Dissolution Theories & Mechanisms MCQs With Answer

Dissolution Theories & Mechanisms MCQs With Answer

This quiz set on dissolution theories and mechanisms is designed specifically for M.Pharm students preparing for MIP 102T – Pharmaceutical Formulation Development. It covers fundamental and advanced concepts including Noyes‑Whitney, diffusion layer models, Hixson‑Crowell, Higuchi, intrinsic dissolution, hydrodynamics, sink vs non‑sink conditions, role of excipients, supersaturation and precipitation, and related mathematical relationships. The questions emphasize mechanistic understanding and application to formulation problems so students can link theory to experimental design and interpretation of dissolution data. Use these MCQs to test comprehension, prepare for exams, and identify areas needing deeper study.

Q1. Which classical equation describes the rate of dissolution as directly proportional to the surface area, diffusion coefficient, and concentration gradient across a stagnant diffusion layer?

• Noyes‑Whitney equation
• Higuchi equation
• Hixson‑Crowell cube root equation
• Nernst‑Planck equation

Correct Answer: Noyes‑Whitney equation

Q2. The Noyes‑Whitney equation assumes which of the following about the concentration in the bulk solution during dissolution?

• Bulk concentration equals saturation concentration at the solid surface
• Bulk concentration is negligible compared to surface concentration (sink conditions)
• Bulk concentration exceeds surface concentration due to agitation
• Bulk concentration is constant and equal to drug solubility

Correct Answer: Bulk concentration is negligible compared to surface concentration (sink conditions)

Q3. The diffusion layer model (Nernst‑Brunner modification) primarily introduces which concept to improve Noyes‑Whitney?

• Time‑dependent decrease in particle radius due to erosion
• Presence of a stagnant diffusion layer of finite thickness adjacent to the solid
• Chemical degradation in the bulk solution
• Convection dominating diffusion across the entire vessel

Correct Answer: Presence of a stagnant diffusion layer of finite thickness adjacent to the solid

Q4. Which equation relates the change in particle size (cube root of mass) to the amount dissolved and is used for erosion or dissolution of solids losing geometric shape?

• Higuchi equation
• Hixson‑Crowell cube root law
• Noyes‑Whitney equation
• Stokes‑Einstein equation

Correct Answer: Hixson‑Crowell cube root law

Q5. The Higuchi model is most appropriate for describing release kinetics from which type of system?

• Instantly soluble tablets under sink conditions
• Matrix systems where drug diffusion through a porous or swelling matrix predominates
• Systems controlled solely by erosion with changing surface area
• Colloidal micellar solubilization systems

Correct Answer: Matrix systems where drug diffusion through a porous or swelling matrix predominates

Q6. Intrinsic dissolution rate (IDR) is defined as the dissolution rate of a pure drug under constant surface area, agitation, and solvent conditions. Which parameter is obtained directly from IDR experiments?

• Bulk solubility (S∞)
• Diffusion coefficient (D) in the bulk medium
• The intrinsic dissolution constant or rate (dM/dt per unit area)
• Particle size distribution

Correct Answer: The intrinsic dissolution constant or rate (dM/dt per unit area)

Q7. Which factor decreases the thickness of the diffusion layer and thereby typically increases the dissolution rate?

• Lowering temperature of the medium
• Reducing agitation or stirring speed
• Increasing hydrodynamic shear or agitation speed
• Increasing drug crystalline lattice energy

Correct Answer: Increasing hydrodynamic shear or agitation speed

Q8. In the context of dissolution, what is meant by ‘sink conditions’?

• Bulk concentration is equal to saturation concentration
• Bulk concentration is maintained at zero by continuous removal
• Bulk concentration remains below 10–30% of saturation so driving force remains approximately constant
• Bulk concentration exceeds solubility causing precipitation

Correct Answer: Bulk concentration remains below 10–30% of saturation so driving force remains approximately constant

Q9. Which phenomenon can increase apparent solubility and transiently enhance dissolution rate but may lead to subsequent precipitation?

• Hydrophobic aggregation
• Supersaturation
• Surface complexation with ions that reduce solubility permanently
• Complete drug degradation at the surface

Correct Answer: Supersaturation

Q10. Which excipient class is commonly used as a precipitation inhibitor to maintain a supersaturated solution and enhance dissolution?

