Critical point MCQs With Answer

Introduction: Understanding the critical point is essential for B.Pharm students studying physical pharmacy, formulation science, and drug processing. Critical point MCQs With Answer focus on concepts like critical temperature (Tc), critical pressure (Pc), critical volume (Vc), phase behavior, supercritical fluids and critical opalescence. These topics explain how vapor-liquid boundaries vanish, how supercritical CO2 acts as a green solvent, and why thermodynamic models (van der Waals, cubic EOS) predict critical constants. Mastery of critical point principles helps in extraction, particle engineering and solubility prediction. This SEO-friendly guide emphasizes key terms such as critical point, supercritical fluid, Tc, Pc and phase diagram. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the thermodynamic definition of the critical point?

• The temperature and pressure where a solid melts
• The temperature and pressure where gas and liquid phases become identical
• The pressure at which vapor pressure equals atmospheric pressure
• The temperature at which a liquid decomposes

Correct Answer: The temperature and pressure where gas and liquid phases become identical

Q2. At the critical point which of the following occurs?

• Surface tension between liquid and vapor becomes maximum
• Two distinct phases (liquid and vapor) coexist indefinitely
• The meniscus between liquid and vapor disappears
• The solid phase appears spontaneously

Correct Answer: The meniscus between liquid and vapor disappears

Q3. Which combination defines the critical constants for a pure substance?

• Boiling point and melting point
• Critical temperature, critical pressure, critical volume
• Vapor pressure at 25°C and density
• Heat capacity and refractive index

Correct Answer: Critical temperature, critical pressure, critical volume

Q4. The critical temperature (Tc) is:

• The temperature above which a gas cannot be liquefied by pressure alone
• The temperature at which a solid vaporizes
• The temperature where vapor pressure equals zero
• The freezing point under atmospheric pressure

Correct Answer: The temperature above which a gas cannot be liquefied by pressure alone

Q5. In the van der Waals equation, the critical volume Vc is related to parameter b as:

• Vc = b
• Vc = 2b
• Vc = 3b
• Vc = 6b

Correct Answer: Vc = 3b

Q6. For a pure substance at the critical point, which mathematical conditions on the isotherm hold?

• dP/dT = 0 and d2P/dT2 = 0
• dP/dV = 0 and d2P/dV2 = 0
• dV/dP = 0 and d2V/dP2 = 0
• dT/dV = 0 and d2T/dV2 = 0

Correct Answer: dP/dV = 0 and d2P/dV2 = 0

Q7. The compressibility factor Z at the critical point for the van der Waals equation is:

• 0.188
• 0.375
• 1.000
• 0.527

Correct Answer: 0.375

Q8. Which relation correctly gives the van der Waals critical pressure Pc in terms of a and b?

• Pc = a / (27 b^2)
• Pc = 27 a / b^2
• Pc = a b / 27
• Pc = 8 a / 27 b

Correct Answer: Pc = a / (27 b^2)

Q9. The law of corresponding states uses which reduced variables?

• Reduced density and reduced enthalpy
• Reduced temperature, reduced pressure, reduced volume
• Reduced viscosity and reduced diffusivity
• Reduced heat capacity and reduced entropy

Correct Answer: Reduced temperature, reduced pressure, reduced volume

Q10. Reduced temperature (Tr) is defined as:

• Tr = T × Tc
• Tr = Tc / T
• Tr = T / Tc
• Tr = T – Tc

Correct Answer: Tr = T / Tc

Q11. Which phenomenon near the critical point causes a fluid to scatter light strongly?

• Critical opalescence
• Supercooling
• Boiling
• Crystallization

Correct Answer: Critical opalescence

Q12. Supercritical fluids typically exhibit which combination of properties?

• Low density, low diffusivity, high viscosity
• Liquid-like density, gas-like diffusivity
• Solid-like rigidity, liquid-like flow
• High surface tension, low solvation power

Correct Answer: Liquid-like density, gas-like diffusivity

Q13. Which of the following is a common supercritical solvent used in pharmaceutical extraction?

• Supercritical water at 25°C
• Supercritical helium
• Supercritical carbon dioxide (scCO2)
• Supercritical nitrogen

Correct Answer: Supercritical carbon dioxide (scCO2)

Q14. The critical temperature of CO2 is approximately:

• −78.5 °C
• 0.01 °C
• 31.1 °C
• 100 °C

Correct Answer: 31.1 °C

Q15. The critical pressure of CO2 is approximately:

• 1 bar
• 7.38 MPa (≈73.8 bar)
• 101.3 kPa
• 0.5 MPa

Correct Answer: 7.38 MPa (≈73.8 bar)

Q16. Which of the following makes scCO2 attractive for pharmaceutical processing?

• High toxicity and strong residual solvent issues
• Non-flammability, low toxicity, tunable solvating power
• High boiling point and low diffusivity
• Reactiveness with most drug molecules

Correct Answer: Non-flammability, low toxicity, tunable solvating power

Q17. Rapid Expansion of Supercritical Solutions (RESS) is used in pharmacy primarily for:

• High-temperature sterilization
• Micronization and particle formation
• Lyophilization of proteins
• Tablet compression

Correct Answer: Micronization and particle formation

Q18. Which equation of state predicts critical constants using parameters a and b and yields Zc = 3/8?

