MCQs on interfacial phenomena: Liquid Interface, Surface & Interfacial Tension GPAT / NIPER / Pharmacist Recruitment (AIIMS / HSSB / SSB / RRB / Pattern) Exam

1. Which one is the correct surface tension definition?

A. Force that acts perpendicular to the surface per unit area
B. Force that acts tangentially per unit length at the liquid surface
C. Internal pressure inside a liquid molecule
D. Viscous force between two layers in a liquid

Answer: B. Force that acts tangentially per unit length at the liquid surface

Explanation:

Surface tension is defined as the tangential force acting along the surface of a liquid per unit length, which tends to minimize the surface area of the liquid.

Mathematically:

γ = F​ / L

Where:

• gamma (γ) = surface tension

• (F) = tangential force

• (L) = length over which force acts

This force arises due to the unbalanced cohesive forces experienced by molecules at the liquid surface.

Why are other options incorrect?

• A. Force perpendicular per unit area: relates more to pressure/stress, not surface tension.

• C. Internal pressure inside a liquid molecule: incorrect concept.

• D. Viscous force between liquid layers: definition of viscosity, not surface tension.

GPAT/NIPER Tip:

A very common trap in competitive exams is the phrase

• “per unit length” → Surface tension

• “per unit area” → Pressure/Stress

Remember:

Surface tension acts along the surface (tangentially), not perpendicular to it.

Quick Memory Trick:

“Surface tension stretches the surface like a tight film.”

So the force acts along the surface → tangentially.

Expected Exam Pattern:

Questions are frequently asked on

• Definition of surface tension

• Units and dimensions

• Difference between surface tension and interfacial tension

• Capillary rise method

• Du Noüy ring and stalagmometer methods

• Effect of surfactants on surface tension

Surface tension is defined as

A. The resistance of a liquid to mixing with other liquids

B. The tendency of molecules to move from the surface into the bulk liquid

C. The force that causes liquids to expand their surface area indefinitely

D. The attraction between liquid molecules and container walls that prevents evaporation

E. The force per unit length acting along the surface of a liquid that minimizes its surface area due to cohesive forces among molecules

Answer E

2. The SI unit of surface tension is:

A. Pascal
B. Dyne/cm
C. Newton/meter
D. Joule/m³

Answer: C. Newton/meter

Explanation:

Surface tension is defined as the force acting tangentially per unit length at the liquid surface.

Therefore, its SI unit is:

Surface Tension = Force / Length = N / m

• Newton per meter (N/m)

Why are other options incorrect?

• A. Pascal (Pa): Unit of pressure/stress
  1 Pa = 1 N/m²

• B. Dyne/cm: This is the CGS unit of surface tension, not the SI unit.

• D. Joule/m³: Unit of energy density or pressure.

GPAT/NIPER Tip:

A very important exam concept is the relation:
1 N/m = 1000 dyne/cm

Questions often ask:

• SI vs CGS units

• Conversion between dyne/cm and N/m

• Dimensional formula of surface tension

Quick Recall Table:

Quantity SI Unit

Surface tension N/m

Viscosity Pa·s

Pressure Pa

Surface free energy (J/m²)

Memory Trick:

Surface tension = Force ÷ Length
So think directly: N/m.

3. Surface free energy is numerically equal to:

A. Density × viscosity
B. Surface tension
C. Osmotic pressure
D. Dielectric constant

Answer: B. Surface tension

Explanation:

Surface free energy is defined as the work required to increase the surface area of a liquid by one unit area. Numerically, it equals surface tension.

Surface Free Energy = Surface Tension

• Surface tension is expressed as force per unit length (N/m).

• Surface free energy is expressed as energy per unit area (J/m²).

Since:

1N/m = 1 J/m²

both are numerically equivalent.

GPAT/NIPER Tip:

Questions often test the relationship between surface tension and surface free energy. Remember:

• Liquids → Surface tension term commonly used

• Solids → Surface free energy term commonly used

So, both represent the same physical concept but are used in different contexts.

4. Liquid surface tension:

A. Increases with an increase in temperature
B. Decreases with an increase in temperature
C. Remains unaffected by temperature
D. First increases, then decreases

Answer: B. Decreases with an increase in temperature

Explanation:

As the temperature increases, the kinetic energy of liquid molecules also increases. This reduces the cohesive forces between molecules at the liquid surface, leading to a decrease in surface tension.

