Partially miscible liquids MCQs With Answer

Partially miscible liquids are pairs of solvents that are soluble in each other only to a limited extent, forming two liquid phases over certain compositions and temperatures. For B.Pharm students, understanding partially miscible liquids is essential for solvent selection, liquid-liquid extraction, formulation design, and purification methods. Key keywords include phase diagram, binodal curve, spinodal curve, tie line, upper critical solution temperature (UCST), lower critical solution temperature (LCST), activity coefficients, Gibbs free energy of mixing, and partition coefficient. Grasping these thermodynamic and practical concepts helps predict solubility behavior and optimize pharmaceutical separation processes. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What defines a pair of partially miscible liquids?

• They are completely miscible at all compositions and temperatures
• They are immiscible and form a single phase
• They are miscible only up to a limited composition or temperature and form two liquid phases
• They form vapour-liquid equilibria only

Correct Answer: They are miscible only up to a limited composition or temperature and form two liquid phases

Q2. What does the binodal curve on a liquid-liquid phase diagram represent?

• The compositions where vaporization occurs
• The limit of stability where spinodal decomposition starts
• The boundary separating single-phase and two-phase regions (coexistence curve)
• The temperature at which solids precipitate

Correct Answer: The boundary separating single-phase and two-phase regions (coexistence curve)

Q3. What is the spinodal curve indicative of?

• Compositions where mixture is metastable and phase separation requires nucleation
• Compositions where mixture is unstable and separates spontaneously by spinodal decomposition
• The equilibrium vapour composition over the liquid
• The temperature of crystallization

Correct Answer: Compositions where mixture is unstable and separates spontaneously by spinodal decomposition

Q4. What is the Upper Critical Solution Temperature (UCST)?

• The highest temperature above which two liquids are completely immiscible
• The lowest temperature below which two liquids are completely miscible
• The highest temperature above which two liquids are completely miscible
• The temperature at which vapour pressure is maximum

Correct Answer: The highest temperature above which two liquids are completely miscible

Q5. Which of the following best describes the Lower Critical Solution Temperature (LCST)?

• Temperature below which liquids are completely immiscible
• Temperature above which liquids become completely miscible
• Temperature above which liquids are immiscible and below which they are miscible
• Temperature at which chemical reaction occurs

Correct Answer: Temperature above which liquids are immiscible and below which they are miscible

Q6. In a partially miscible binary system with UCST, increasing temperature generally causes:

• Phase separation to increase
• Two phases to become a single homogeneous phase above UCST
• Immediate precipitation of solute
• No change in miscibility

Correct Answer: Two phases to become a single homogeneous phase above UCST

Q7. Tie lines in a phase diagram connect:

• Compositions of vapour and liquid phases
• Compositions of coexisting liquid phases at equilibrium
• Temperature and pressure at triple point
• Points of maximum solubility only

Correct Answer: Compositions of coexisting liquid phases at equilibrium

Q8. The lever rule is used to:

• Calculate vapor pressure of a solution
• Determine relative amounts of coexisting phases from phase diagram compositions
• Measure activity coefficients directly
• Estimate boiling point elevation

Correct Answer: Determine relative amounts of coexisting phases from phase diagram compositions

Q9. Why are activity coefficients important in partially miscible liquids?

• They directly give the partition coefficient
• They quantify non-ideal interactions affecting miscibility and phase equilibrium
• They are only needed for gases
• They are irrelevant in liquid-liquid systems

Correct Answer: They quantify non-ideal interactions affecting miscibility and phase equilibrium

Q10. Which thermodynamic quantity must have a common value in coexisting phases at equilibrium?

• Gibbs free energy per mole of pure component only
• Chemical potential of each component
• Temperature only
• Volume only

Correct Answer: Chemical potential of each component

Q11. In a binary partially miscible system, phase separation is indicated by a double minimum in which plot?

• Gibbs free energy of mixing vs temperature
• Gibbs free energy of mixing vs composition
• Viscosity vs composition
• Vapour pressure vs temperature

Correct Answer: Gibbs free energy of mixing vs composition

Q12. What effect does adding a salt (salting out) typically have on aqueous-organic partially miscible systems?