• Ideal gas law
• van der Waals equation
• Redlich-Kwong equation
• Peng-Robinson equation

Correct Answer: van der Waals equation

Q19. The Clapeyron equation describes the slope of a phase boundary. At the critical point, the vapor-liquid boundary:

• Continues with infinite slope
• Ends and Clapeyron’s finite-slope concept becomes irrelevant
• Is defined by the ideal gas law
• Is described by Raoult’s law

Correct Answer: Ends and Clapeyron’s finite-slope concept becomes irrelevant

Q20. Which experimental method is commonly used to determine Tc and Pc for volatile compounds?

• Differential scanning calorimetry at atmospheric pressure only
• PVT measurements in a high-pressure cell
• UV-Vis spectroscopy
• Thin-layer chromatography

Correct Answer: PVT measurements in a high-pressure cell

Q21. How does adding a non-volatile solute to a pure solvent generally affect the solvent’s critical temperature?

• Always increases Tc
• Always decreases Tc
• Can shift Tc depending on interactions and concentration
• Has no effect on Tc

Correct Answer: Can shift Tc depending on interactions and concentration

Q22. Critical opalescence arises because of:

• Quantum tunneling near Tc
• Large-scale density fluctuations and correlations
• Chemical reactions between phases
• Complete absence of molecular motion

Correct Answer: Large-scale density fluctuations and correlations

Q23. Which practical limitation is important when working very close to the critical point in experiments?

• No change in physical properties
• Large property fluctuations complicate measurements
• Complete chemical inertness
• Infinite surface tension

Correct Answer: Large property fluctuations complicate measurements

Q24. In phase diagrams, the critical point lies at the end of which curve?

• Sublimation curve
• Solid-liquid equilibrium curve
• Vapor-liquid coexistence curve
• Triple point line

Correct Answer: Vapor-liquid coexistence curve

Q25. Which statement about supercritical fluid density is correct when pressure increases at constant temperature above Tc?

• Density decreases with increasing pressure
• Density increases with increasing pressure, improving solvation
• Density remains constant regardless of pressure
• Density becomes zero

Correct Answer: Density increases with increasing pressure, improving solvation

Q26. Which cubic equation of state is often preferred for hydrocarbon critical predictions in engineering?

• Ideal gas law
• Peng-Robinson equation
• Arrhenius equation
• Henry’s law

Correct Answer: Peng-Robinson equation

Q27. The concept of reduced variables helps to:

• Scale properties to compare different substances at corresponding states
• Eliminate the need for experimental data
• Predict crystallization kinetics only
• Describe quantum behavior of molecules

Correct Answer: Scale properties to compare different substances at corresponding states

Q28. Which of the following best describes a supercritical extraction process?

• Extraction using a solvent below its boiling point at atmospheric pressure
• Extraction using a supercritical fluid with tunable solvating power by pressure/temperature adjustment
• Extraction by distillation at ambient pressure
• Extraction by cold maceration only

Correct Answer: Extraction using a supercritical fluid with tunable solvating power by pressure/temperature adjustment

Q29. Which property of drugs often improves in supercritical CO2 compared to air or solvents?

• Thermal decomposition susceptibility
• Solubility of nonpolar drugs
• Hydrophilicity
• Chemical reactivity with water

Correct Answer: Solubility of nonpolar drugs

Q30. The disappearance of a distinct liquid-gas meniscus signals you have reached:

• The triple point
• The critical point
• Absolute zero
• Melting point

Correct Answer: The critical point

Q31. Which critical exponent concept is relevant near continuous critical points?

• Empirical boiling exponent
• Scaling exponents describing how properties diverge (e.g., heat capacity ∝ |T−Tc|−α)
• Avogadro exponent
• Henry exponent

Correct Answer: Scaling exponents describing how properties diverge (e.g., heat capacity ∝ |T−Tc|−α)

Q32. Which statement about surface tension approaching the critical point is true?

• Surface tension increases to a maximum at Tc
• Surface tension decreases and tends to zero at Tc
• Surface tension oscillates unpredictably at Tc
• Surface tension remains the same as at room temperature

Correct Answer: Surface tension decreases and tends to zero at Tc

Q33. Which parameter combination from an equation of state is primarily tuned to match experimental critical properties?

• Chemical functional groups only
• Parameters controlling attractive and repulsive forces (e.g., a and b)
• Optical constants
• Color and odor descriptors

Correct Answer: Parameters controlling attractive and repulsive forces (e.g., a and b)

Q34. In a binary mixture, the locus of critical points as composition varies is called:

• Triple line
• Critical line or critical curve
• Boiling plateau
• Spinodal point only

Correct Answer: Critical line or critical curve

Q35. Which of the following best describes the spinodal relative to the critical point?