Temperature↑ = Surface Tension↓

At the critical temperature, surface tension becomes nearly zero because the distinction between liquid and vapor phases disappears.

GPAT/NIPER Tip:

A very common conceptual question:

• Higher temperature → lower intermolecular attraction → lower surface tension

• The same trend is also seen with viscosity for most liquids.

Quick Recall:

• Cold water → higher surface tension

• Hot water → lower surface tension

5. Which one possesses the highest surface tension at room temperature?

A. Benzene
B. Ethanol
C. Ether
D. Water

Answer: D. Water

Explanation:

Water possesses the highest surface tension among the given liquids because of strong hydrogen bonding between water molecules.

Approximate surface tension values at room temperature:

• Water → ~72 dynes/cm

• Ethanol → ~22 dynes/cm

• Benzene → ~29 dynes/cm

• Ether → ~17 dynes/cm

Surface Tension: Water>Benzene>Ethanol>Ether

Strong intermolecular attraction in water causes molecules at the surface to be pulled inward more strongly, resulting in high surface tension.

GPAT/NIPER Tip:

Remember:

• Hydrogen bonding ↑ = Surface tension ↑

• Water generally shows unusually high values for:

  • boiling point

  • viscosity

  • surface tension

because of extensive hydrogen bonding.

6. Interfacial tension exists between:

A. Solid and gas
B. Two miscible liquids
C. Two immiscible liquids
D. Gas and vacuum

Answer: C. Two immiscible liquids

Explanation:

Interfacial tension is the force acting at the interface between two immiscible liquids due to unequal intermolecular attractions.

Examples:

• Oil and water

• Mercury and water

Interfacial Tension = exists between two immiscible liquids

If the liquids are miscible, they mix completely, and no stable interface exists, so interfacial tension is not observed.

GPAT/NIPER Tip:

• Surface tension: between liquid and air

• Interfacial tension: between two immiscible phases

A common exam trick is to confuse surface tension with interfacial tension.

7. The unit “dyne/cm” is commonly used for:

A. Diffusion coefficient
B. Viscosity
C. Density
D. Surface tension

Answer: D

Correct Answer: D. Surface tension

Explanation:

The unit dyne/cm is the CGS unit of surface tension.

Surface Tension = Force / Length

CGS unit → dyne/cm

• SI unit → N/m

Here:

• dyne = unit of force

• cm = unit of length

GPAT/NIPER Tip:

Frequently asked unit-based question:

Property Common Unit

Surface tension dyne/cm or N/m

Viscosity poise or centipoise

Density g/cm³

Diffusion coefficient cm²/s

Remember:

• 1 N/m = 1000 dyne/cm

8. Surface-active agents reduce surface tension due to

A. Increase in cohesive forces
B. Increase in density
C. Increase in viscosity
D. Accumulation at interfaces

Answer: D. Accumulation at interfaces

Explanation:

Surface-active agents (surfactants) reduce surface tension because they preferentially accumulate at the liquid interface (surface). Their presence disrupts cohesive forces between liquid molecules.

Surfactant accumulation at interface ⇒ Surface Tension↓

Surfactants contain:

• Hydrophilic (water-loving) part

• Hydrophobic (oil-loving) part

They orient themselves at the interface and lower the energy required to expand the surface.

GPAT/NIPER Tip:

Key conceptual point:

• Greater adsorption of surfactant at the interface: greater reduction in surface tension.

• This principle is important in:

  • emulsification

  • wetting

  • detergency

  • micelle formation.

9. Which method is used to determine surface tension?

A. Stalagmometer
B. Pycnometer
C. Polarimeter
D. Ostwald viscometer

Answer A

Explanation:

A stalagmometer is an instrument used to measure surface tension by the drop count or drop weight method.

The principle is that the number or weight of drops formed depends on the surface tension of the liquid.

