Coarse Dispersions: Suspensions, Stokes’ Law, Sedimentation & Flocculation

Coarse Dispersions: Suspensions, Stokes’ Law, Sedimentation & Flocculation

Coarse dispersions are heterogeneous systems in which the particle size of the dispersed phase is relatively large (generally > 1 μm). Among them, suspensions are the most important pharmaceutical dosage forms.

1. Suspensions

A suspension is a biphasic system in which solid particles are dispersed in a liquid medium but remain undissolved.

Key features:

• Particle size: > 1 μm

• Thermodynamically unstable but kinetically stable

• Requires shaking before use

• Common in oral, topical, and parenteral formulations

Good suspensions must show slow sedimentation and easy redispersibility.

2. Stokes’ Law

Sedimentation of particles in a suspension is governed by Stokes’ Law, which explains the settling velocity of spherical particles under gravity.

v=2r²(ρp−ρf)g/9η

Where:

• v = sedimentation velocity

• r = particle radius

• ρp = density of particle

• ρf = density of fluid

• η = viscosity of medium

• g = gravitational acceleration

Key implication:
Sedimentation increases with particle size and density difference but decreases with higher viscosity.

3. Sedimentation

Sedimentation refers to the settling of suspended particles under gravity.

Factors affecting sedimentation:

• Particle size (larger → faster settling)

• Viscosity of medium (higher → slower settling)

• Density difference between particle and medium

• Degree of flocculation

Sedimentation can be either:

• Flocculated system → loose aggregates, fast settling, easy redispersion

• Deflocculated system → slow settling but forms hard cake

4. Flocculation

Flocculation is the process where suspended particles form loose aggregates (flocs) instead of remaining as separate entities.

Advantages of controlled flocculation:

• Prevents formation of hard cake

• Improves redispersibility

• Ensures uniform dosing after shaking

Mechanism:

• Reduction of zeta potential

• Addition of electrolytes or polymers

• Weak interparticle attraction

Summary

In pharmaceutical suspensions, stability depends on balancing sedimentation behavior and flocculation control. Proper formulation ensures uniform distribution, easy redispersion, and acceptable shelf life, making suspensions effective and patient-friendly dosage forms.

1. A pharmaceutical suspension definition:

A. A liquid dosage form
B. A biphasic system of insoluble solid particles dispersed in a liquid medium
C. A solution having dissolved drug molecules
D. A colloidal dispersion with particle size below 1 nm

Answer: B

Correct Answer: B. A biphasic system of insoluble solid particles dispersed in a liquid medium

Explanation:

A pharmaceutical suspension is a biphasic liquid dosage form in which insoluble solid drug particles are dispersed uniformly in a liquid vehicle (dispersion medium).

• The solid phase = insoluble drug particles

• The liquid phase = dispersion medium (usually water)

Since the drug does not dissolve completely, particles remain suspended and may settle on standing.

Why Other Options Are Incorrect:

• A. A liquid dosage form

  • Too broad. Suspensions are liquid dosage forms, but this definition is incomplete.

• C. A solution having dissolved drug molecules

  • This describes a solution, not a suspension.

• D. A colloidal dispersion with particle size below 1 nm

  • Colloidal particles are much smaller. Suspension particles are generally larger and visible under a microscope.

Remember the keyword.

“Suspension = Insoluble particles dispersed in liquid.”

Points About Suspensions

• Biphasic system

• Requires shaking before use

• Sedimentation occurs on standing

• Stability follows v=2r²(ρs−ρo)g / 9η (Stokes' law)

• Flocculated and deflocculated suspensions

These are commonly asked topics.

2. The coarse dispersions' particle size ranges:

A. Less than 1 nm
B. 1 to 500 nm
C. Greater than 0.5 µm
D. Exactly 10 nm

Answer: C Greater than 0.5 µm

Explanation:

Coarse dispersions contain particles that are relatively large in size, generally greater than 0.5 µm (500 nm).

Examples include:

• Suspensions

• Emulsions

These particles are large enough to:

• settle on standing,

• scatter light strongly,

• and often is seen under a microscope.

Why Other Options Are Incorrect:

• A. Less than 1 nm

  • This range belongs to true solutions.

• B. 1 to 500 nm

  • This is the particle size range of colloidal dispersions.

• D. Exactly 10 nm

  • Too specific and incorrect for coarse dispersions.

Important Classification for GPAT & NIPER

• System Particle Size

• True solution Less than 1 nm

• Colloidal dispersion 1–500 nm

• Coarse dispersion Greater than 500 nm (0.5 µm)

Preparation Tips:

• Learn the Size-Based Classification

This is a high-frequency conceptual topic.

• Easy Memory Trick

  • Solution → Smallest

  • Colloid → Intermediate

  • Coarse dispersion → Largest

• Frequently Asked Associated Topics

  • Brownian movement

  • Tyndall effect

  • Sedimentation

  • Stokes’ law

  • Flocculation and deflocculation

The sedimentation rate in suspensions is explained by:
v=2r²(ρs−ρo)g/9η

where:

• v = sedimentation velocity

• r = particle radius

• η (eta) = viscosity of medium

3. Stokes’ law states that sedimentation velocity is directly proportional to:

A. Medium Viscosity
B. Square of particle radius
C. Medium Density
D. Surface tension

Answer: B Square of particle radius

Explanation:

According to Stokes’ law, the sedimentation velocity of particles in a suspension is:

v=2r²(ρs−ρo)g/9η

From the equation:

• v ∝ r²

This means the sedimentation velocity increases with the square of the particle radius.

Even a small increase in particle size greatly increases the settling rate.

So:

• Larger particles settle much faster than smaller particles.

Why Other Options Are Incorrect:

• A. Medium Viscosity

  • Sedimentation velocity is inversely proportional to viscosity η (eta).
    Higher viscosity = slower settling.

• C. Medium Density

  • Velocity depends on the difference between the particle density and the medium density, not on the medium density alone.

• D. Surface tension

  • Surface tension is not part of Stokes’ law.

Preparation Tips:

Concept

In competitive exams, questions are commonly asked on:

• factors affecting sedimentation rate,

• methods to reduce settling,

• and formulation stability.

