answer: Reynolds number

Q10. Which statement about colloid mills is correct?

• They rely solely on thermal degradation to reduce particle size
• They use high shear between a rotor and stator to produce fine dispersions
• They are only suitable for dry powder milling
• They cannot be used for producing creams or lotions

Correct Answer: They use high shear between a rotor and stator to produce fine dispersions

Q11. What is the main mechanism of size reduction in ball mills?

• Evaporation and condensation cycles
• Compression and sliding without any impact
• Impact and attrition between grinding media and particles
• Centrifugal separation based on density only

Correct Answer: Impact and attrition between grinding media and particles

Q12. Which factor most contributes to re-agglomeration of nanoparticles after milling?

• Excessive zeta potential magnitude
• Insufficient stabilizer adsorption and attractive van der Waals forces
• Complete surface coverage by steric stabilizers
• Low solids concentration during storage

Correct Answer: Insufficient stabilizer adsorption and attractive van der Waals forces

Q13. When selecting a mixer for producing a shear-sensitive protein suspension, which equipment is most appropriate?

• High-speed rotor-stator homogenizer with maximum rpm
• Low-shear planetary or paddle mixer with gentle circulation
• Pin mill to ensure rapid size reduction
• Ball mill operated at high energy input

Correct Answer: Low-shear planetary or paddle mixer with gentle circulation

Q14. Which measurement technique is commonly used to characterize particle size distribution after milling?

• Infrared spectroscopy
• Laser diffraction
• pH titration
• Karl Fischer titration

Correct Answer: Laser diffraction

Q15. What is the purpose of adding a dispersant during milling of suspensions?

• To cause immediate aggregation of particles
• To provide electrostatic or steric stabilization and prevent particle aggregation
• To increase the density of the continuous phase arbitrarily
• To reduce the chemical stability of the active ingredient

Correct Answer: To provide electrostatic or steric stabilization and prevent particle aggregation

Q16. Which parameter should be closely monitored in high-energy wet mills to avoid degradation of thermo-labile compounds?

• Ambient pressure only
• Temperature rise in the milling slurry
• Color of the milling media alone
• Electrical conductivity of the continuous phase only

Correct Answer: Temperature rise in the milling slurry

Q17. In a stirred media mill, what is the role of the grinding media size and density?

• They only influence the color of the product
• Smaller and denser media increase collision frequency and energy, improving size reduction
• Larger, lighter media always provide better nanoscale grinding
• Media size and density have no effect on milling kinetics

Correct Answer: Smaller and denser media increase collision frequency and energy, improving size reduction

Q18. Which of the following is a disadvantage of jet milling?

• It generates high levels of heat unsuitable for all materials
• It cannot produce particles below 10 micrometers
• It requires high energy and has lower throughput for coarse feedstocks
• It always introduces solvent contamination into the product

Correct Answer: It requires high energy and has lower throughput for coarse feedstocks

Q19. What is the significance of the specific surface area of particles produced by mills in dissolution of poorly soluble drugs?

• Higher specific surface area typically decreases dissolution rate
• Specific surface area has no relation to dissolution
• Higher specific surface area increases dissolution rate by providing greater contact with solvent
• Specific surface area only affects tablet hardness and nothing else

Correct Answer: Higher specific surface area increases dissolution rate by providing greater contact with solvent

Q20. Which control strategy is most important during scale-up of a wet milling process to maintain similar product quality?

• Maintaining identical mixer geometry regardless of power input
• Controlling energy density or power per unit volume and maintaining similar residence time distributions
• Keeping feed concentration arbitrarily higher without monitoring
• Switching to completely different milling principle without validation

Correct Answer: Controlling energy density or power per unit volume and maintaining similar residence time distributions

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