• Increases mutual solubility between phases
• Decreases mutual solubility, promoting phase separation
• Has no effect on phase behavior
• Causes immediate emulsification

Correct Answer: Decreases mutual solubility, promoting phase separation

Q13. Which process commonly uses partially miscible liquids in pharmaceutical separation?

• Steam distillation
• Liquid-liquid extraction (solvent extraction)
• Centrifugation of solids only
• Gas chromatography

Correct Answer: Liquid-liquid extraction (solvent extraction)

Q14. A binodal curve and spinodal curve coincide at which special point?

• Triple point
• Cryoscopic point
• Critical consolute point (critical solution point)
• Boiling point

Correct Answer: Critical consolute point (critical solution point)

Q15. The partition coefficient (distribution ratio) between two partially miscible liquids depends on:

• Only temperature
• Composition of each phase, temperature, and solute interactions
• Only pressure
• Only the presence of solids

Correct Answer: Composition of each phase, temperature, and solute interactions

Q16. Which statement about UCST behavior is correct?

• Systems with UCST are miscible at low temperatures and immiscible at high temperatures
• Systems with UCST are immiscible at all temperatures
• Systems with UCST are immiscible at low temperatures and miscible above UCST
• UCST applies only to polymer blends

Correct Answer: Systems with UCST are immiscible at low temperatures and miscible above UCST

Q17. Which experimental technique can map liquid-liquid phase diagrams for partially miscible systems?

• Differential scanning calorimetry alone
• Cloud point determination and titration at controlled temperature
• IR spectroscopy only
• Polarimetry

Correct Answer: Cloud point determination and titration at controlled temperature

Q18. What does a positive deviation from Raoult’s law indicate in binary liquid mixtures?

• Stronger unlike interactions than like interactions leading to complete miscibility
• Weaker unlike interactions than like interactions, favoring phase separation
• No deviation; ideal behavior
• Solid formation

Correct Answer: Weaker unlike interactions than like interactions, favoring phase separation

Q19. In pharmaceutical extraction, why is understanding miscibility important for solvent selection?

• To ensure solvents react chemically with drug molecules
• To predict phase behavior, maximize solute transfer, and avoid unwanted emulsions
• Because miscible solvents are always preferred
• Because immiscible solvents never extract drug molecules

Correct Answer: To predict phase behavior, maximize solute transfer, and avoid unwanted emulsions

Q20. Which quantity must be minimized for spontaneous mixing at constant T and P?

• Enthalpy of reaction
• Gibbs free energy of mixing (ΔGmix)
• Entropy of mixing
• Pressure

Correct Answer: Gibbs free energy of mixing (ΔGmix)

Q21. For two partially miscible liquids, at a given temperature inside the two-phase region, compositions of the two phases are found:

• At the intersections of a horizontal tie line with the binodal curve
• At the critical point only
• By finding the vapor pressures only
• At random compositions

Correct Answer: At the intersections of a horizontal tie line with the binodal curve

Q22. Which factor generally increases mutual solubility of two liquids?

• Increasing disparity in polarity between the liquids
• Strong specific interactions like hydrogen bonding between molecules
• Adding large amounts of salt
• Lowering temperature for systems with UCST always

Correct Answer: Strong specific interactions like hydrogen bonding between molecules

Q23. In partially miscible systems, emulsification is most likely to occur when:

• Interfacial tension is very high
• Interfacial tension is very low and there is mechanical agitation
• Temperature is below freezing
• No mixing is performed

Correct Answer: Interfacial tension is very low and there is mechanical agitation

Q24. Which of the following describes the consolute point?

• Point where three phases coexist
• Temperature and composition where two phases become identical and system becomes single phase
• Temperature at which a solute decomposes
• Maximum vapour pressure condition

Correct Answer: Temperature and composition where two phases become identical and system becomes single phase

Q25. Which pair is a classic example of partially miscible liquids?