Important Relationships from Stokes’ Law

• Factors Affecting Sedimentation

• Particle size ↑ Velocity ↑

• Viscosity ↑ Velocity ↓

• Density difference ↑ Velocity ↑

Application in Pharmaceutics

To prepare stable suspensions:

• Reduce particle size

• Increase viscosity using suspending agents

• Use flocculating agents when required

4. Brownian movement is more significant in

A. Large particles
B. Fine particles
C. Flocculated particles only
D. Dense particles only

Answer: B. Fine particles

Explanation:

Brownian movement is the continuous random zig-zag motion of suspended particles caused by collisions with molecules of the dispersion medium.

It is more significant in fine (small-sized) particles because

• smaller particles are lighter,

• less affected by gravity,

• and more easily kept in motion by molecular bombardment.

This movement helps reduce sedimentation in colloidal and finely divided systems.

Why Other Options Are Incorrect:

• A. Large particles

  • Large particles settle quickly due to gravity and show less Brownian movement.

• C. Flocculated particles only

  • Brownian movement depends mainly on particle size, not flocculation alone.

• D. Dense particles only

  • Dense particles tend to sediment faster.

Preparation Tips:

Key Concept

Brownian movement is important in:

• colloidal dispersions,

• suspension stability,

• and prevention of particle settling.

Important Relationship

Particle Size vs Brownian Movement

• Particle Size Brownian Movement

• Very small particles More significant

• Large particles Less significant

Easy Memory Trick

“Brownian motion belongs to tiny particles.”

Tiny particles:

• move randomly,

• resist settling,

• remain dispersed longer.

Related important Topics

• Tyndall effect

• Sedimentation

• Diffusion

• Zeta potential

• Stokes’ law

Sedimentation rate in suspensions follows Stokes’ law

Smaller particles have lower sedimentation velocity and therefore show more noticeable Brownian movement.

5. Brownian movement:

A. Increases caking
B. Prevents sedimentation of very small particles
C. Increases particle aggregation
D. Reduces viscosity

Answer: B. Prevents sedimentation of very small particles

Explanation:

Brownian movement is the random zig-zag movement of fine particles caused by continuous collision with molecules of the dispersion medium.

This movement helps keep very small particles suspended and, therefore:

• opposes gravitational settling,

• reduces sedimentation,

• and improves dispersion stability.

Hence, it helps prevent sedimentation of very fine particles.

Why Other Options Are Incorrect:

• A. Increases caking

  • Brownian movement helps maintain dispersion and does not promote caking.

• C. Increases particle aggregation

  • Aggregation mainly depends on interparticle forces and zeta potential, not Brownian movement alone.

• D. Reduces viscosity

  • Brownian movement has no direct effect on viscosity.

Important Concept

Brownian movement is especially significant in:

• colloidal dispersions,

• nanosized particles,

• and finely divided suspensions.

Key Relationship

Smaller Particle → Greater Brownian Movement → Less Sedimentation

Stokes’ law explains sedimentation tendency.

As particle radius decreases:

• sedimentation velocity decreases,

• Brownian motion becomes more effective.

Memory Trick

“Tiny particles dance instead of settling.”

That “dance” is Brownian movement.

6. Which one is a desirable property of a pharmaceutical suspension?

A. Rapid caking
B. Difficult redispersion
C. Uniform sedimentation
D. Easy to redisperse after shaking

Answer: D. Easy to redisperse after shaking

Explanation:

An ideal pharmaceutical suspension should form sediment slowly, and, if settling occurs, the sediment should be easily redispersed on gentle shaking.

This ensures:

• uniform dosing,

• better patient compliance,

• and physical stability of the formulation.

A good suspension should not form a hard cake.

Why Other Options Are Incorrect:

• A. Rapid caking

  • Caking is undesirable because the sediment becomes hard and difficult to redisperse.

• B. Difficult redispersion

  • Indicates poor suspension stability.

• C. Uniform sedimentation

  • Sedimentation itself is expected, but the key desirable feature is easy redispersion.

Preparation Tips:

Characteristics of an Ideal Suspension

Remember these points:

• Slow sedimentation

• No hard cake formation

• Easy redispersion

• Uniform particle distribution

• Elegant appearance

Concept

Flocculated vs Deflocculated Suspensions

• Property Flocculated Deflocculated

• Sedimentation Fast Slow

• Cake formation Minimal More

• Redispersion Easy Difficult

Easy Memory Trick

“Good suspension = Shake and use easily.”

If shaking restores uniformity quickly, the suspension is pharmaceutically stable.

Related Topics

• Sedimentation volume

• Degree of flocculation

• Suspensing agents

• Stokes’ law

• Brownian movement

Sedimentation behavior is governed by Stokes’ law

7. A sediment that cannot be easily redispersed is known as:

A. Deflocculates
B. Flocculates
C. Cakes
D. Emulsions

Answer: C. Cakes

Explanation:

A hard sediment that forms at the bottom of a suspension and cannot be easily redispersed by shaking is called a cake or caking.

Caking occurs when particles settle closely together and form a compact mass due to strong attractive forces.

This is undesirable because:

• uniform dosing becomes difficult,

• product stability decreases,

• and patient compliance may be affected.

Why Other Options Are Incorrect:

• A. Deflocculates

  • Deflocculated suspensions contain separate particles and are more prone to caking, but the hard sediment itself is called cake.

• B. Flocculates

  • Floccules are loosely bound aggregates that are usually easy to redisperse.

• D. Emulsions

  • Emulsions are dispersions of one liquid in another liquid.

Concept

Caking is associated mainly with:

• deflocculated suspensions,

• very fine particles,

• prolonged storage.

Flocculated vs Deflocculated Suspension

• Property Flocculated Deflocculated

• Sediment Loose Compact

• Redispersion Easy Difficult

• Caking Rare Common

Easy Memory Trick

“Cake becomes hard.”

If sediment becomes hard and stubborn → think caking.

Pharmacists reduce caking by:

• controlled flocculation,

• increasing viscosity,

• and proper particle size adjustment.

8. Select a correct statement. Particles settle in a deflocculated suspension:

A. Rapidly to form a loose cake
B. Slowly to form a compact cake
C. Rapidly form a cake
D. Slowly to form loose sedimentation

Answer: B. Slowly to form a compact cake

Explanation:

In a deflocculated suspension, particles remain as separate individual particles rather than forming loose aggregates.

Because the particles are very fine:

• they settle slowly,

• but over time they pack closely together,

• forming a compact sediment (cake) that is difficult to redisperse.