Other Instruments:

• Pycnometer: measures density/specific gravity

• Polarimeter: measures optical rotation

• Ostwald viscometer: measures viscosity

GPAT/NIPER Tip:

Common instrument-based questions:

Instrument/Method Measurement

Stalagmometer Surface tension

Ostwald viscometer Viscosity

Pycnometer Density

Polarimeter Optical activity

10. The principle of the stalagmometer is:

A. Osmosis
B. Capillary rise
C. Sedimentation
D. Drop count/drop weight

Answer: D

Explanation:

A stalagmometer works on the drop count or drop weight principle for determining surface tension.

The weight or number of drops formed from a capillary tip is related to the liquid’s surface tension.

• Higher surface tension: larger/heavier drops

• Lower surface tension: smaller/lighter drops

GPAT/NIPER Tip:

Remember the association:

• Stalagmometer → drop count/drop weight

• The capillary rise method is another method for surface tension determination. But it is not the operating principle of a stalagmometer in this context.

11. The capillary rise method for surface tension measurement is used in:

A. Stoke’s law
B. Poiseuille’s law
C. Raoult’s law
D. Jurin’s law

Answer: D. Jurin’s law

Explanation:

The capillary rise method for measuring surface tension is based on Jurin’s law, which relates the height of liquid rise in a capillary tube to its surface tension.

h=rρg2γcosθ​

Where:

• h = height of capillary rise

• γ (gamma) = surface tension

• r = radius of capillary tube

• ρ (rho) = density of liquid

• g = acceleration due to gravity

• θ (theta) = angle of contact

  Other Laws:

  Stoke’s law → sedimentation of particles

  Poiseuille’s law → flow of liquids through capillaries

  Raoult’s law → vapor pressure of solutions

GPAT/NIPER Tip:

Associate these terms:

• Capillary rise → Jurin’s law

• Viscosity flow → Poiseuille’s law

• Sedimentation → Stokes' law

12. The spreading coefficient (S) equation is expressed by:

S=γB​−(γA​+γAB​)

A. Difference between vapor pressures
B. Relation between density and pressure
C. Relation among surface and interfacial tensions
D. Ratio of viscosities

Answer: C

Explanation:

The spreading coefficient (S) expresses the relationship between surface tension and interfacial tension of two phases.

S=γB​−(γA​+γAB​)

Where:

• γB (gamma_B)​ = surface tension of phase B

• γA (gamma_A)​ = surface tension of phase A

• γAB (gamma_{AB})​ = interfacial tension between A and B

The spreading coefficient predicts whether one liquid will spread over another liquid's surface.

• S > 0 → spontaneous spreading occurs

• S < 0 → spreading does not occur completely

GPAT/NIPER Tip:

Spreading coefficient is important in:

• emulsions

• wetting

• ointment spreading

• surfactant behavior

A common confusing question is linking a positive spreading coefficient with better spreading/wetting. Such as

A liquid will spread spontaneously over another surface when the spreading coefficient (S) is

A. Equal to zero
B. Negative
C. Positive
D. Infinite

Answer: C. Positive

Explanation:

A positive spreading coefficient indicates that adhesive forces are greater than cohesive forces, resulting in better spreading and wetting.

S>0 = Spontaneous spreading and better wetting

• S > 0: complete spreading occurs

• S < 0: incomplete spreading occurs

GPAT/NIPER Tip:

Remember:

• Positive spreading coefficient = good wetting/spreading

• Important in:

  • emulsions

  • ointments

  • tablet coating

  • surfactant action

13. A positive spreading coefficient indicates:

A. No spreading occurs
B. Complete spreading of the liquid over another
C. Formation of precipitate
D. Increased viscosity

Answer: B. Complete spreading of the liquid over another

Explanation:

A positive spreading coefficient (S > 0) indicates that the liquid spreads spontaneously and completely over another surface or liquid.

S>0⇒Complete spontaneous spreading

This occurs when adhesive forces between the two phases are greater than the cohesive forces within the liquid.

GPAT/NIPER Tip:

• Positive S → better wetting and spreading

• Negative S → incomplete spreading

Applications include:

• emulsions

• pharmaceutical creams and ointments

• coating processes

• surfactant systems

14. Which factor increases surface tension?

A. Electrostatic repulsion
B. Adhesive forces
C. Gravitational force
D. Cohesive forces

Answer: D

Explanation:

Surface tension arises because of cohesive forces between molecules of a liquid. Stronger intermolecular attraction pulls surface molecules inward, increasing surface tension.