• Water and ethanol (completely miscible)
• Water and benzene (partially miscible with limited solubility)
• Hexane and heptane (completely miscible)
• Mercury and water (immiscible)

Correct Answer: Water and benzene (partially miscible with limited solubility)

Q26. The slope of the Gibbs free energy curve at a given composition corresponds to:

• The activity coefficient only
• The chemical potential (partial molar Gibbs free energy) of the component
• The total pressure
• The temperature derivative

Correct Answer: The chemical potential (partial molar Gibbs free energy) of the component

Q27. For a binary liquid showing LCST behavior, raising the temperature above LCST typically:

• Makes the mixture more miscible
• Causes the single phase to separate into two phases
• Has no effect on miscibility
• Always leads to crystallization

Correct Answer: Causes the single phase to separate into two phases

Q28. Which is true about tie-line slopes in an asymmetric binary phase diagram?

• Tie lines are always vertical
• Tie lines are horizontal at constant temperature but their endpoints on the binodal are asymmetric about composition
• Tie lines indicate temperature changes
• Tie lines are irrelevant for calculations

Correct Answer: Tie lines are horizontal at constant temperature but their endpoints on the binodal are asymmetric about composition

Q29. Which intermolecular interaction tends to reduce miscibility between two organic liquids?

• Strong hydrogen bonding between unlike molecules
• Large differences in polarity and absence of favorable interactions
• Good π-π interactions between aromatic rings
• Formation of specific solute-solvent complexes

Correct Answer: Large differences in polarity and absence of favorable interactions

Q30. How does pressure usually affect liquid-liquid miscibility in simple organic systems?

• Pressure has a negligible effect compared to temperature for most liquids
• Pressure always increases miscibility dramatically
• Pressure always decreases miscibility dramatically
• Pressure converts liquids to gases

Correct Answer: Pressure has a negligible effect compared to temperature for most liquids

Q31. In designing an extraction for a drug from aqueous phase to organic phase, which property is crucial?

• Partition coefficient of the drug between the two phases
• Colour of the solvent
• Magnetic susceptibility
• Dielectric breakdown strength

Correct Answer: Partition coefficient of the drug between the two phases

Q32. Which statement about Gibbs free energy of mixing (ΔGmix) is correct for a spontaneously mixing ideal solution?

• ΔGmix > 0
• ΔGmix = 0
• ΔGmix < 0 indicating spontaneous mixing
• ΔGmix is irrelevant for miscibility

Correct Answer: ΔGmix < 0 indicating spontaneous mixing

Q33. What is the practical importance of tie-line endpoints in pharmaceutical extraction?

• They define equilibrium compositions of each phase for solute partitioning
• They define boiling points
• They specify reaction kinetics
• They are used only in gas phase analysis

Correct Answer: They define equilibrium compositions of each phase for solute partitioning

Q34. Which phenomenon complicates liquid-liquid extraction when droplets become very small?

• Enhanced phase separation
• Formation of stable emulsions that hinder phase disengagement
• Instant crystallization
• Complete miscibility

Correct Answer: Formation of stable emulsions that hinder phase disengagement

Q35. A miscibility gap in a phase diagram indicates:

• Range of pressure where gas forms
• Range of compositions and temperatures where two liquid phases coexist
• Region where solid forms
• Complete miscibility region only

Correct Answer: Range of compositions and temperatures where two liquid phases coexist

Q36. Which effect can increase miscibility of polymer-solvent systems but is analogous to small-molecule LCST/UCST behavior?

• Adding non-solvent salts always
• Changing temperature to alter entropy and enthalpy contributions to ΔGmix
• Applying magnetic field
• Changing container shape

Correct Answer: Changing temperature to alter entropy and enthalpy contributions to ΔGmix

Q37. In pharmaceutical formulations, partially miscible solvents can be used to:

• Promote unwanted degradation only
• Control drug delivery, create microphases, and aid in extraction or purification
• Eliminate the need for testing
• Replace all aqueous media

Correct Answer: Control drug delivery, create microphases, and aid in extraction or purification

Q38. Which mathematical condition indicates the onset of spinodal decomposition?