This phenomenon is called caking.

Why Other Options Are Incorrect:

• A. Rapidly to form a loose cake

  • Deflocculated particles settle slowly, not rapidly.

• C. Rapidly form a cake

  • Rapid settling is characteristic of flocculated systems.

• D. Slowly to form loose sedimentation

  • Loose sediment is characteristic of flocculated suspensions.

Important Comparison

• Property Flocculated Suspension Deflocculated Suspension

• Particle state Aggregated Separate

• Sedimentation rate Fast Slow

• Sediment nature Loose Compact

• Redispersion Easy Difficult

• Caking tendency Low High

Memory Trick

“Deflocculated = Dense deposit”

• Slow settling

• Dense compact cake

• Difficult shaking

Important Related Concept

Sedimentation rate is governed by Stokes' Law

In deflocculated systems:

• smaller particles settle slowly,

• But eventually, pack tightly to form a cake.

9. The sedimentation volume (F) is expressed as:

F = Vu / V_0

Where "Vu" denotes:

A. Suspension's initial volume
B. Sediment ultimate volume
C. Vehicle volume
D. Particles' volume only

Answer: B. Sediment ultimate volume

Explanation:

Sedimentation volume (F) is an important parameter used to evaluate the physical stability of suspensions.

It is expressed as:

F = Vu / V₀

Where:

• (Vu) = ultimate volume of sediment

• (V0) = original total volume of suspension

So, (Vu) represents the final settled sediment volume after sedimentation is complete.

Why Other Options Are Incorrect:

• A. Suspension's initial volume

  • This is (V_0), not (V_u).

• C. Vehicle volume

  • Vehicle volume is not represented by (V_u).

• D. Particles' volume only

  • (V_u) refers to total sediment volume, not only particle volume.

Preparation Tips:

Interpretation of Sedimentation Volume

• Value of F Meaning

• F = 1 No sedimentation

• F close to Suspension stability

• Low F value Poor stability

Higher sedimentation volume generally indicates:

• better flocculation,

• loose sediment formation,

• easier redispersion.

Memory Trick

“U in (Vu) = Ultimate sediment volume”

10. Sedimentation volume F = 1 indicates

A. Complete caking
B. No sedimentation
C. Ultimate sediment volume equals original suspension volume
D. Highly deflocculated system

Answer: C

Correct Answer: C. Ultimate sediment volume equals original suspension volume

Explanation:

Sedimentation volume is defined as

F = Vu / V₀

Where:

• (V_u) = ultimate sediment volume

• (V_0) = original suspension volume

If
F = 1

then:
V_u = V_0

This means the sediment occupies the entire original volume of the suspension.

It usually indicates a highly flocculated system with no clear supernatant.

Why Other Options Are Incorrect:

• A. Complete caking

  • Caking produces compact sediment and usually lowers sedimentation volume.

• B. No sedimentation

  • Sediment may still be present; (F = 1) simply means sediment volume equals total suspension volume.

• D. Highly deflocculated system

  • Deflocculated systems generally show lower sedimentation volume and compact sediment.

Preparation Tips:

Important Interpretation of Sedimentation Volume

• Sedimentation Volume (F) Interpretation

• F = 1 (V_u = V_0)

• F < 1 Sediment occupies a smaller volume

• Higher F Better flocculation

• Lower F Greater compaction/caking tendency

Memory Trick

“F = 1: Full volume sediment”

The sediment occupies the full original volume.

11. Which suspension shows the highest tendency for cake formation?

A. Deflocculated suspension
B. Structured vehicle suspension
C. Controlled flocculated suspension
D. Flocculated suspension

Answer: A. Deflocculated suspension

Explanation:

A deflocculated suspension has particles that remain as separate individual particles rather than forming loose aggregates (flocs).

Because these fine particles settle slowly, they eventually pack closely together at the bottom, producing a dense compact sediment (cake) that is difficult to redisperse.

Therefore, deflocculated suspensions show the highest tendency for cake formation.

Why Other Options Are Incorrect:

• B. Structured vehicle suspension

  • Structured vehicles increase viscosity and help reduce sedimentation and caking.

• C. Controlled flocculated suspension

  • Controlled flocculation minimizes caking and improves redispersibility.

• D. Flocculated suspension

  • Flocculated particles form loose sediment that is easily redispersed.

Memory Trick

“Deflocculated = Dense deposit = Difficult to redisperse."

Think:

• separate particles,

• compact packing,

• cake formation.

To reduce cake formation:

• controlled flocculation is used,

• viscosity is increased,

• and particle size is optimized.

12. The objective of flocculation is to:

A. Decrease in particle size
B. Prevent caking
C. Increase rate of dissolution
D. Increase in density difference

Answer: B. Prevent caking

Explanation:

Flocculation is the process in which fine particles form loose aggregates called flocs.

The main objective of flocculation is to:

• produce loose, fluffy sediment,

• avoid compact sediment formation,

• and therefore prevent caking.

Flocculated particles settle rapidly, but the sediment formed is easily redispersed on shaking.

Why Other Options Are Incorrect:

• A. Decrease in particle size

  • Flocculation causes particles to aggregate, not decrease in size.

• C. Increase rate of dissolution

  • Flocculation mainly affects suspension stability, not dissolution rate.

• D. Increase in density difference

  • Density difference is not the objective of flocculation.

Preparation Tips:

Purpose of Flocculation

• Prevent hard cake formation

• Improve redispersibility

• Maintain suspension stability

Flocculated vs Deflocculated Suspensions

• Property Flocculated Deflocculated

• Sediment Loose Compact

• Caking Minimal High

• Redispersion Easy Difficult

• Sedimentation rate Fast Slow

Memory Trick

“Flocculation forms fluffy flocs.”

Fluffy sediment:

• settles quickly,

• but redisperses easily,

• preventing caking.

Flocculated particles behave like larger particles and settle faster, but they form non-compact sediment.

13. Controlled flocculation is achieved on

A. Removal of electrolytes completely
B. Maintenance of optimum zeta potential
C. Increase in particle density
D. Decrease in viscosity to zero

Answer: B. Maintenance of optimum zeta potential

Explanation:

Controlled flocculation is achieved by maintaining an optimum zeta potential so that particles form loose flocs without becoming completely deflocculated or excessively aggregated.

