Understanding solubility of liquids in liquids is essential for B.Pharm students focused on formulation, extraction, and purification. This topic explores miscibility, phase diagrams, Raoult’s law, activity coefficients, azeotropes, Gibbs free energy of mixing, and Hildebrand solubility parameters to predict liquid-liquid interactions. You will learn about ideal and non-ideal behavior, positive and negative deviations, critical solution temperatures (UCST/LCST), and principles of liquid-liquid extraction and partition coefficient. Practical emphasis on solvent selection, cosolvency, temperature effects, and intermolecular forces prepares you for formulation design, process scale-up, and troubleshooting. Clear conceptual knowledge bridges theory with pharmaceutical applications. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What term best describes full mixing of two liquids to form a single homogeneous phase?
- Immiscibility
- Miscibility
- Partial solvation
- Heterogeneous dispersion
Correct Answer: Miscibility
Q2. Which law gives the vapor pressure of a component in an ideal liquid mixture as the product of its mole fraction and vapor pressure of the pure component?
- Henry’s law
- Raoult’s law
- Gibbs-Duhem equation
- Le Chatelier’s principle
Correct Answer: Raoult’s law
Q3. Positive deviation from Raoult’s law indicates which of the following?
- Strong attraction between unlike molecules
- Weaker interactions between unlike molecules than like molecules
- Formation of complex associates
- Complete immiscibility
Correct Answer: Weaker interactions between unlike molecules than like molecules
Q4. An azeotrope is characterized by which property?
- The liquid and vapor compositions are identical at the azeotropic point
- The solution follows ideal behavior at all compositions
- The vapor pressure is always lower than that of pure components
- The components are completely immiscible
Correct Answer: The liquid and vapor compositions are identical at the azeotropic point
Q5. Which parameter quantifies the tendency of a solute to distribute between two immiscible liquid phases?
- Solubility product (Ksp)
- Partition coefficient (P)
- Vapor pressure
- Boiling point elevation
Correct Answer: Partition coefficient (P)
Q6. The Hildebrand solubility parameter primarily measures which interaction?
- Electrostatic interactions
- Dispersion (cohesive) energy density
- Hydrogen bonding strength only
- Acid-base reactivity
Correct Answer: Dispersion (cohesive) energy density
Q7. Which phase diagram feature represents compositions of coexisting liquid phases in a partially miscible binary system?
- Critical point
- Binodal curve (coexistence curve)
- Spinodal curve
- Triple point
Correct Answer: Binodal curve (coexistence curve)
Q8. Upper critical solution temperature (UCST) describes what behavior?
- Two liquids become miscible above UCST
- Two liquids become immiscible above UCST
- Complete miscibility at all temperatures
- No temperature dependence of miscibility
Correct Answer: Two liquids become miscible above UCST
Q9. Which of the following most strongly promotes miscibility between two liquids?
- Large difference in polarity
- Similar intermolecular forces and polarity
- High molecular weight disparity
- Opposite pH values
Correct Answer: Similar intermolecular forces and polarity
Q10. Which expression relates Gibbs free energy change of mixing (∆Gmix) to spontaneity of mixing two liquids?
- Mixing is spontaneous if ∆Gmix > 0
- Mixing is spontaneous if ∆Gmix = 0
- Mixing is spontaneous if ∆Gmix < 0
- Spontaneity is independent of ∆Gmix
Correct Answer: Mixing is spontaneous if ∆Gmix < 0
Q11. What does an activity coefficient different from unity indicate in a liquid mixture?
- Ideal solution behavior
- Non-ideal solution behavior
- Zero vapor pressure
- Complete phase separation
Correct Answer: Non-ideal solution behavior
Q12. In liquid-liquid extraction, increasing temperature usually has what effect on distribution coefficient for most solutes?
- Always increases distribution coefficient
- Always decreases distribution coefficient
- May increase or decrease depending on enthalpy of transfer
- Has no effect
Correct Answer: May increase or decrease depending on enthalpy of transfer
Q13. Which pair of liquids is most likely to form an azeotrope?
- Two completely nonpolar hydrocarbons
- A polar protic solvent and a nonpolar hydrocarbon with weak interactions
- Two chemically identical liquids
- Two liquids with significantly different vapor pressures and non-ideal interactions
Correct Answer: Two liquids with significantly different vapor pressures and non-ideal interactions
Q14. The lever rule in phase diagrams is used to determine what?
- Temperature at which azeotrope forms
- Relative amounts of coexisting phases at equilibrium
- Vapor pressure of each component
- Molecular weight of mixture
Correct Answer: Relative amounts of coexisting phases at equilibrium
Q15. Which intermolecular force is most important for miscibility between ethanol and water?
- London dispersion forces
- Hydrogen bonding
- Ionic bonding
- Metallic bonding
Correct Answer: Hydrogen bonding
Q16. Which statement about partially miscible liquids is true?
