Mechanism of liquid mixing MCQs With Answer

Understanding the mechanism of liquid mixing is essential for B. Pharm students involved in formulation development, process design, and quality control. This concise, keyword-rich introduction covers fundamental concepts such as molecular diffusion, turbulent mixing, convective transport, impeller action, mixing time, mass transfer coefficients (kLa), Reynolds and Peclet numbers, and scale-up principles for stirred tanks and homogenizers. Emphasis on mixing mechanisms, dispersion, droplet breakup, and interfacial phenomena helps clarify how formulation properties like viscosity, non-Newtonian behavior, and surface tension influence mixing efficiency. Solid grounding in these topics improves experimental design and troubleshooting in pharmaceutical manufacturing. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary mechanism responsible for mass transfer at the molecular scale during liquid mixing?

• Molecular diffusion due to concentration gradients
• Turbulent eddy diffusion
• Convective bulk flow only
• Mechanical shearing without diffusion

Correct Answer: Molecular diffusion due to concentration gradients

Q2. Which dimensionless number compares inertial forces to viscous forces in a mixing system?

• Peclet number
• Reynolds number
• Schmidt number
• Froude number

Correct Answer: Reynolds number

Q3. In a stirred tank, what effect do baffles primarily provide?

• Reduce shear near the impeller
• Suppress vortex formation and improve mixing
• Increase thermal conductivity
• Decrease power consumption

Correct Answer: Suppress vortex formation and improve mixing

Q4. Which impeller type is most associated with high radial flow and strong shear suitable for gas dispersion?

• Axial flow hydrofoil impeller
• Rushton turbine
• Pitched-blade turbine angled at 45°
• Anchor impeller

Correct Answer: Rushton turbine

Q5. Mixing time is best defined as:

• Time to reach 50% of final concentration uniformly
• Time for complete chemical equilibrium to be achieved
• Time required to reach an acceptable degree of homogeneity
• Time taken for the impeller to complete one revolution

Correct Answer: Time required to reach an acceptable degree of homogeneity

Q6. The Peclet number in mixing processes quantifies the ratio of:

• Viscous to inertial forces
• Convective transport to diffusive transport
• Turbulent to laminar flow fractions
• Kinematic to molecular viscosity

Correct Answer: Convective transport to diffusive transport

Q7. Which law describes molecular diffusion flux proportional to the concentration gradient?

• Raoult’s law
• Fick’s first law
• Henry’s law
• Hooke’s law

Correct Answer: Fick’s first law

Q8. In turbulent mixing, eddy diffusion primarily enhances which process?

• Bulk convection only
• Microscopic chemical reactions only
• Macroscopic transport and effective diffusivity
• Viscosity reduction

Correct Answer: Macroscopic transport and effective diffusivity

Q9. Which parameter is directly used to calculate power consumption of an impeller?

• Power number (Np)
• Peclet number
• Schmidt number
• Prandtl number

Correct Answer: Power number (Np)

Q10. For scale-up from lab to production, one common rule is to keep which parameter constant?

• Geometric similarity only
• Constant power per unit volume
• Constant impeller diameter irrespective of tank size
• Constant Reynolds number for non-Newtonian fluids only

Correct Answer: Constant power per unit volume

Q11. Which process is dominant in homogenizers for reducing droplet size in emulsions?

• Electrostatic separation
• High shear and cavitation
• Simple convective mixing
• Temperature cycling

Correct Answer: High shear and cavitation

Q12. The mass transfer coefficient combined with interfacial area is expressed as:

• kRe
• kLa
• Np
• Sh

Correct Answer: kLa

Q13. Taylor dispersion in tubular flow refers to:

• Chemical reaction rate enhancement by mixing
• Enhanced axial dispersion due to combined convection and radial diffusion
• Formation of Taylor vortices in stirred tanks
• Interfacial tension reduction during emulsification

Correct Answer: Enhanced axial dispersion due to combined convection and radial diffusion

Q14. Which mixing index quantifies homogeneity based on tracer concentration variance?

• Reynolds number
• Coefficient of variation or variance-based mixing index
• Peclet number
• Froude number

Correct Answer: Coefficient of variation or variance-based mixing index

Q15. In laminar mixing, which mechanism becomes more important relative to turbulent mixing?

• Bulk turbulent eddies
• Molecular diffusion and laminar shear
• Cavitation-induced dispersion
• Acoustic streaming

Correct Answer: Molecular diffusion and laminar shear

Q16. Which factor most strongly increases mixing time in a stirred vessel?