• If the zeta potential is too high = particles repel each other strongly = deflocculation occurs.

• If the zeta potential is reduced to an optimum level = particles form weakly bonded flocs = controlled flocculation occurs.

This helps:

• prevent caking,

• improve redispersibility,

• and maintain suspension stability.

Why Other Options Are Incorrect:

• A. Removal of electrolytes completely

  • Electrolytes are often added intentionally to adjust zeta potential and promote flocculation.

• C. Increase in particle density

  • Density affects sedimentation rate but does not control flocculation directly.

• D. Decrease in viscosity to zero

  • Lower viscosity increases sedimentation and destabilizes suspension.

Preparation Tips:

Concept: Zeta Potential

Zeta potential represents the electrical charge around dispersed particles.

• Zeta Potential Result

• High Deflocculation

• Optimum Controlled Flocculation

• Very low Coagulation

Memory Trick

“Optimum charge gives optimum flocculation.”

Neither too much repulsion nor complete neutralization is desirable.

Important Related Topics

• Electrical double layer

• DLVO theory

• Flocculating agents

• Suspension stability

• Sedimentation volume

14. Which one acts as a flocculating agent?

A. Surfactants
B. Electrolytes
C. Polymers
D. All of the above

Answer: D. All of the above

Explanation:

Flocculation in pharmaceutical suspensions can be achieved using different types of flocculating agents, including:

• Electrolytes

  • Reduce zeta potential and decrease particle repulsion.

• Surfactants

  • Modify surface properties and help controlled aggregation.

• Polymers

  • Produce bridging between particles to form flocs.

Therefore, all of these can act as flocculating agents.

Why Each Option Works

• Flocculating Agent Mechanism

• Electrolytes Reduce electrical repulsion

• Surfactants Alter interfacial properties

• Polymers Bridge particles together

Preparation Tips:

Concept

Flocculating agents are used to:

• prevent caking,

• improve redispersibility,

• and produce stable suspensions.

Controlled flocculation depends on optimum zeta potential.

Sedimentation rate follows Stokes' Law

Where increasing particle aggregation effectively increases particle size and changes sedimentation behavior.

Memory Trick

“ESP causes flocculation."

• E = Electrolytes

• S = Surfactants

• P = Polymers

15. Electrolyte addition produces flocculation on:

A. Decrease in sedimentation
B. Decrease in zeta potential
C. Increase in Brownian movement
D. Increase of particle charge

Answer: B. Decrease in zeta potential

Explanation:

Electrolytes produce flocculation by reducing the zeta potential of suspended particles.

Normally, particles in suspension carry similar charges and repel each other.
When electrolytes are added:

• Oppositely charged ions compress the electrical double layer.

• Repulsive forces decrease,

• Particles come closer, And

• Loose flocs are formed.

Thus, electrolyte-induced flocculation occurs due to a decrease in zeta potential.

Why Other Options Are Incorrect:

• A. Decrease in sedimentation

  • Flocculation usually increases sedimentation rate because flocs are larger.

• C. Increase in Brownian movement

  • Brownian movement depends mainly on particle size, not electrolyte addition.

• D. Increase of particle charge

  • Electrolytes reduce effective surface charge, not increase it.

Preparation Tips:

Mechanism

Electrolytes = Reduce zeta potential = Controlled flocculation

This is one of the most frequently asked suspension concepts in GPAT, NIPER, and Pharmacist recruitment exams

Memory Trick

“Electrolytes eliminate excess charge.”

Less repulsion = particles join loosely = flocculation occurs.

16. Which polymer acts as a suspending agent?

A. Gelatin
B. Acacia
C. Sodium Carboxymethyl Cellulose
D. All of the above

Answer: D. All of the above

Explanation:

Suspending agents are substances that increase the viscosity of the dispersion medium and help keep particles uniformly dispersed.

All the listed substances can act as suspending agents:

• Gelatin: protective colloid and viscosity enhancer

• Acacia: natural gum used as suspending and emulsifying agent

• Sodium Carboxymethyl Cellulose (NaCMC): widely used hydrophilic polymer suspending agent

Therefore, all of them are used in pharmaceutical suspensions.

Why Suspending Agents Are Important

They help to:

• reduce sedimentation,

• improve physical stability,

• prevent caking,

• and improve redispersibility.

Preparation Tips:

Classification of Suspending Agents

• Type Examples

• Natural polymers Acacia, Tragacanth, Gelatin

• Semisynthetic polymers Sodium CMC, Methylcellulose

• Synthetic polymers Carbopol, PVP,

• Clays Bentonite, Veegum

Memory Trick

“GAS keeps particles suspended."

• G = Gelatin

• A = Acacia

• S = Sodium CMC

Concept

Suspending agents increase viscosity, thereby reducing sedimentation velocity, according to Stokes' Law

17. Structured vehicles. The purpose is to:

A. Develop caking
B. Reduction in viscosity
C. Retard particles sedimentation
D. Increase in sedimentation rate

Answer: C

Correct Answer: C. Retard particles sedimentation

Explanation:

A structured vehicle is a viscous dispersion medium used in suspensions to slow down the settling of suspended particles.

Its main purpose is to

• Increase viscosity,

• Reduce sedimentation rate,

• Improve physical stability And

• Help maintain uniform distribution of particles.

Thus, structured vehicles retard particle sedimentation.

Why Other Options Are Incorrect:

• A. Develop caking

  • Structured vehicles help reduce caking, not develop it.

• B. Reduction in viscosity

  • They actually increase viscosity.

• D. Increase in sedimentation rate

  • Increased viscosity slows sedimentation.

Preparation Tips:

Concept

Structured vehicles are commonly prepared using:

• Sodium CMC

• Methylcellulose

• Bentonite

• Tragacanth

These agents create a network structure that keeps particles suspended.

Important Relationship

Sedimentation rate is inversely proportional to viscosity.

Memory Trick

“Structured vehicle = Structured stability”

More structure in vehicle = slower settling.

18. The degree of flocculation (β) is:

β = F ​/ F∞​

A higher β value means

A. Rapid flocculation
B. Rapid caking
C. Smaller particles
D. Low sedimentation

Answer: A. Rapid flocculation

Explanation:

The degree of flocculation β (beta) compares the sedimentation volume of a flocculated suspension with that of a deflocculated suspension.