- They are immiscible at all temperatures
- They are miscible at some temperatures and immiscible at others
- They form a single phase regardless of composition
- They always form azeotropes
Correct Answer: They are miscible at some temperatures and immiscible at others
Q17. In a binary liquid mixture showing negative deviation from Raoult’s law, what is typical?
- Higher vapor pressures than predicted
- Lower vapor pressures than predicted
- No change in vapor pressure
- Complete immiscibility
Correct Answer: Lower vapor pressures than predicted
Q18. Which method is commonly used to determine mutual solubility curves experimentally?
- Refractometry or cloud-point determination
- Mass spectrometry
- X-ray crystallography
- Infrared spectroscopy exclusively
Correct Answer: Refractometry or cloud-point determination
Q19. Which of the following best describes cosolvency in pharmaceutical formulations?
- Use of a co-solvent to reduce mutual solubility of liquids
- Use of a second solvent to increase solubility of a solute in a primary solvent
- Removal of solvents by distillation
- Formation of an azeotrope intentionally
Correct Answer: Use of a second solvent to increase solubility of a solute in a primary solvent
Q20. Which property of solvents is most relevant when selecting an extraction solvent for a nonpolar drug from water?
- High dielectric constant
- Low polarity and immiscibility with water
- Ability to form hydrogen bonds with water
- High acidity
Correct Answer: Low polarity and immiscibility with water
Q21. When two liquids have identical solubility parameters, what is the expected behavior?
- They will be immiscible
- They are likely to be miscible
- They will form an azeotrope always
- They will react chemically
Correct Answer: They are likely to be miscible
Q22. Which thermodynamic quantity is directly related to activity coefficients in a solution?
- Enthalpy of fusion
- Chemical potential
- Boiling point
- Molar mass
Correct Answer: Chemical potential
Q23. In liquid-liquid extraction, multiple extractions with smaller volumes of solvent are preferred because:
- Total extracted amount decreases
- They give better overall recovery due to partitioning
- They form azeotropes more easily
- They reduce miscibility
Correct Answer: They give better overall recovery due to partitioning
Q24. Which descriptor defines the affinity of a solute for the organic phase relative to the aqueous phase?
- Distribution ratio (D)
- Boiling point elevation
- Viscosity
- Surface tension
Correct Answer: Distribution ratio (D)
Q25. What effect does adding a salt to an aqueous phase usually have on partitioning of a neutral organic solute?
- It always decreases partitioning into organic phase
- It can increase partitioning into organic phase by salting-out
- It converts the solute into its ionic form
- It causes formation of azeotrope
Correct Answer: It can increase partitioning into organic phase by salting-out
Q26. The spinodal curve in a phase diagram indicates what region?
- Metastable mixtures only
- Absolute instability where spontaneous demixing occurs
- Single-phase ideal behavior
- Temperature independence
Correct Answer: Absolute instability where spontaneous demixing occurs
Q27. Which is a consequence of a large positive free energy of mixing at a given composition?
- Complete miscibility
- Phase separation
- Formation of a homogeneous ideal solution
- Decrease in vapor pressure
Correct Answer: Phase separation
Q28. For ideal binary liquids, which of the following is true about enthalpy of mixing (∆Hmix)?
- ∆Hmix is strongly positive
- ∆Hmix is strongly negative
- ∆Hmix is zero
- ∆Hmix is undefined
Correct Answer: ∆Hmix is zero
Q29. Which analytical technique can quantify composition of two coexisting liquid phases accurately?
- Nuclear magnetic resonance (NMR)
- Thermogravimetric analysis only
- Polarimetry only
- Gas chromatography only for volatile components
Correct Answer: Nuclear magnetic resonance (NMR)
Q30. What role does interfacial tension play in liquid-liquid systems?
- It prevents phase separation
- It determines droplet formation and stability in emulsions
- It controls vapor pressure directly
- It converts immiscible liquids to miscible
Correct Answer: It determines droplet formation and stability in emulsions
Q31. Which effect describes increased solubility of a drug in a mixture of solvents compared to each solvent alone?
- Synergistic solubility (cosolvency)
- Salting-in
- Hydrotropy
- Crystallization
Correct Answer: Synergistic solubility (cosolvency)
Q32. In a titration-based liquid-liquid extraction of an ionizable drug, adjusting pH affects solubility by:
- Changing drug’s volatility only
- Altering the drug’s ionization state and aqueous solubility
- Changing solvent polarity permanently
- Creating azeotropes
Correct Answer: Altering the drug’s ionization state and aqueous solubility
Q33. Which term describes two liquids that separate into layers due to density difference but are miscible?
- Partially miscible
- Metastable emulsion
- Completely immiscible
- Miscible — cannot separate
Correct Answer: Partially miscible
Q34. What is the significance of the common tangent construction on Gibbs free energy vs composition curve?
- Identifies compositions of coexisting phases (equilibrium)
- Measures vapor pressure directly
- Predicts boiling point elevation
- Determines solubility parameter numerically
Correct Answer: Identifies compositions of coexisting phases (equilibrium)
Q35. Which factor can lead to formation of an excipient-induced phase separation in a drug formulation?