Cohesive Forces↑ = Surface Tension↑

Examples:

• Water has high surface tension due to strong hydrogen bonding.

• Liquids with weaker intermolecular forces show lower surface tension.

GPAT/NIPER Tip:

Remember:

• Cohesive forces: attraction between similar molecules

• Adhesive forces: attraction between unlike molecules

High cohesive force generally means

• higher surface tension

• stronger droplet formation

• less spreading/wetting

15. What type of force does liquid bulk face inside?

A. Adhesive force
B. Balanced intermolecular forces
C. Zero cohesive force
D. Unbalanced inward force

Answer: B

Explanation:

Molecules inside the bulk of a liquid are surrounded uniformly by neighboring molecules in all directions. Therefore, the intermolecular attractive forces acting on them are balanced.

Inside liquid bulk = Balanced intermolecular forces

In contrast:

• Molecules at the surface experience an unbalanced inward pull, which gives rise to surface tension.

GPAT/NIPER Tip:

A frequently tested concept:

• Bulk molecules → balanced forces

• Surface molecules → unbalanced inward cohesive force → surface tension

16. Which surfactant property is responsible for the wetting?

A. Increase in viscosity
B. Increase in density
C. Increase in osmotic pressure
D. Reduction in surface tension

Answer: B

Explanation:

Surfactants promote wetting primarily by reducing the surface tension of liquids. Lower surface tension allows the liquid to spread more easily over a solid surface.

Surface Tension↓ = Wetting↑

This improves:

• spreading

• penetration

• contact between liquid and surface

GPAT/NIPER Tip:

Wetting agents are commonly used in:

• suspension formulation

• tablet granulation

• ointments

• detergents

Remember:

• Lower surface tension → lower contact angle → better wetting.

17. The angle of contact for a perfect wetting of liquid molecules is:

A. 0°
B. 45°
C. 90°
D. 180°

Answer: A. 0°

Explanation:

For perfect wetting, the liquid spreads completely over the surface, and the angle of contact becomes zero.

θ=0∘⇒Perfect wetting

• Smaller contact angle → better wetting

• Larger contact angle → poor wetting

Examples:

• Water on clean glass → very low contact angle

• Mercury on glass → high contact angle

GPAT/NIPER Tip:

Quick recall:

• θ<90∘ = good wetting

• θ>90∘ = poor wetting

• θ=0° = complete/perfect wetting

18. Which statement about interfacial tension is correct?

A. Independent of temperature
B. Exists only in solids
C. Lower than the surface tension when surfactants are added
D. Greater than surface tension

Answer: C. Lower than the surface tension when surfactants are added

Explanation:

Surfactants accumulate at the interface between two immiscible liquids and reduce the interfacial tension significantly.

\text{Surfactants} \uparrow \Rightarrow \text{Interfacial Tension} \downarrow

This reduction helps in:

• emulsification

• wetting

• detergency

• stabilization of dispersed systems

Why are other options incorrect:

• A. Independent of temperature: Incorrect; interfacial tension decreases with increasing temperature.

• B. Exists only in solids: Incorrect; it mainly exists between two immiscible liquids.

• D. Greater than surface tension: Incorrect; interfacial tension is usually lower, especially in the presence of surfactants.

GPAT/NIPER Tip:

A common exam concept:

• Surfactants reduce both the

  • surface tension

  • interfacial tension

This is the basis of emulsion and suspension stabilization.

19. The surfactant's detergent action is due to

A. Increase in viscosity
B. Increase in pH
C. Reduction in interfacial tension
D. Increase in surface free energy

Answer: C. Reduction in interfacial tension

Explanation:

Surfactants exhibit detergent action by reducing the interfacial tension between oil/grease and water. This allows oily dirt to detach from surfaces and become dispersed in water.

Interfacial Tension↓ = Detergency↑

Surfactant molecules:

• surround grease particles,

• forms micelles,

• and help emulsify oily substances in water.

GPAT/NIPER Tip:

Key functions of surfactants:

• Wetting

• Detergency

• Emulsification

• Solubilization

Remember:

• Detergency mainly depends on lowering the interfacial tension between oil and water.