• Second derivative of Gibbs free energy of mixing with respect to composition is positive
• Second derivative of Gibbs free energy of mixing with respect to composition is negative
• Second derivative equals zero at spinodal boundary
• First derivative is maximum

Correct Answer: Second derivative equals zero at spinodal boundary

Q39. What role does entropy of mixing play in liquid miscibility?

• Entropy always disfavors mixing
• Entropy of mixing favors mixing and can counteract unfavorable enthalpy
• Entropy is irrelevant for liquids
• Entropy determines vapor pressure only

Correct Answer: Entropy of mixing favors mixing and can counteract unfavorable enthalpy

Q40. Which action can be taken to resolve a problematic emulsion during extraction?

• Ignore it and proceed
• Add a salt, change temperature, or use centrifugation or a demulsifier
• Always add a catalyst
• Boil the mixture vigorously

Correct Answer: Add a salt, change temperature, or use centrifugation or a demulsifier

Q41. What is the significance of the common tangent construction on ΔGmix vs composition curves?

• It locates compositions of coexisting phases by equalizing chemical potentials
• It measures viscosity differences
• It identifies boiling points
• It predicts crystal structure

Correct Answer: It locates compositions of coexisting phases by equalizing chemical potentials

Q42. Which solvent property often correlates with miscibility with water?

• Low molecular weight only
• Polarity and hydrogen-bonding ability
• Colour
• Density only

Correct Answer: Polarity and hydrogen-bonding ability

Q43. In liquid-liquid extraction, multiple stages are used to:

• Reduce total solvent consumption and increase overall recovery of solute
• Increase emulsion formation intentionally
• Make the process slower without benefits
• Only change temperature

Correct Answer: Reduce total solvent consumption and increase overall recovery of solute

Q44. Which description fits a miscibility gap shrinking with increasing temperature?

• Typical UCST behavior where above UCST gap disappears
• Typical LCST behavior where gap appears at high temperature
• No physical meaning
• Indicates reaction kinetics

Correct Answer: Typical UCST behavior where above UCST gap disappears

Q45. Which instrumental method can characterize composition of each phase in a partially miscible system?

• Gas chromatography, HPLC, or NMR after phase separation
• Only visual inspection
• Only pH paper
• X-ray diffraction of liquids

Correct Answer: Gas chromatography, HPLC, or NMR after phase separation

Q46. Which is a likely sign that two liquids are approaching the consolute point?

• Sharp precipitation of solids
• Interfacial tension goes to zero and phases become indistinguishable
• Immediate combustion
• No change visually

Correct Answer: Interfacial tension goes to zero and phases become indistinguishable

Q47. For drug partitioning, which thermodynamic parameter is directly related to the partition coefficient?

• Difference in chemical potentials (or standard free energies) of the drug in the two phases
• Boiling point difference only
• Viscosity ratio
• Dielectric constant alone

Correct Answer: Difference in chemical potentials (or standard free energies) of the drug in the two phases

Q48. Which strategy helps improve selectivity in liquid-liquid extraction involving partially miscible solvents?

• Using a solvent with identical polarity to the feed
• Tuning pH, adding complexing agents, or choosing solvents with favorable specific interactions
• Always increasing temperature to maximum
• Reducing contact time to zero

Correct Answer: Tuning pH, adding complexing agents, or choosing solvents with favorable specific interactions

Q49. In partially miscible systems, increasing the molecular size of a component generally:

• Always increases miscibility with water
• May decrease entropy of mixing and reduce miscibility
• Does not affect miscibility
• Converts liquid to gas

Correct Answer: May decrease entropy of mixing and reduce miscibility

Q50. Which practical consideration is essential when using partially miscible solvents in pharmaceutical manufacturing?

• Ignoring regulatory and toxicity profiles of solvents
• Ensuring solvent compatibility, safety, phase separation efficiency, and removal procedures
• Using the cheapest solvent irrespective of behavior
• Assuming complete miscibility at all conditions

Correct Answer: Ensuring solvent compatibility, safety, phase separation efficiency, and removal procedures

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