• Increasing impeller speed
• Decreasing fluid viscosity
• Removing baffles
• Using multiple impellers

Correct Answer: Removing baffles

Q17. The Schmidt number (Sc) is defined as the ratio of:

• Viscosity to thermal diffusivity
• Momentum diffusivity to mass diffusivity
• Convective to conductive heat transfer
• Inertial to gravitational forces

Correct Answer: Momentum diffusivity to mass diffusivity

Q18. Which tracer technique is commonly used to experimentally determine mixing time in tanks?

• Colorimetric or pH conductivity tracer study
• Mass spectrometry of bulk fluid
• Infrared spectroscopy only
• Viscometry

Correct Answer: Colorimetric or pH conductivity tracer study

Q19. For non-Newtonian fluids, which property must be considered for accurate mixing predictions?

• Constant viscosity as in Newtonian fluids
• Shear-dependent viscosity and yield stress
• Only density variations
• Electrical conductivity

Correct Answer: Shear-dependent viscosity and yield stress

Q20. Which phenomenon can hinder mixing by creating stable layers within the vessel?

• Complete turbulent flow
• Density stratification
• Excessive impeller power
• High mass transfer coefficient

Correct Answer: Density stratification

Q21. In gas-liquid mixing, gas hold-up primarily affects:

• Only the viscosity of the liquid
• Interfacial area and mass transfer rates
• Electrical properties only
• Color of the solution

Correct Answer: Interfacial area and mass transfer rates

Q22. Which impeller characteristic most directly influences tip speed?

• Impeller blade material
• Impeller diameter and rotational speed
• Tank diameter only
• Baffle width

Correct Answer: Impeller diameter and rotational speed

Q23. Droplet breakup in liquid-liquid mixing is largely controlled by the balance between:

• Viscous heating and cooling
• Inertial/shear forces and interfacial tension
• Mass transfer coefficient and diffusion only
• Gravitational settling and buoyancy only

Correct Answer: Inertial/shear forces and interfacial tension

Q24. Which technique is effective for mixing highly viscous or yield-stress pharmaceutical slurries?

• High-speed Rushton turbine only
• Anchor or helical ribbon agitators
• Only static mixers for low viscosity fluids
• Purely magnetic stirring

Correct Answer: Anchor or helical ribbon agitators

Q25. Micro-mixing refers to:

• Large-scale bulk circulation in the tank
• Molecular scale mixing and rapid local strain affecting reaction kinetics
• Only heating and cooling of fluid
• Gas dispersion techniques

Correct Answer: Molecular scale mixing and rapid local strain affecting reaction kinetics

Q26. Which property reduces droplet coalescence in emulsification?

• Increasing interfacial tension
• Using effective surfactants to stabilize interfaces
• Decreasing shear rate to zero
• Removing dissolved solids

Correct Answer: Using effective surfactants to stabilize interfaces

Q27. Residence time distribution (RTD) studies help to evaluate:

• Only temperature profiles
• Mixing uniformity and flow patterns in reactors
• Viscosity of the fluid
• Impeller material properties

Correct Answer: Mixing uniformity and flow patterns in reactors

Q28. Which dimensionless number relates convective mass transport to diffusive mass transport for a specific species?

• Schmidt number
• Peclet number
• Mach number
• Biot number

Correct Answer: Peclet number

Q29. In mixing scale-up, maintaining constant tip speed is preferred when:

• Heat transfer scaling is the main concern
• Shear-sensitive products must not be over-sheared
• Maximizing turbulence is the only goal
• Geometric similarity cannot be achieved

Correct Answer: Shear-sensitive products must not be over-sheared

Q30. Which of the following increases effective diffusivity in turbulent flow?

• Decreasing Reynolds number
• Turbulent eddies that enhance mixing
• Eliminating all flow disturbances
• Only molecular diffusion without turbulence

Correct Answer: Turbulent eddies that enhance mixing

Q31. CFD (Computational Fluid Dynamics) is useful in mixing design because it can:

• Replace all experimental validation
• Predict flow patterns, dead zones, and scale effects
• Only calculate temperature distribution
• Prevent droplet breakup mechanically

Correct Answer: Predict flow patterns, dead zones, and scale effects

Q32. Which mixing configuration is best for rapid gas transfer into a liquid?

• Slow-moving anchor impeller without gas sparger
• High-shear impeller with proper gas sparging and dispersion
• Static mixer with no agitation
• Only baffle installation without impeller

Correct Answer: High-shear impeller with proper gas sparging and dispersion

Q33. Which factor lowers mass transfer coefficient for a solute in liquid mixing?

• Increasing turbulence
• Increasing viscosity
• Increasing interfacial area
• Higher impeller speed

Correct Answer: Increasing viscosity

Q34. Dead zones in mixing vessels are characterized by:

• High local turbulence and rapid mixing
• Poor circulation and slow mass transfer
• Enhanced heat transfer only
• Regions of cavitation

Correct Answer: Poor circulation and slow mass transfer

Q35. Which measurement method is commonly used to assess droplet size distribution in emulsions?