20. Surface free energy is the

A. Energy to increase surface area
B. Energy to decrease density
C. Energy for Internal heat
D. Energy released for evaporation

Answer: A. Energy to increase surface area

Explanation:

Surface free energy is defined as the amount of energy required to increase the surface area of a liquid by one unit area.

Surface Free Energy = Work Done / Increase in Surface Area

It arises because molecules at the surface possess higher energy than molecules inside the bulk due to unbalanced cohesive forces.

GPAT/NIPER Tip:

Important relation:

• Surface free energy is numerically equal to surface tension.

Also remember:

• Liquids naturally try to minimize surface free energy by reducing surface area, which is why liquid droplets tend to become spherical.

21. Which pharmaceutical preparation requires low interfacial tension for stability?

A. Emulsions
B. Tablets
C. Capsules
D. Powders

Answer: B. Emulsions

Explanation:

Emulsions require low interfacial tension between the oil and water phases for proper formation and stability.

Interfacial Tension↓ = Emulsion Stability↑

Emulsifying agents (surfactants) reduce interfacial tension, making it easier to disperse one liquid into another and preventing coalescence of droplets.

GPAT/NIPER Tip:

Key role of surfactants in emulsions:

• reduce interfacial tension

• stabilize droplets

• improve dispersion

Commonly tested concept:

• Lower interfacial tension: easier emulsion formation and greater stability.

22. The rise of liquid in a capillary tube is inversely proportional to:

h ∝ 1/r

A. Liquid Density
B. Capillary Radius
C. Surface tension
D. Gravitational acceleration

Answer: B. Capillary Radius

Explanation:

According to Jurin’s law, the height of rise of a liquid in a capillary tube is inversely proportional to the radius of the capillary tube.

h \propto \frac{1}{r}

This means:

• Smaller capillary radius: greater capillary rise

• Larger capillary radius: lower capillary rise

The complete equation is:

h = 2γcosθ​/rρg

Where:

• h = height of rise

• r = radius of capillary

• γ (gamma) = surface tension

• ρ (rho) = density

• g = gravitational acceleration

GPAT/NIPER Tip:

Remember the proportionality:

• h↑ when r↓r \downarrowr↓

• Thin capillary tubes show maximum capillary rise.

23. Which one lowers the water surface tension most effectively?

A. Potassium chloride
B. Common Salt
C. Glucose
D. Soap solution

Answer: D. Soap solution

Explanation:

Soap solution contains surfactants that accumulate at the water surface and greatly reduce surface tension.

Surfactants (Soap) = Surface Tension of Water↓

In contrast:

• Electrolytes like sodium chloride and potassium chloride generally do not significantly reduce surface tension.

• Glucose has only a minor effect compared to surfactants.

GPAT/NIPER Tip:

Substances that strongly reduce surface tension:

• soaps

• detergents

• surfactants

• wetting agents

This property is important in:

• emulsification

• detergency

• suspension formulation

• wetting action

24. Surface tension can be expressed as:

A. Density per unit mass
B. Pressure per unit volume
C. Viscosity per unit length
D. Work done per unit surface area

Answer: D

Explanation:

Surface tension can also be defined as the amount of work or energy required to increase the surface area of a liquid by one unit area.

Surface Tension = Work Done / Surface Area​

Thus, surface tension is numerically equal to surface free energy.

It may also be expressed as:

Surface Tension = Force / Length

GPAT/NIPER Tip:

Two important expressions of surface tension:

• Force per unit length

• Work done (energy) per unit surface area

Both are numerically equivalent:

• 1 N/m = 1 J/m²

25. Which one is responsible for the spherical shape of liquid droplets?

A. Adsorption
B. Diffusion
C. Surface tension
D. Interfacial tension

Answer: C

Explanation:

Liquid droplets tend to acquire a spherical shape because surface tension tries to minimize the surface area for a given volume. A sphere has the minimum possible surface area.

Surface Tension = Minimum Surface Area = Spherical Droplets

Surface molecules experience an inward pull due to cohesive forces, causing the liquid to contract into a spherical form.

GPAT/NIPER Tip:

Common conceptual point:

• Higher surface tension → more perfectly spherical droplets

• Soap or surfactants lower surface tension and reduce this effect.