It is expressed as:

β=F​/F∞​

Where:

• (F) = sedimentation volume of flocculated suspension

• F∞​(Finfty) = sedimentation volume of deflocculated suspension

A higher β (beta) value indicates

• Greater flocculation,

• Formation of loose flocs,

• Improved redispersibility, and

• Lower tendency for caking.

Thus, higher β (beta) means greater or more effective flocculation.

Why Other Options Are Incorrect:

• B. Rapid caking

  • Flocculation actually helps prevent caking.

• C. Smaller particles

  • Flocculation forms larger aggregates (flocs).

• D. Low sedimentation

  • Flocculated particles usually sediment faster because of increased particle size.

Preparation Tips:

Important Interpretation

• Degree of Flocculation β (beta) Meaning

• Higher β (beta) Better flocculation

• Lower β (beta) More deflocculated system

Memory Trick

“Higher β = Better flocculation."

Think:

• Bigger flocs,

• Easy redispersion,

• Less caking.

Sedimentation rate follows Stokes' Law
19. Which statement about flocculated suspensions is correct?

A. Slow Sedimentation
B. Difficult to redisperse sediment.
C. Easy to redisperse sediment
D. Physically unstable

Answer: C. Easy to redisperse sediment

Explanation:

In a flocculated suspension, particles form loose aggregates called flocs.

These flocs:

• Settle rapidly,

• Form a loose and porous sediment, and

• Can be easily redispersed on shaking.

Therefore, flocculated suspensions are preferred because they minimize hard cake formation.

Why Other Options Are Incorrect:

• A. Slow sedimentation

  • Flocculated particles are larger aggregates, so they settle faster.

• B. Difficult to redisperse sediment

  • This is characteristic of deflocculated suspensions.

• D. Physically unstable

  • Controlled flocculated suspensions are comparatively more stable against caking.

Memory Trick

“Flocs are fluffy.”

Fluffy sediment:

• Settles fast,

• But shakes back easily.

Flocculated particles behave as larger aggregates, increasing sedimentation velocity but preventing compact cake formation.

20. The suspended particles' settling velocity increases with:

A. Increased viscosity
B. Decreased particle size
C. Increased particle radius
D. Decreased gravity

Answer: C

21. Sedimentation velocity is inversely proportional to

A. Viscosity of medium
B. Density difference
C. Particle radius
D. Gravity

Answer: A

22. Which of the following methods reduces sedimentation in suspensions?

A. Increase in particle size
B. Reduction in viscosity
C. Use of structured vehicles
D. Increased density difference

Answer: C. Use of structured vehicles

Explanation:

Structured vehicles increase the viscosity of the dispersion medium, which slows down the settling of suspended particles and reduces sedimentation. (Stokes’ law)

Sedimentation velocity is inversely proportional to viscosity.

Therefore:

• Increasing viscosity using structured vehicles decreases the sedimentation rate.

Examples of structured vehicles:

• Sodium CMC

• Methylcellulose

• Bentonite

• Tragacanth

Why Other Options Are Incorrect:

• A. Increase in particle size

  • Larger particles settle faster because (v \propto r^2).

• B. Reduction in viscosity

  • Lower viscosity increases sedimentation.

• D. Increased density difference

  • Greater density difference increases settling rate.

Preparation Tips:

Factors Affecting Sedimentation

• Factor Effect on Sedimentation

• Particle size ↑ Sedimentation ↑

• Viscosity ↑ Sedimentation ↓

• Density difference ↑ Sedimentation ↑

Memory Trick

"A viscous vehicle prevents velocity.”

Higher viscosity = slower settling.

23. Brownian movement is caused by

A. Gravitational pull
B. Collision of particles with molecules of dispersion medium
C. Electrostatic attraction
D. Sedimentation force

Answer: B. Collision of particles with molecules of dispersion medium

Explanation:

Brownian movement is the continuous random zig-zag movement of suspended particles caused by their collision with the molecules of the dispersion medium.

This phenomenon is especially significant in:

• colloidal particles,

• very fine suspensions,

• and nanosized systems.

Brownian movement helps keep small particles suspended and opposes sedimentation.

Why Other Options Are Incorrect:

• A. Gravitational pull

  • Gravity causes sedimentation, not Brownian movement.

• C. Electrostatic attraction

  • Electrostatic forces influence flocculation and stability, not Brownian motion directly.

• D. Sedimentation force

  • Sedimentation results from gravity acting on particles.

GPAT Preparation Tips:

Important Features of Brownian Movement

• More prominent in small particles

• Opposes sedimentation

• Caused by molecular bombardment

• Important in colloidal stability

Memory Trick

“Molecules make particles move.”

• Tiny molecules continuously hit suspended particles, causing random motion.

• Sedimentation behavior follows Stokes' Law

• Smaller particles show greater Brownian movement and lower sedimentation velocity.

24. A suspension with a high zeta potential is likely to be

A. Flocculated
B. Deflocculated
C. Coagulated
D. Creaming

Answer: B. Deflocculated

Explanation:

Zeta potential is the electrical charge surrounding suspended particles.

When zeta potential is high:

• particles strongly repel each other,

• aggregation is prevented,

• particles remain separate,

• resulting in a deflocculated suspension.

Thus, high zeta potential favors deflocculation.

Why Other Options Are Incorrect:

• A. Flocculated

  • Flocculation occurs when the zeta potential is reduced to an optimum level.

• C. Coagulated

  • Coagulation occurs when repulsive forces are almost completely neutralized.

• D. Creaming

  • Creaming is related to emulsions, not suspensions.

Memory Trick

“High charge keeps particles apart.”

More repulsion = less aggregation = deflocculation.

Concept

• Electrolytes produce flocculation by lowering the zeta potential.

• Sedimentation behavior is described by Stokes' Law

• Deflocculated particles settle slowly but tend to form a compact cake on standing.

25. The ideal pharmaceutical suspension should possess:

A. Slow sedimentation and easy redispersion
B. Complete absence of sedimentation
C. Rapid sedimentation and hard cake
D. High crystal growth

Answer: A. Slow sedimentation and easy redispersion

Explanation:

An ideal pharmaceutical suspension should:

• Sediment slowly,

• Form no hard cake, and

• Redisperse easily on gentle shaking.

These properties ensure

• uniform dosing,

• physical stability,

• and better patient acceptability.

Some sedimentation is unavoidable in suspensions, but the sediment should remain loose and easily redispersible.