• Strong acids
• Hydrophilic polymers (e.g., PVP, HPMC)
• High melting point excipients
• Nonpolar oils

Correct Answer: Hydrophilic polymers (e.g., PVP, HPMC)

Q11. Which parameter in the Noyes‑Whitney equation is most affected by particle size reduction (e.g., micronization)?

• Diffusion coefficient (D)
• Thickness of diffusion layer (h)
• Effective surface area (A)
• Bulk solubility (Cs)

Correct Answer: Effective surface area (A)

Q12. The Stokes‑Einstein relation links the diffusion coefficient (D) to which of the following?

• Temperature, viscosity of medium and hydrodynamic radius of the diffusing species
• Drug melting point and pH of medium
• Surface energy and crystallinity of the solid
• Rotation speed of dissolution apparatus only

Correct Answer: Temperature, viscosity of medium and hydrodynamic radius of the diffusing species

Q13. Which hydrodynamic factor is especially important when comparing USP Apparatus 1 (basket) and Apparatus 2 (paddle) dissolution profiles?

• Electrical conductivity of the medium
• Flow patterns and shear rates near the dosage form
• Molecular weight of the drug
• Lipid solubility of excipients

Correct Answer: Flow patterns and shear rates near the dosage form

Q14. In poorly soluble weakly basic drugs, dissolution in gastric fluid is often high but decreases in intestinal pH. What mechanism explains this behavior?

• Salt formation in alkaline media
• pH‑dependent ionization causing higher solubility at low pH and precipitation at higher pH
• Increased diffusion coefficient at higher pH
• Enhanced wetting in intestinal fluid leading to slower dissolution

Correct Answer: pH‑dependent ionization causing higher solubility at low pH and precipitation at higher pH

Q15. Micellar solubilization by surfactants affects dissolution primarily by which mechanism?

• Reducing the diffusion coefficient of the drug drastically
• Increasing apparent solubility by transferring drug into micelles, thus increasing the concentration gradient
• Increasing crystalline lattice energy of the API
• Forming irreversible covalent bonds with the drug

Correct Answer: Increasing apparent solubility by transferring drug into micelles, thus increasing the concentration gradient

Q16. Which of the following best describes the role of polymers like HPMC in controlled release formulations regarding dissolution?

• They always increase dissolution rate by decreasing viscosity
• They form a gel barrier that controls drug diffusion and erosion, slowing release
• They convert crystalline drug into a more soluble salt
• They precipitate the drug to prevent absorption

Correct Answer: They form a gel barrier that controls drug diffusion and erosion, slowing release

Q17. When a drug forms a soluble complex with a counterion in the medium, what is the likely effect on dissolution?

• Apparent solubility decreases and dissolution stops
• Apparent solubility increases and dissolution rate may increase
• Diffusion layer thickness becomes infinite
• Hydrodynamics become the only determinant of dissolution rate

Correct Answer: Apparent solubility increases and dissolution rate may increase

Q18. Which experimental observation suggests dissolution is diffusion‑controlled rather than surface‑area controlled?

• Dissolution rate is proportional to surface area throughout the test
• A plot of amount released versus square root of time is linear (Higuchi behavior)
• Dissolution rate increases with particle enlargement
• Rate is independent of diffusion coefficient

Correct Answer: A plot of amount released versus square root of time is linear (Higuchi behavior)

Q19. Which change would most likely decrease the diffusion coefficient of a drug in the dissolution medium?

• Raising the temperature
• Decreasing the medium viscosity
• Adding a high molecular weight polymer to increase viscosity
• Reducing the hydrodynamic radius of the drug molecule

Correct Answer: Adding a high molecular weight polymer to increase viscosity

Q20. During method development, which approach helps distinguish whether incomplete dissolution is due to poor solubility or poor wetting of the solid?

• Measuring melting point of the drug
• Performing dissolution with and without a surfactant or wetting agent in the medium
• Using a lower paddle speed only
• Increasing compression force during tablet manufacturing without testing

Correct Answer: Performing dissolution with and without a surfactant or wetting agent in the medium

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