Examples:

• Raindrops

• Mercury droplets

• Oil droplets in emulsions

26. Which one measures interfacial tension by using the force to detach a ring from the surface?

A. Ostwald viscometer
B. Brookfield viscometer
C. Du Nouy tensiometer
D. Pycnometer

Answer: C

Correct Answer: C. Du Nouy tensiometer

Explanation:

The Du Nouy tensiometer measures surface and interfacial tension using the ring detachment method. The force required to pull a platinum ring away from the liquid surface or interface is measured.

Greater force required → higher surface/interfacial tension.

Other Instruments:

• Ostwald viscometer: viscosity measurement

• Brookfield viscometer: viscosity of non-Newtonian systems

• Pycnometer: density/specific gravity determination

GPAT/NIPER Tip:

Remember instrument associations:

• Stalagmometer: drop count method

• Du Nouy tensiometer: ring detachment method

• Viscometers: viscosity measurement

27. Wetting agents are added mainly for:

A. Improvement of contact between the liquid and the solid
B. Increasing particle distribution
C. Increase crystallization
D. Increasing the tablet granules' hardness

Answer: A. Improvement of contact between the liquid and the solid

Explanation:

Wetting agents are added to improve the contact between a liquid and a solid surface by reducing surface tension and contact angle.

Surface Tension↓⇒Wetting↑

This helps the liquid spread more easily over the solid surface and penetrate porous materials.

Applications include:

• suspension preparation

• tablet granulation

• dispersion of powders

• emulsification

GPAT/NIPER Tip:

A common concept:

• Wetting agents improve the dispersion of hydrophobic powders in liquids.

• Lower contact angle = better wetting and spreading.

28. Which statement is TRUE for surfactants?

A. They increase surface free energy
B. They increase cohesive forces
C. They increase solubility
D. They concentrate on interfaces

Answer: D. They concentrate on interfaces

Explanation:

Surfactants are surface-active agents that preferentially accumulate or concentrate at interfaces (such as liquid-air or oil-water interfaces).

Surfactants: Concentrate at interfaces

By accumulating at the interface, they:

• reduce surface tension

• reduce interfacial tension

• improve wetting and emulsification

Why are other options incorrect?

• A. Increase surface free energy: Incorrect; surfactants decrease surface free energy.

• B. Increase cohesive forces: Incorrect; they reduce cohesive attraction at the surface.

• C. Increase solubility: May occur in some cases (solubilization), but it is not the defining property of surfactants.

GPAT/NIPER Tip:

Core property of surfactants:

• Adsorption at interfaces is responsible for:

  • detergency

  • wetting

  • emulsification

  • foaming

  • solubilization

29. The capillary rise method is not accurate when:

A. High Surface tension
B. Low Density
C. Contact angle about zero
D. Wetting of the capillary wall

Answer: B

Correct Answer: D. Wetting of the capillary wall

Explanation:

The capillary rise method assumes ideal wetting conditions and accurate formation of the meniscus. If excessive wetting or irregular wetting of the capillary wall occurs, measurement errors can arise, reducing accuracy.

h = 2γcosθ / rρg

For accurate measurements:

• The capillary tube must be clean.

• the contact angle should be well-defined,

• and wetting behavior should remain consistent.

Why are other options incorrect?

• High surface tension: generally gives a better measurable rise.

• Low density: increases capillary rise and does not inherently reduce accuracy.

• Contact angle of about zero: actually favors proper capillary rise calculations.

GPAT/NIPER Tip:

Common sources of error in the capillary rise method:

• dirty capillary tube

• irregular wetting

• inaccurate radius measurement

• temperature variation

30. The major application of interfacial tension is in

A. Solubilisation
B. Emulsion formation
C. Lyophilization
D. Encapsulation

Answer: B

Correct Answer: B. Emulsion formation

Explanation:

Interfacial tension plays a major role in the formation and stability of emulsions. Emulsifying agents reduce the interfacial tension between oil and water phases, making dispersion easier.

Interfacial Tension↓⇒Emulsion Formation↑

Lower interfacial tension:

• reduces the energy required for droplet formation,

• prevents coalescence,

• improves emulsion stability.

GPAT/NIPER Tip:

Key association:

• Surface tension: liquid-air interface

• Interfacial tension: oil-water systems and emulsions

Very common exam statement:

• “Emulsifying agents stabilize emulsions by lowering interfacial tension.”