- Use of a perfectly matched solvent pair
- Addition of polymers that change solvent interactions
- Sterilization at low temperature
- Inert gas purging
Correct Answer: Addition of polymers that change solvent interactions
Q36. What is the primary cause of negative deviations from Raoult’s law?
- Weaker unlike interactions than like interactions
- Stronger unlike interactions than like interactions
- No interactions between molecules
- Phase separation tendency
Correct Answer: Stronger unlike interactions than like interactions
Q37. Which property is NOT directly predicted by Hildebrand solubility parameter?
- Miscibility tendency due to cohesive energy
- Specific hydrogen bonding contributions
- General solubility trends for nonpolar systems
- Qualitative solvent selection guidance
Correct Answer: Specific hydrogen bonding contributions
Q38. Which scenario most likely yields formation of an azeotrope during distillation?
- A truly ideal binary mixture with identical interactions
- Strong non-ideal interactions causing vapor and liquid compositions to coincide
- Completely immiscible liquids
- Two solids dissolved in one solvent
Correct Answer: Strong non-ideal interactions causing vapor and liquid compositions to coincide
Q39. The Nernst distribution law assumes which ideal condition?
- The solute reacts in both phases
- The solute is present in a single molecular form and partitions equally at equilibrium
- Phase volumes are equal
- Temperature varies during extraction
Correct Answer: The solute is present in a single molecular form and partitions equally at equilibrium
Q40. Which experimental observation indicates formation of microemulsion rather than simple immiscible layers?
- Immediate sharp interface between layers
- Transparent or translucent single-phase appearance
- Complete phase separation within seconds
- Solid precipitation
Correct Answer: Transparent or translucent single-phase appearance
Q41. In designing a solvent system for extraction, which safety and regulatory factor must be considered along with miscibility?
- Azeotrope formation only
- Toxicity, flammability, and residual solvent limits
- Only solvent color
- Boiling point alone
Correct Answer: Toxicity, flammability, and residual solvent limits
Q42. Which of the following best explains why temperature can reverse miscibility in LCST systems?
- Entropy effect dominates at low temperatures causing miscibility
- Enthalpic interactions weaken with temperature increase leading to demixing
- Pressure changes cause phase inversion
- Polarity of solvents becomes zero at LCST
Correct Answer: Enthalpic interactions weaken with temperature increase leading to demixing
Q43. When selecting solvents for drug purification by liquid-liquid extraction, which combination is generally ideal?
- Solvents with identical polarity and high mutual miscibility
- One solvent highly polar and miscible with aqueous phase
- Two solvents immiscible with appropriate partition coefficient for the drug
- Two reactive solvents that form chemical bonds with drug
Correct Answer: Two solvents immiscible with appropriate partition coefficient for the drug
Q44. Which mathematical model can describe non-ideal liquid mixtures and predict activity coefficients?
- Ideal gas law
- Margules, Wilson, or NRTL models
- Arrhenius equation
- Beer-Lambert law
Correct Answer: Margules, Wilson, or NRTL models
Q45. In practice, how is the partition coefficient (log P) of a drug commonly measured?
- By measuring solubility in a single solvent
- Using octanol-water partitioning experiments
- By NMR chemical shift only
- From vapor pressure data
Correct Answer: Using octanol-water partitioning experiments
Q46. Which effect describes increased miscibility of two otherwise immiscible liquids upon addition of a third component?
- Tyndall effect
- Mutual solubilization via a third component (cosolvent or surfactant)
- Boiling point elevation
- Hydrophobic effect only
Correct Answer: Mutual solubilization via a third component (cosolvent or surfactant)
Q47. For a binary liquid with UCST behavior, what is observed below the UCST?
- Complete miscibility at all compositions
- Phase separation into two phases within a miscibility gap
- Immediate formation of azeotrope
- Solution becomes gaseous
Correct Answer: Phase separation into two phases within a miscibility gap
Q48. Which factor most strongly influences the depth of a miscibility gap in a binary liquid system?
- Color of the liquids
- Magnitude and sign of interaction parameter (χ) or relative enthalpic interactions
- Atmospheric pressure only
- Presence of inert solids
Correct Answer: Magnitude and sign of interaction parameter (χ) or relative enthalpic interactions
Q49. In pharmaceutical extractions, why is back-extraction (stripping) used?
- To remove solvent permanently
- To recover solute from organic phase into fresh aqueous phase or vice versa
- To form azeotropes
- To purify solvent only
Correct Answer: To recover solute from organic phase into fresh aqueous phase or vice versa
Q50. Which statement best summarizes the role of intermolecular forces in liquid-liquid solubility?
- Only dispersion forces determine solubility
- Balance of dispersion, dipole-dipole, and hydrogen bonding dictates miscibility and solubility
- Intermolecular forces are irrelevant for liquids
- Solubility is determined solely by molecular weight
Correct Answer: Balance of dispersion, dipole-dipole, and hydrogen bonding dictates miscibility and solubility
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.
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