Why Other Options Are Incorrect:

• B. Complete absence of sedimentation

  • Practically difficult because suspended particles eventually settle under gravity.

• C. Rapid sedimentation and hard cake

  • Hard cake formation is undesirable and leads to poor redispersibility.

• D. High crystal growth

  • Crystal growth increases particle size and may destabilize the suspension.

Preparation Tips:

Characteristics of an Ideal Suspension

• Slow sedimentation

• Easy redispersion

• No caking

• Uniform particle distribution

• Elegant appearance

Related Concept

Sedimentation velocity is governed by Stokes' Law

To obtain an ideal suspension:

• Particle size is reduced,

• Viscosity is increased, and

• Controlled flocculation is used.

Memory Trick

“Ideal suspension = Settle slowly, shake easily.”

26. Which one is most useful to evaluate the physical stability of suspensions?

A. Sedimentation volume
B. Partition coefficient
C. Refractive index
D. Surface tension

Answer: A. Sedimentation volume

Explanation:

Sedimentation volume is one of the most important parameters used to evaluate the physical stability of pharmaceutical suspensions.

It indicates:

• the extent of sedimentation,

• degree of flocculation,

• and ease of redispersion.

It is expressed as:

F = Vu / V₀}

Where:

• Vu = ultimate sediment volume

• V₀ = original suspension volume

A higher sedimentation volume generally indicates better suspension stability and less compact sediment formation.

Why Other Options Are Incorrect:

• B. Partition coefficient

  • Used to evaluate drug lipophilicity and distribution, not suspension stability.

• C. Refractive index

  • Mainly used in optical characterization of liquids.

• D. Surface tension

  • Important in wetting and emulsions, but not the primary parameter for suspension stability.

Importance of Sedimentation Volume

It helps assess:

• Flocculation behavior

• Redispersibility

• Caking tendency

• Physical stability

Memory Trick

“F tells how firmly particles settle.”

Higher (F) = loose sediment = better redispersion.

27. Controlled structure flocculation combines

A. Deflocculation and electrolysis
B. Flocculation with structured vehicles
C. Emulsification and solubilization
D. Coagulation and precipitation

Answer: B. Flocculation with structured vehicles

Explanation:

Controlled structured flocculation is a formulation approach in which:

• Particles are first flocculated to prevent caking, and

• Then a structured vehicle is added to reduce the sedimentation rate.

This combination provides:

• Easy redispersion

• Reduced caking

• Slower sedimentation

• Improved physical stability

Thus, it combines flocculation with structured vehicles.

Why Other Options Are Incorrect:

• A. Deflocculation and electrolysis

  • Deflocculation promotes compact cake formation.

• C. Emulsification and solubilization

  • These are related to emulsions and solutions, not suspension stabilization.

• D. Coagulation and precipitation

  • Coagulation may destabilize the suspension.

Purpose of Controlled Structured Flocculation

• Component Function

• Flocculation Prevents caking

• Structured vehicle Slows sedimentation

This is considered one of the best approaches for preparing stable pharmaceutical suspensions.

Memory Trick

“Flocs + Structure = Stable suspension”

• Flocks prevent a compact cake

• Structured vehicle slows settling

28. Which suspending agent increases viscosity by forming a network structure?

A. Tragacanth
B. Bentonite
C. Sodium alginate
D. All of the above

Answer: D

Explanation:
All three agents — tragacanth, bentonite, and sodium alginate — act as suspending agents by increasing viscosity and helping form a structured network in the dispersion medium, which reduces sedimentation of suspended particles.

• Tragacanth: Natural gum forming a viscous colloidal network

• Bentonite: Swelling clay producing a thixotropic gel structure

• Sodium alginate: Hydrophilic polymer forming a viscous network in water

These structured vehicles improve the physical stability of pharmaceutical suspensions.

29. Which condition is suitable for sedimentation according to Stokes’ law?

A. Higher viscosity
B. Smaller particle size
C. Increased Brownian movement
D. Greater density difference between particle and medium

Answer: D

According to Stokes’ law:

A greater density difference increases the sedimentation rate.

Why are other options incorrect?

• A. Higher viscosity: decreases sedimentation

• B. Smaller particle size: decreases sedimentation

• C. Increased Brownian movement: opposes settling of particles

30. The main disadvantage of a deflocculated suspension:

A. Rapid sedimentation
B. Poor elegance
C. Low bioavailability
D. Formation of compact sediment

Answer: D. Formation of compact sediment

Explanation:
In a deflocculated suspension, particles remain as separate entities and settle slowly. However, during settling, they pack closely together and form a hard, compact cake (caking), which is difficult to redisperse.

Characteristics of deflocculated suspensions:

• Slow sedimentation

• Clear supernatant

• Formation of compact sediment (cake)

• Difficult redispersion

In contrast, flocculated suspensions sediment rapidly but form loose, easily redispersible sediment.

31. Which of the following is the best explanation of controlled flocculation?

A. Increase in crystal growth
B. Complete elimination of sedimentation
C. Permanent aggregation of particles
D. Formation of weak particle aggregates with easy redispersion

Answer: D. Formation of weak particle aggregates with easy redispersion

Explanation:
Controlled flocculation is the process in which particles form loosely bound aggregates (flocs). These flocs settle rapidly but do not form a hard cake, making the suspension easy to redisperse on shaking.

Advantages:

• Prevents caking

• Improves redispersibility

• Maintains physical stability of suspension

Why others are incorrect:

• A. Increase in crystal growth: Related to Ostwald ripening, not flocculation

• B. Complete elimination of sedimentation: Impossible in most suspensions

• C. Permanent aggregation: Describes coagulation, not controlled flocculation

32. Viscosity enhancers are added to:

A. Increase density difference
B. Increase dissolution rate
C. Minimize particle settling
D. Improve sedimentation rate

Answer: C Minimize particle settling

Explanation:
Viscosity enhancers (suspending agents) increase the viscosity of the dispersion medium, thereby reducing the sedimentation rate of suspended particles according to Stokes’ law.

As viscosity increases, sedimentation velocity decreases.