31. Which factor decreases surface tension?

A. Low temperature
B. Surfactants
C. Decrease in Interfacial tension
D. Percentage purity of additives

Answer: B. Surfactants

Explanation:

Surfactants decrease surface tension by accumulating at the liquid surface and reducing cohesive forces between liquid molecules.

Surfactants = Surface Tension↓

Examples:

• soaps

• detergents

• wetting agents

• emulsifying agents

Why are other options incorrect?

• A. Low temperature → generally increases surface tension.

• C. Decrease in interfacial tension: It does not directly cause a decrease in surface tension.

• D. Percentage purity of additives: not a general determining factor.

GPAT/NIPER Tip:

Main factors affecting surface tension:

• Temperature ↑: surface tension ↓

• Surfactants added: surface tension ↓

• Strong cohesive forces: surface tension ↑

32. A liquid spreads over another liquid under the condition:

A. S = 0
B.S. < 0
C. S > 0
D. S = 1

Answer: C. S > 0

Explanation:

A liquid spreads spontaneously over another liquid when the spreading coefficient (S) is positive.

S>0: Spontaneous spreading occurs

This means the adhesive forces between the two liquids are greater than the cohesive forces within the spreading liquid.

If:

• S > 0 → complete spreading

• S < 0 → incomplete or no spreading

GPAT/NIPER Tip:

A positive spreading coefficient indicates the following:

• better wetting

• better spreading

• improved emulsification behavior

Commonly asked in:

• surface chemistry

• pharmaceutics

• surfactant systems

33. The excess pressure inside a spherical droplet is given by:

ΔP=2γ/r​

A. Stoke’s law
B. Fick’s law
C. Bernoulli equation
D. Laplace equation

Answer: D. Laplace equation

Explanation:

The excess pressure inside a spherical liquid droplet is described by the Laplace equation.

ΔP=2γ​/r

Where:

• ΔP = excess pressure inside the droplet

• γ (gamma) = surface tension

• r = radius of the droplet

This shows:

• Smaller droplet radius: higher internal pressure

• Higher surface tension: higher excess pressure

Why are other options incorrect?

• Stoke’s law → sedimentation of particles

• Fick’s law → diffusion

• Bernoulli equation → fluid flow and pressure relation

GPAT/NIPER Tip:

For spherical systems:

• Droplet: ΔP=2γ /r

• Soap bubble (two surfaces): ΔP=4γ / r​

A very common competitive exam concept.

34. Which property of a liquid minimizes surface area?

A. Capillary action
B. Osmotic pressure
C. Partition coefficient
D. Surface tension

Answer: A

Correct Answer: D. Surface tension

Explanation:

Surface tension causes a liquid to minimize its surface area because surface molecules experience inward cohesive forces.

Surface Tension: Minimum Surface Area

As a result:

• Liquid droplets tend to become spherical,

• The surface contracts to attain the lowest energy state.

Why are other options incorrect?

• Capillary action: rise or fall of liquid in narrow tubes

• Osmotic pressure: movement of solvent across a semipermeable membrane

• Partition coefficient: distribution of solute between two immiscible phases

GPAT/NIPER Tip:

Remember:

• A sphere has the minimum surface area for a given volume.

• Surface tension is directly responsible for spherical droplet formation.

35. Surfactants improve emulsification due to

A. Increase in viscosity only
B. Low interfacial tension between oil and water
C. Increase in flow

D. Increase in particle density

Answer: B

Correct Answer: B. Low interfacial tension between oil and water

Explanation:

Surfactants improve emulsification by lowering the interfacial tension between oil and water phases, making it easier to disperse one liquid into another.

Interfacial Tension↓⇒Emulsification↑

This helps:

• formation of smaller droplets,

• prevention of droplet coalescence,

• improved emulsion stability.

Why are other options incorrect?

• A. Increase in viscosity only: viscosity may aid stability but is not the primary mechanism.

• C. Increase in flow: not directly responsible for emulsification.

• D. Increase in particle density: unrelated to emulsion formation.