• Spinodal is where the metastable region ends and instability begins; it meets the coexistence curve at the critical point
• Spinodal is the same as the triple point
• Spinodal only exists for ideal gases
• Spinodal represents maximum surface tension

Correct Answer: Spinodal is where the metastable region ends and instability begins; it meets the coexistence curve at the critical point

Q36. Which advantage does scCO2 have over organic solvents in drug processing?

• Leaves toxic residues more often
• Allows solvent-free drying of extracts and easy solvent removal by depressurization
• Is always a better solvent for polar compounds without modifiers
• Cannot be compressed or expanded easily

Correct Answer: Allows solvent-free drying of extracts and easy solvent removal by depressurization

Q37. How does temperature need to be adjusted relative to Tc to obtain a supercritical fluid?

• Temperature must be below Tc and pressure above Pc
• Temperature must be above Tc and pressure above Pc
• Temperature must be equal to absolute zero
• Temperature can be any value if pressure is zero

Correct Answer: Temperature must be above Tc and pressure above Pc

Q38. In supercritical chromatography, what is an advantage of using supercritical CO2 as the mobile phase?

• Very high viscosity leading to slow flow
• Low diffusivity and poor mass transfer
• Tunable solvating strength and lower viscosity than liquids improving efficiency
• Inability to mix with modifiers

Correct Answer: Tunable solvating strength and lower viscosity than liquids improving efficiency

Q39. Which of the following is true about the critical point in mixtures versus pure substances?

• Mixtures cannot have a critical point
• Mixtures can exhibit critical lines or consolute critical temperatures depending on composition
• Pure substances have no critical temperature
• Critical points in mixtures are identical to those in pure components

Correct Answer: Mixtures can exhibit critical lines or consolute critical temperatures depending on composition

Q40. Which measurement directly indicates approach to the critical point in scattering experiments?

• Decrease in light scattering intensity
• Increase in light scattering intensity and correlation length
• No change in scattering but change in color
• Only infrared absorption changes

Correct Answer: Increase in light scattering intensity and correlation length

Q41. For van der Waals constants a and b, Tc is given by which expression?

• Tc = a / bR
• Tc = 8a / 27bR
• Tc = a b / 8R
• Tc = 27 bR / 8a

Correct Answer: Tc = 8a / 27bR

Q42. In formulation, why might one choose to perform supercritical drying rather than conventional drying?

• To increase surface tension and collapse porous structures
• To avoid capillary forces and preserve porous microstructure (e.g., aerogels)
• Because it always reduces process cost
• Because it requires no equipment control

Correct Answer: To avoid capillary forces and preserve porous microstructure (e.g., aerogels)

Q43. The acentric factor is used to improve corresponding states predictions by accounting for:

• Molecular shape and polarity deviations from spherical behavior
• Only molar mass
• Color and odor of substances
• Number of chiral centers

Correct Answer: Molecular shape and polarity deviations from spherical behavior

Q44. Which of the following best describes a supercritical antisolvent (SAS) process?

• Using scCO2 to dissolve a solute directly for spray drying
• Using scCO2 as an antisolvent to precipitate solute from an organic solution
• Using water as the antisolvent at room temperature
• Using supercritical nitrogen as a solvent

Correct Answer: Using scCO2 as an antisolvent to precipitate solute from an organic solution

Q45. Near-critical enhancement of solubility is largely due to:

• Increase in molecular weight at Tc
• Large increase in density and solvent power as pressure rises near Tc
• Decrease in diffusivity to zero
• Complete immiscibility of phases

Correct Answer: Large increase in density and solvent power as pressure rises near Tc

Q46. Which statement about the triple point versus the critical point is correct?

• Triple point is where all three phases coexist; critical point is where two phases become indistinguishable
• Both describe identical conditions for any substance
• Triple point always lies above the critical point in temperature
• Critical point involves solid, liquid and gas coexistence

Correct Answer: Triple point is where all three phases coexist; critical point is where two phases become indistinguishable

Q47. Which technique can be used to tune solubility in scCO2 for polar drugs?

• Adding cosolvents/modifiers like ethanol
• Reducing the pressure only
• Increasing CO2 purity to 100% without modifiers
• Cooling below −100 °C

Correct Answer: Adding cosolvents/modifiers like ethanol

Q48. What is the primary reason real gases deviate from ideal behavior near the critical point?

• Negligible intermolecular interactions
• Strong intermolecular interactions and finite molecular volume become significant
• Ideal gas law predicts critical opalescence correctly
• Molecules become non-existent at the critical point

Correct Answer: Strong intermolecular interactions and finite molecular volume become significant

Q49. In the context of process safety, why is knowledge of Pc and Tc important for handling compressed fluids?

• They determine color of storage vessels
• They help predict conditions where phase change, density, and pressure hazards occur
• They decide the price of the chemical only
• They allow ignoring pressure relief systems

Correct Answer: They help predict conditions where phase change, density, and pressure hazards occur

Q50. Which statement about the van der Waals isotherm at temperatures above Tc is true?

• It shows a loop indicating coexistence of phases
• It is monotonic and has no oscillatory region corresponding to phase separation
• It crosses the temperature axis at three points
• It becomes discontinuous

Correct Answer: It is monotonic and has no oscillatory region corresponding to phase separation

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