Examples of viscosity enhancers:

• Tragacanth

• Sodium alginate

• Methylcellulose

• Bentonite

Why others are incorrect:

• A. Increase density difference: would increase settling

• B. Increase dissolution rate: not the primary role in suspensions

• D. Improve sedimentation rate: opposite effect of viscosity enhancers

33. If the particle radius of a suspension is doubled, the sedimentation velocity will be:

A. Four times
B. Eight times
C. Double
D. Half

Answer: . Four times

Explanation:

According to Stokes’ law:

v=2r²(ρs−ρo)g/9η

Sedimentation velocity (v) is directly proportional to the square of particle radius (r²).

If the particle radius is doubled:

v ∝ (2r)²

v ∝ 4r²

Therefore, the sedimentation velocity becomes four times.

Why Other Options Are Incorrect:

• B. Eight times

  • Would occur if velocity were proportional to r³, which is incorrect.

• C. Double

  • Velocity depends on the square of the radius, not directly on the radius.

• D. Half

  • Opposite of the correct relationship.

Preparation Tips:

Numerical Concept

Remember: v ∝ r²

• Change in Radius Change in Sedimentation

• Velocity Radius doubled Velocity ×4

• Radius tripled Velocity ×9

• Radius halved Velocity ×1/4

Memory Trick

“Radius squared rules settling speed.”

Small increase in particle size = major increase in sedimentation.

Related Concepts

To reduce sedimentation:

• Reduce particle size,

• Increase viscosity,

• Use structured vehicles.

34. A suspension contains particles with a radius of 2 μm. If the particle radius is reduced to 1 μm, the sedimentation velocity will be

A. Four times
B. Double
C. Half
D. One-fourth

Answer: D. One-fourth

Explanation:

According to Stokes’ law:

v=2r²(ρs−ρo)g/9η

Sedimentation velocity (v) is directly proportional to the square of particle radius:

v∝r2

Initial radius:
r1​=2μm

Final radius:
r₂=1 μm

Therefore:

v2/v1=(1/2)²

v2/v1=1/4

So, the sedimentation velocity becomes one-fourth of the original value.

Why Other Options Are Incorrect:

• A. Four times

  • Velocity decreases, not increases.

• B. Double

  • Incorrect proportional relationship.

• C. Half

  • Velocity depends on the square of the radius, not directly on the radius.

Easy Memory Trick

“Half radius = quarter settling.”

Smaller particles settle much more slowly.

35. Which condition produces the highest physical stability in a suspension?

A. Complete deflocculation

B. High zeta potential with low viscosity
C. Controlled flocculation with structured vehicle
D. Rapid sedimentation with hard cake formation

Answer: C. Controlled flocculation with structured vehicle

Explanation:

The highest physical stability in a pharmaceutical suspension is achieved by combining:

• Controlled flocculation and

• A structured vehicle.

This system provides:

• Loose, easily redispersible sediment

• Reduced caking tendency

• Slower sedimentation

• Improved uniformity during storage

Controlled flocculation prevents compact cake formation, while the structured vehicle increases viscosity and slows settling.

Why Other Options Are Incorrect:

• A. Complete deflocculation

  • Leads to compact cake formation and difficult redispersion.

• B. High zeta potential with low viscosity

  • Causes deflocculation and faster sedimentation due to low viscosity.

• D. Rapid sedimentation with hard cake formation

  • Represents poor physical stability.

Preparation Tips:

Ideal Suspension Strategy

“Flocculate + Structure = Stability”

• Flocculation: Prevents caking

• Structured vehicle: Retards sedimentation

This combination is considered the best approach for stable suspensions.

Memory Trick

“Controlled flocs in a viscous vehicle give maximum stability.”

36. In Stokes’ law, the settling velocity becomes zero if:

A. Increase in particle size to very large
B. Increase in gravitational force
C. Low viscosity
D. Density of particle and density of medium are equal

Answer: D. Density of particle and density of medium are equal

Explanation:

According to Stokes’ law:

v=2r²(ρs−ρo)g/9η

Where:

• v = sedimentation velocity

• ρs (rho)​ = density of particle

• ρo (rho)​ = density of dispersion medium

If the density of the particle becomes equal to the density of the medium:

ρs−ρo=0

Therefore:

v=0

So, no sedimentation occurs.

Why Other Options Are Incorrect:

• A. Increase in particle size to very large

  • Larger particles settle faster.

• B. Increase in gravitational force

  • Increases sedimentation velocity.

• C. Low viscosity

  • Lower viscosity increases settling rate.

Memory Trick

“No density difference = No settling.”

37. The sedimentation volume of a suspension was found to be 0.8. This indicates that:

A. Final sediment occupies 80% of original suspension volume
B. 20% of drug is degraded
C. Suspension is completely unstable
D. No sedimentation occurs

Answer: A

Correct Answer: A. Final sediment occupies 80% oftoriginal suspension volume

Explanation:

Sedimentation volume (F) is defined as:

F = Vu/Vo

Where:

• (Vu) = Ultimate sediment volume

• (Vo) = Original suspension volume

If
F = 0.8

then:
Vu = 0.8 X Vo = 0.8 X 100.

This means the final sediment occupies 80% of the original suspension volume.

Why Other Options Are Incorrect:

• B. 20% of the drug is degraded

  • Sedimentation volume has no relation to chemical degradation.

• C. Suspension is completely unstable

  • A value of 0.8 usually indicates reasonably good suspension characteristics.

• D. No sedimentation occurs

  • No sedimentation would correspond to (F = 1).

Preparation Tips:

Interpretation of Sedimentation Volume

• Sedimentation Volume (F) Meaning

• F = 1 Sediment occupies full original volume

• High F Better flocculation and redispersibility

• Low F Compact sediment and greater caking tendency

Memory Trick

“F value directly tells the fraction of settled volume.”

38. A suspension has an ultimate sediment volume of 40 mL and an original suspension volume of 100 mL. Calculate sedimentation volume (F).

A. 0.2
B. 0.4
C. 2.5
D. 4.0

Answer: B. 0.4

Sedimentation volume (F) is calculated by:

F=Vu/Vo

Where:

• (Vu) = ultimate sediment volume = 40 mL

• (Vo) = original suspension volume = 100 mL

F=40/100=0.4

Therefore, the sedimentation volume is 0.4.

39. Which statement best explains the role of Brownian movement in suspensions?

A. It causes irreversible aggregation
B. It prevents the movement of particles
C. It opposes the sedimentation of fine particles
D. It increases the density difference

Answer: C. It opposes the sedimentation of fine particles

Explanation:
Brownian movement is the continuous random motion of small particles caused by collisions with molecules of the dispersion medium. This random motion helps keep fine particles suspended and opposes their settling under gravity.

