Surface tension MCQs With Answer

Surface tension MCQs With Answer is a focused resource designed for B. Pharm students to master the fundamentals of surface phenomena in pharmaceutics. This introduction covers key concepts such as surface tension, interfacial tension, measurement methods (Du Noüy ring, Wilhelmy plate, pendant drop), surfactant action, Gibbs adsorption and clinical relevance in emulsions, aerosols and tablet wetting. Clear, concise explanations emphasize units, equations (Young–Laplace, capillary rise), and practical implications for formulation and quality control. The questions below reinforce theory and application, helping you prepare for exams and real-world formulation tasks. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is surface tension?

• The energy required to increase the surface area of a liquid by a unit amount
• The force acting along the surface making it behave like a stretched membrane
• The pressure difference across a curved interface
• Both the energy per unit area and tangential force per unit length at a liquid surface

Correct Answer: Both the energy per unit area and tangential force per unit length at a liquid surface

Q2. What are the SI units commonly used for surface tension in pharmaceutics?

• Newton per meter (N/m) or milliNewton per meter (mN/m)
• Joule per mole (J/mol)
• Pascal second (Pa·s)
• Newton per square meter (N/m²)

Correct Answer: Newton per meter (N/m) or milliNewton per meter (mN/m)

Q3. Approximate surface tension of pure water at 20–25°C used in many calculations is:

• 0.0728 N/m (72.8 mN/m)
• 0.5 N/m (500 mN/m)
• 0.00728 N/m (7.28 mN/m)
• 1.0 N/m (1000 mN/m)

Correct Answer: 0.0728 N/m (72.8 mN/m)

Q4. Which method measures surface tension by pulling a ring from a liquid surface?

• Wilhelmy plate method
• Du Noüy ring method
• Pendant drop method
• Capillary rise method

Correct Answer: Du Noüy ring method

Q5. The Wilhelmy plate method directly measures:

• The contact angle only
• The force on a thin plate in contact with the liquid to calculate surface tension
• Droplet shape to infer interfacial tension
• Capillary rise height only

Correct Answer: The force on a thin plate in contact with the liquid to calculate surface tension

Q6. The pendant drop method determines surface or interfacial tension by:

• Measuring the capillary rise in a tube
• Fitting the shape of a suspended drop to the Young–Laplace equation
• Using a ring and applying a correction factor
• Measuring the electrical conductivity of the solution

Correct Answer: Fitting the shape of a suspended drop to the Young–Laplace equation

Q7. Young–Laplace equation relates pressure difference across a curved interface to:

• Surface tension and curvature (radius) of the interface
• Contact angle and density only
• Temperature and pressure of the bulk liquid
• Surfactant concentration directly

Correct Answer: Surface tension and curvature (radius) of the interface

Q8. For a spherical droplet, Laplace pressure ΔP is given by:

• ΔP = γ·r
• ΔP = 2γ / r
• ΔP = γ / (2r)
• ΔP = r / (2γ)

Correct Answer: ΔP = 2γ / r

Q9. Capillary rise height h for a liquid in a tube depends inversely on:

• Surface tension
• Tube radius
• Contact angle of the tube material
• Liquid viscosity

Correct Answer: Tube radius

Q10. Young’s equation for contact angle involves which interfacial tensions?

• Solid–vapor and vapor–liquid only
• Solid–vapor, solid–liquid and liquid–vapor
• Liquid–liquid and liquid–vapor only
• Solid–liquid and solid–gas only

Correct Answer: Solid–vapor, solid–liquid and liquid–vapor

Q11. A contact angle less than 90° indicates:

• Non-wetting
• Partial wetting
• Complete wetting
• Hydrophobicity

Correct Answer: Partial wetting

Q12. Which statement is true about surface tension and temperature?

• Surface tension increases with increasing temperature
• Surface tension is independent of temperature
• Surface tension generally decreases with increasing temperature
• Surface tension oscillates with temperature

Correct Answer: Surface tension generally decreases with increasing temperature

Q13. Primary action of surfactants at an interface is to:

• Increase surface tension by aligning polar groups outward
• Decrease surface tension by adsorbing at the interface
• Eliminate all interfacial phenomena
• Convert liquids into solids

Correct Answer: Decrease surface tension by adsorbing at the interface

Q14. Critical micelle concentration (CMC) is defined as:

• The surfactant concentration where surface tension starts to rise
• The concentration above which micelles form and surface tension plateaus
• The minimum concentration needed to wet a surface
• The concentration at which surfactant degrades

Correct Answer: The concentration above which micelles form and surface tension plateaus

Q15. According to Gibbs adsorption isotherm, an increase in surfactant concentration at the interface results in:

• An increase in surface tension
• No change in surface tension
• A decrease in surface tension related to surface excess
• Immediate precipitation of surfactant

Correct Answer: A decrease in surface tension related to surface excess

Q16. Surface excess concentration (Γ) from Gibbs isotherm has units of:

• mol·m²
• mol·m⁻²
• m²·mol⁻¹
• kg·mol⁻¹

Correct Answer: mol·m⁻²

Q17. Which technique is least affected by small evaporation losses when measuring surface tension?