GPAT/NIPER Tip:

Core mechanism of emulsifying agents:

• adsorption at the oil–water interface

• lowering interfacial tension

• stabilizing dispersed droplets

36. Which one is an interface?

A. Solid and liquid
B. Water and air
C. Water and oil
D. All of the above

Answer: D. All of the above

Explanation:

An interface is the boundary separating two different phases. Interfaces can exist between:

• Solid and liquid: e.g., tablet in water

• Liquid and gas: e.g., water and air

• Two immiscible liquids: e.g., water and oil

An interface is a boundary between two different phases

GPAT/NIPER Tip:

Types of interfaces commonly asked:

• Liquid–air: surface tension

• Liquid–liquid: interfacial tension

• Solid–liquid: wetting and adsorption phenomena

Hence, all the given examples represent interfaces.

37. Surface tension is highest in:

A. Solution
B. Non-polar liquids
C. Strongly hydrogen-bonded liquids
D. Highly volatile liquids

Answer: C. Strongly hydrogen-bonded liquids

Explanation:

Liquids with strong hydrogen bonding possess strong cohesive intermolecular forces, resulting in high surface tension.

Hydrogen Bonding↑⇒Surface Tension↑

Example:

• Water has a very high surface tension because of extensive hydrogen bonding between water molecules.

Why are other options incorrect?

• Solutions may have higher or lower surface tension depending on solutes.

• Non-polar liquids generally have weaker intermolecular forces and lower surface tension.

• Highly volatile liquids usually have weaker cohesive forces and lower surface tension.

GPAT/NIPER Tip:

Remember:

• Strong intermolecular attraction = high surface tension

• Hydrogen bonding is a major contributor to unusually high surface tension in liquids like water.

38. The force that is responsible for capillary rise:

A. Centrifugal force
B. Gravitational force
C. Electrostatic force only
D. Adhesive force between liquid and wall

Answer: D. Adhesive force between liquid and wall

Explanation:

Capillary rise occurs mainly due to the adhesive force between the liquid and the capillary wall. When adhesive forces are greater than cohesive forces, the liquid rises in the capillary tube.

Adhesive Force > Cohesive Force: Capillary Rise

For example:

• Water rises in a glass capillary because water adheres strongly to glass.

Why are other options incorrect?

• Centrifugal force: unrelated to capillary action

• Gravitational force: actually opposes capillary rise

• Electrostatic force only: not the primary explanation

GPAT/NIPER Tip:

Key concept:

• Adhesive force: attraction between unlike molecules

• Cohesive force: attraction between similar molecules

Capillary rise occurs when:

• adhesive force > cohesive force.

39. Which one is highly dependent on the wetting phenomenon?

A. Suspension preparation
B. Encapsulation
C. Lyophilization
D. Film coating

Answer: A. Suspension preparation

Explanation:

Wetting is highly important in suspension preparation because solid particles must be properly wetted by the dispersion medium to ensure uniform dispersion and prevent floating or clumping.

Better Wetting: Better Particle Dispersion in Suspensions

Hydrophobic powders are often difficult to disperse in water, so wetting agents (surfactants) are added to reduce surface tension and improve contact between the liquid and solid particles.

Why are other options incorrect?

• Encapsulation → mainly involves enclosing drugs in shells or coatings.

• Lyophilization → based on freezing and sublimation.

• Film coating → involves coating processes but is less directly dependent on wetting compared with suspension dispersion.

GPAT/NIPER Tip:

Common applications of wetting agents:

• suspension formulation

• powder dispersion

• granulation

• emulsification

A classic exam point:

• Poor wetting: floating of powder on liquid surface.

40. The surface tension reduction by surfactants helps to:

A. Improve spreading and wetting
B. Increase crystal growth
C. Increase dissolution
D. Reduce dryness

Answer: A. Improve spreading and wetting

Explanation:

Surfactants reduce surface tension, allowing liquids to spread more easily over solid surfaces and improving wetting.

Surface Tension↓⇒Spreading and Wetting↑

This enhances:

• penetration of liquids,

• dispersion of powders,

• emulsification,

• detergency.

Why are other options incorrect?

• B. Increase crystal growth: not a primary effect of surfactants.

• C. Increase dissolution: may occur indirectly, but wetting/spreading is the main direct effect.

• D. Reduce dryness: not related to surface tension reduction.

GPAT/NIPER Tip:

A common conceptual sequence:

• Surfactant added: surface tension decreases: contact angle decreases: wetting increases.

Dr. Alok Singh