• Optical microscopy or laser diffraction
• pH titration only
• Conductivity probes exclusively
• Viscometry without imaging

Correct Answer: Optical microscopy or laser diffraction

Q36. For a given impeller, increasing fluid viscosity generally causes:

• Decreased power draw and faster mixing
• Increased power requirement and slower mixing
• No change to mixing behavior
• Immediate transition to turbulent flow

Correct Answer: Increased power requirement and slower mixing

Q37. Which type of static mixer element creates dispersive mixing through shear and elongation?

• Helical or corrugated plate elements
• Open paddle agitators
• Anchor impeller blades
• High-speed rotor-stator only

Correct Answer: Helical or corrugated plate elements

Q38. During emulsification, the Sauter mean diameter (D32) describes:

• The largest droplet observed
• The volume-to-surface mean droplet diameter relevant to interfacial area
• The arithmetic mean diameter only
• Droplet charge distribution

Correct Answer: The volume-to-surface mean droplet diameter relevant to interfacial area

Q39. Which mixing phenomenon is especially important for fast chemical reactions during mixing?

• Global macro-mixing only
• Micromixing at molecular scales
• Temperature homogenization only
• Electrical stirring

Correct Answer: Micromixing at molecular scales

Q40. In stirred vessels, multiple impellers are often used to:

• Increase dead zones near the bottom
• Provide axial flow and uniform mixing in tall tanks
• Reduce fluid shear near the free surface only
• Eliminate the need for baffles

Correct Answer: Provide axial flow and uniform mixing in tall tanks

Q41. Coalescence rate in dispersed systems increases when:

• Interfacial tension is lowered by surfactant
• Film drainage between droplets is slower
• Droplet collisions are more frequent with sufficient contact time
• Viscosity of continuous phase is extremely high preventing collisions

Correct Answer: Droplet collisions are more frequent with sufficient contact time

Q42. The Kolmogorov microscale in turbulent flow indicates:

• Largest eddy size only
• Smallest scales of turbulence where viscous dissipation occurs
• Average particle size in suspension
• Mixing time for laminar flow

Correct Answer: Smallest scales of turbulence where viscous dissipation occurs

Q43. In mixing of suspensions, what promotes uniform solid dispersion?

• Low impeller speed with no circulation
• Appropriate impeller type and sufficient upward flow to suspend particles
• Allowing particles to settle before agitation
• High static head without agitation

Correct Answer: Appropriate impeller type and sufficient upward flow to suspend particles

Q44. Which outcome indicates successful micro-mixing for an instantaneous competitive reaction?

• Reaction selectivity controlled by coarse mixing only
• Observed product distribution matches predictions for rapid local mixing
• Complete absence of reaction
• Only heat transfer is improved

Correct Answer: Observed product distribution matches predictions for rapid local mixing

Q45. When mixing a viscous polymer solution, which is a common approach to reduce mixing energy?

• Increase temperature to lower viscosity
• Reduce impeller diameter while increasing RPM proportionally
• Remove baffles entirely
• Use a smaller tank

Correct Answer: Increase temperature to lower viscosity

Q46. In a batch reactor, the term macro-mixing refers to:

• Molecular diffusion inside eddies
• Large-scale convective transport and circulation patterns
• Only heat conduction at walls
• Electromagnetic stirring effects

Correct Answer: Large-scale convective transport and circulation patterns

Q47. Which factor most strongly affects interfacial area for gas-liquid contact?

• Tank color
• Bubble size and gas dispersion method
• Impeller shaft material
• Ambient humidity

Correct Answer: Bubble size and gas dispersion method

Q48. Which experimental evidence suggests poor mixing in a pharmaceutical tank?

• Uniform tracer concentration quickly achieved
• Persistent concentration gradients and long mixing times
• Consistent product quality across batches
• Rapid dissolution of solutes

Correct Answer: Persistent concentration gradients and long mixing times

Q49. For a chemical reaction limited by mass transfer, improving mixing will primarily:

• Decrease reaction rate indefinitely
• Increase reactant transport to the reaction zone and raise apparent rate
• Only change product color
• Remove the need for catalysts

Correct Answer: Increase reactant transport to the reaction zone and raise apparent rate

Q50. Which strategy best reduces foaming during vigorous mixing of protein solutions?

• Increase aeration and sparging
• Add controlled antifoam agents and reduce surface shear
• Use only Rushton turbines at maximum speed
• Lower viscosity dramatically by dilution with air

Correct Answer: Add controlled antifoam agents and reduce surface shear

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