• Du Noüy ring method
• Wilhelmy plate method (with controlled environment)
• Pendant drop method (image analysis)
• Capillary rise method in open air

Correct Answer: Pendant drop method (image analysis)

Q18. Interfacial tension differs from surface tension in that it specifically refers to:

• The tension at a liquid–gas interface only
• The tension between two immiscible liquids or liquid–solid
• Tension in solids only
• Tension that does not affect emulsions

Correct Answer: The tension between two immiscible liquids or liquid–solid

Q19. In formulation, lowering interfacial tension primarily helps to:

• Make droplets larger and less stable
• Facilitate droplet breakup and formation of smaller droplets (emulsification)
• Eliminate the need for homogenization
• Increase sedimentation of particles

Correct Answer: Facilitate droplet breakup and formation of smaller droplets (emulsification)

Q20. Surface tension and surface energy are often used interchangeably because:

• They have different physical meanings and units
• Surface tension (force/length) equals surface free energy (energy/area) numerically
• One is mechanical and the other chemical with no relation
• Surface energy is measured in volts, not related

Correct Answer: Surface tension (force/length) equals surface free energy (energy/area) numerically

Q21. Which factor does NOT typically lower surface tension of water?

• Anionic surfactant at concentrations above CMC
• Nonionic surfactant at moderate concentration
• Increase in temperature
• Adding a small amount of a non-adsorbing salt

Correct Answer: Adding a small amount of a non-adsorbing salt

Q22. Which statement best describes dynamic surface tension?

• The equilibrium value measured after long times
• Surface tension measured as a function of time as surfactant adsorbs to a newly created interface
• Surface tension of solids
• The surface tension measured at absolute zero

Correct Answer: Surface tension measured as a function of time as surfactant adsorbs to a newly created interface

Q23. A major correction factor in the Du Noüy ring method accounts for:

• Viscosity of the liquid
• Buoyancy of the ring and shape differences between ring and ideal line force
• Temperature fluctuations only
• Presence of surfactant micelles

Correct Answer: Buoyancy of the ring and shape differences between ring and ideal line force

Q24. Which property is directly improved by lowering the surface tension during tablet coating?

• Rate of chemical degradation of API
• Wetting and spreading of coating solution over the tablet surface
• Core density of tablet
• Tablet hardness intrinsically

Correct Answer: Wetting and spreading of coating solution over the tablet surface

Q25. In pulmonary formulations, surfactant-like excipients can influence:

• Only the taste of the aerosol
• Aerosol droplet formation, stability and deposition in the respiratory tract
• The crystalline form of the API inside capsules only
• Bulk density of inhaler device

Correct Answer: Aerosol droplet formation, stability and deposition in the respiratory tract

Q26. Which measurement principle uses the equation F = P·γ·cosθ (where P is perimeter)?

• Du Noüy ring method
• Wilhelmy plate method
• Pendant drop method
• Goniometric drop volume method

Correct Answer: Wilhelmy plate method

Q27. Contact angle hysteresis refers to the difference between:

• Surface tension and interfacial tension
• Static and dynamic surface tension
• Advancing and receding contact angles
• Equilibrium adsorption and kinetic adsorption

Correct Answer: Advancing and receding contact angles

Q28. A surfactant molecule at the air–water interface typically orients with:

• Hydrophobic tail in water and polar head in air
• Hydrophilic head in water and hydrophobic tail towards air
• Both head and tail entirely in the bulk water
• Head buried in the bulk oil phase

Correct Answer: Hydrophilic head in water and hydrophobic tail towards air

Q29. Which parameter is reduced most dramatically when moving from a pure solvent to a surfactant solution above CMC?

• Bulk density
• Surface tension (which then plateaus at CMC)
• Viscosity to zero
• pH to neutral

Correct Answer: Surface tension (which then plateaus at CMC)

Q30. Which method can be used to determine CMC besides surface tension?

• Tensiometry only
• Conductivity, light scattering, or fluorescence probe techniques
• Optical microscopy only
• pH titration exclusively

Correct Answer: Conductivity, light scattering, or fluorescence probe techniques

Q31. Spreading coefficient S = γ_sv – γ_sl – γ_lv. If S > 0, then:

• The liquid spreads spontaneously on the solid
• The liquid forms a perfect sphere
• The liquid does not wet the solid at all
• The surface tension values are invalid

Correct Answer: The liquid spreads spontaneously on the solid

Q32. In emulsions, a low interfacial tension helps to:

• Hinder formation of small droplets
• Facilitate formation of small droplets during homogenization
• Prevent surfactant adsorption
• Increase droplet coalescence instantly

Correct Answer: Facilitate formation of small droplets during homogenization

Q33. Which surfactant type is most sensitive to added electrolytes affecting CMC?

• Nonionic surfactants
• Anionic surfactants
• Zwitterionic surfactants only
• Neutral polymers

Correct Answer: Anionic surfactants

Q34. Which physical phenomenon is primarily responsible for Ostwald ripening in emulsions?