Key point:

• More significant in very small particles (colloidal range)

• Helps improve stability of suspensions by reducing sedimentation

Why others are incorrect:

• A. Irreversible aggregation is related to coagulation/flocculation

• B. Brownian movement causes motion, not prevents it

• D. It does not affect density difference

40. A suspension becomes difficult to redisperse after storage due to:

A. Controlled flocculation
B. Formation of compact cake
C. Increased Brownian movement
D. Decrease in particle interaction

Answer: B

Correct Answer: B. Formation of compact cake

Explanation:

A suspension becomes difficult to redisperse when particles settle and form a hard, compact sediment called a "cake."

This compact cake:

• Packs tightly at the bottom,

• Resists shaking, and

• Prevents uniform redistribution of particles.

Caking commonly occurs in deflocculated suspensions.

Why Other Options Are Incorrect:

• A. Controlled flocculation

  • Controlled flocculation helps prevent caking and improves redispersibility.

• C. Increased Brownian movement

  • Brownian movement helps keep small particles suspended.

• D. Decrease in particle interaction

  • Reduced interaction does not directly cause hard-cake formation.

Preparation Tips:

Causes of Difficult Redispersion

• Compact sediment formation

• Deflocculated particles

• Long storage

• Crystal growth

Memory Trick

“Cake causes shaking problems.”

Hard cake = difficult redispersion.

41. Which parameter can be increased to decrease sedimentation without changing particle size?

A. Density difference
B. Viscosity of medium
C. Gravitational force
D. Particle radius

Answer: B. Viscosity of medium

According to Stokes’ law:

Sedimentation velocity (v) is inversely proportional to viscosity . Therefore, increasing the viscosity of the dispersion medium decreases sedimentation without changing particle size.

Why others are incorrect:

• A. Density difference: increasing it increases sedimentation

• C. Gravitational force: increasing it increases settling

• D. Particle radius: changing it alters particle size, which the question excludes

42. If the viscosity of the dispersion medium is increased 3-fold, sedimentation velocity according to Stokes’ law will

A. Increase three times
B. Become one-third
C. Remain unchanged
D. Become nine times less

Answer: B. Become one-third

According to Stokes’ law:

v=2r²(ρp−ρm)g/9η

Sedimentation velocity (v) is inversely proportional to viscosity.

If viscosity increases 3 times:

v ∝ 1​/η

vnew = v / 3

Therefore, the sedimentation velocity becomes one-third of the original value.

43. In a deflocculated suspension, particles possess:

A. Strong attractive forces
B. High zeta potential
C. No surface charge
D. Large floccules

Answer: B. High zeta potential

In a deflocculated suspension, particles remain separate and do not form aggregates because they have a high zeta potential, which creates strong electrostatic repulsion between particles and prevents flocculation.

Why others are incorrect:

• A. Strong attractive forces → leads to flocculation, not deflocculation

• C. No surface charge → not true; particles usually carry charge

• D. Large floccules → characteristic of flocculated systems, not deflocculated ones

44. Which of the following combinations gives optimum suspension stability?

A. Fine particles + low viscosity + high density difference
B. Controlled flocculation + structured vehicle
C. Large particles + low zeta potential
D. Deflocculation + low viscosity

Answer: B

The correct answer is:

B. Controlled flocculation + structured vehicle

Explanation:

Optimum suspension stability is achieved when:

• Particles are in a controlled flocculated state (prevents caking, ensures easy redispersion)

• The medium is a structured vehicle (high-viscosity system) (reduces sedimentation rate)

This combination ensures

• No hard cake formation

• Slow but controlled settling

• Easy redispersion after shaking

• Overall physical stability

Why others are incorrect:

• A. Fine particles + low viscosity + high density difference → increases sedimentation and instability

• C. Large particles + low zeta potential → promotes rapid settling and poor stability

• D. Deflocculation + low viscosity → leads to caking and poor redispersibility

45. A suspension exhibits crystal growth during storage. This phenomenon is called

A. Peptization
B. Ostwald ripening
C. Flocculation
D. Coacervation

Answer: B. Ostwald ripening

Explanation:

Ostwald ripening is the process in which:

• smaller particles dissolve,

• and redeposit onto larger particles during storage.

As a result:

• Crystal size increases,

• Suspension stability decreases, and

• Caking tendency may increase.

This occurs because smaller particles have higher surface free energy and greater solubility than larger particles.

Why Other Options Are Incorrect:

• A. Peptization

  • Conversion of precipitate into colloidal dispersion.

• C. Flocculation

  • Formation of loose particle aggregates (flocs).

• D. Coacervation

  • Phase separation process used in microencapsulation.

Preparation Tips:

Effects of Ostwald Ripening

• Increase in crystal size

• Increased sedimentation

• Possible caking

• Reduced suspension stability

Memory Trick

“Small crystals sacrifice themselves to grow bigger crystals.”

46. The major advantage of structured vehicles in suspensions is that they:

A. Eliminate sedimentation completely
B. Increase particle density
C. Retard settling of particles by increasing viscosity
D. Reduce zeta potential to zero

Answer: C

Correct Answer: C. Retard settling of particles by increasing viscosity

Explanation:

Structured vehicles are viscous dispersion media used in pharmaceutical suspensions to slow down sedimentation.

They work by:

• increasing viscosity of the medium,

• reducing particle settling rate,

• improving physical stability,

• and maintaining uniform dispersion.

According to Stokes’ law:

• Sedimentation velocity is inversely proportional to viscosity.
  Therefore, increasing viscosity retards particle settling.

Why Other Options Are Incorrect:

• A. Eliminate sedimentation completely

  • Sedimentation can only be reduced, not completely eliminated.

• B. Increase particle density

  • Structured vehicles mainly affect viscosity, not density.

• D. Reduce zeta potential to zero

  • Zeta potential adjustment relates to flocculation, not structured vehicles.

Preparation Tips:

Common Structured Vehicle Agents

• Sodium CMC

• Methylcellulose

• Bentonite

• Tragacanth

These agents improve suspension stability by slowing sedimentation.

Memory Trick

“Structured vehicle = Slower settling through viscosity.”

Dr Alok Singh