• Decrease in surface tension with temperature
• Solubility difference causing net mass transfer from small to large droplets due to Laplace pressure
• Electrical conductivity changes
• Change in color of the emulsion

Correct Answer: Solubility difference causing net mass transfer from small to large droplets due to Laplace pressure

Q35. Which of the following is a common tensiometer technique for routine QC in industry?

• Dynamic light scattering
• Du Noüy ring or Wilhelmy plate tensiometer
• Nuclear magnetic resonance
• Gas chromatography

Correct Answer: Du Noüy ring or Wilhelmy plate tensiometer

Q36. Which change typically indicates contamination of a pure liquid surface?

• Rapid decrease in measured surface tension over time
• Slow increase in vapor pressure only
• Immediate solidification
• No change in any measurable property

Correct Answer: Rapid decrease in measured surface tension over time

Q37. Surface rheology describes:

• Only the viscosity of the bulk liquid
• Viscoelastic properties of the interfacial layer affecting foam and emulsion stability
• Thermal conductivity of the surface
• Electrical resistance across an interface

Correct Answer: Viscoelastic properties of the interfacial layer affecting foam and emulsion stability

Q38. A high surface elasticity at an interface typically contributes to:

• Reduced foam stability
• Increased foam and emulsion stability against disturbances
• Decreased droplet surface area
• Instant mixing of immiscible phases

Correct Answer: Increased foam and emulsion stability against disturbances

Q39. Which effect describes the pinning of a contact line on surface defects leading to hysteresis?

• Marangoni effect
• Contact line pinning
• Brownian motion
• Viscosity-driven wetting

Correct Answer: Contact line pinning

Q40. Marangoni flow at an interface is driven by:

• Gravitational waves only
• Gradients in surface tension (e.g., due to temperature or concentration differences)
• Homogeneous surface tension everywhere
• Magnetic fields only

Correct Answer: Gradients in surface tension (e.g., due to temperature or concentration differences)

Q41. Which parameter is commonly used to express the temperature dependence of surface tension?

• dn/dT
• dγ/dT (temperature coefficient of surface tension)
• dρ/dT
• dμ/dT

Correct Answer: dγ/dT (temperature coefficient of surface tension)

Q42. Numerically, mN/m is equivalent to which of the following energy units useful in thin film energetics?

• Joule per cubic meter (J/m³)
• MilliJoule per square meter (mJ/m²)
• Watt per meter (W/m)
• Newton per square meter (N/m²)

Correct Answer: MilliJoule per square meter (mJ/m²)

Q43. Which experimental artifact can cause overestimation of surface tension in ring or plate methods?

• Complete wetting of the probe
• Contamination that reduces surface tension
• Insufficient cleaning leaving residue that increases apparent force
• Using ultrapure water

Correct Answer: Insufficient cleaning leaving residue that increases apparent force

Q44. Which of the following best describes dynamic adsorption of surfactants at an interface?

• Instantaneous uniform adsorption irrespective of concentration
• Time-dependent diffusion and kinetic barriers that determine how quickly surface tension falls
• Adsorption that always creates a crystalline layer
• Adsorption that does not depend on surfactant structure

Correct Answer: Time-dependent diffusion and kinetic barriers that determine how quickly surface tension falls

Q45. In a two-phase oil–water system, lowering interfacial tension with surfactant will most directly:

• Increase the contact angle of water on oil
• Enhance dispersion of one phase into the other to form smaller droplets
• Stop all mass transfer between phases
• Turn the phases into a single homogeneous phase without surfactants

Correct Answer: Enhance dispersion of one phase into the other to form smaller droplets

Q46. Which factor typically increases the CMC of an ionic surfactant?

• Adding counterions that screen headgroup repulsion
• Increasing temperature (dependent) in certain systems
• Decreasing hydrocarbon chain length
• Adding co-surfactants that stabilize micelles

Correct Answer: Decreasing hydrocarbon chain length

Q47. Which method is preferred when no correction factors are desired for interfacial tension measurement?

• Du Noüy ring with Harkins-Jordan correction
• Pendant drop method with image analysis
• Capillary rise with open tube
• Wilhelmy plate with unknown contact angle

Correct Answer: Pendant drop method with image analysis

Q48. Which of the following is a pharmaceutical consequence of high surface tension in a suspension?

• Improved wetting of hydrophobic particles
• Poor wetting and possible aggregation of hydrophobic solid particles
• Spontaneous formation of micelles
• Instant dissolution of insoluble drugs

Correct Answer: Poor wetting and possible aggregation of hydrophobic solid particles

Q49. Adding a small amount of alcohol to water usually:

• Greatly increases surface tension
• Has no effect on surface tension
• Reduces surface tension and can alter spreading behaviour
• Causes immediate solidification

Correct Answer: Reduces surface tension and can alter spreading behaviour

Q50. Why is surface tension measurement important in pharmaceutical quality control?

• It verifies color and odor of formulations
• It assesses interfacial properties relevant to emulsification, wetting, aerosolization and stability
• It measures tablet hardness directly
• It quantifies API potency directly

Correct Answer: It assesses interfacial properties relevant to emulsification, wetting, aerosolization and stability

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