GPAT, NIPER, Pharmacist Recruitment (AIIMS, SSC, HSSC, RRB, ESIC, DSSB) Exams MCQs on Solubility & Distribution Phenomenon.

1. The maximum amount of solute soluble in a given quantity of solvent at a specified temperature is

A. Partition coefficient
B. Solubility
C. Distribution ratio
D. Miscibility

Answer: B. Solubility

Correct Answer: B. Solubility

Explanation:

Solubility is defined as the maximum amount of solute that can dissolve in a given quantity of solvent at a specified temperature to form a saturated solution.

• Partition coefficient: Ratio of distribution of a solute between two immiscible solvents.

• Distribution ratio: Total concentration ratio of all forms of solute between two phases.

• Miscibility: The ability of two liquids to mix in all proportions.

Solubility = Maximum amount of solute dissolved / Given amount of solvent at specified temperature

2. The distribution law states that when a solute distributes itself between two immiscible solvents, the ratio of concentrations remains constant at:

A. Constant Temperature
B. Constant Pressure
C. Constant Volume
D. Constant Solvent density

Answer: A. Temperature remains constant

Explanation:

According to the Nernst Distribution Law, when a solute distributes itself between two immiscible solvents, the ratio of its concentrations in the two phases remains constant provided the temperature remains constant, and the solute exists in the same molecular state in both solvents.

The law is expressed as:

K=C1 / C2​​

Where:

• K = Distribution coefficient

• C1​ and C2​ = Concentrations of solute in the two immiscible solvents at equilibrium.

3. Nernst distribution law is

A. K = C1/C2
B. K = C1 + C2
C. K = C1 X C2
D. K = 1/C1.C2

Answer: A. K = C1/C2

Explanation:

The Nernst Distribution Law states that a solute distributes itself between two immiscible solvents such that the ratio of concentrations in the two solvents remains constant at equilibrium and constant temperature.

The mathematical expression is:

K = C1/C2​​

Where:

• K = Distribution coefficient or partition coefficient

• C1​ = Concentration of solute in first solvent

• C2​ = Concentration of solute in the second solvent

4. Which ONE increases solids' solubility in liquids?

A. Temperature Increase
B. Temperature Decrease
C. Pressure Increase
D. Reduced surface area

Answer: A. Increase in temperature

Explanation:

For most solid solutes, solubility in liquids increases with an increase in temperature because higher temperatures provide more kinetic energy for dissolution.

This relationship is commonly represented conceptually as

For most solids: ↑T = ↑Solubility

Why are other options incorrect?

• B. Temperature Decrease: Usually decreases the solubility of solids.

• C. Pressure Increase: Has little effect on solids dissolved in liquids; pressure mainly affects gases.

• D. Reduced surface area: Slows dissolution rate, not equilibrium solubility

5. The gases' solubility in liquids depends on:

A. Increases with temperature
B. Decreases with temperature
C. Remains unaffected
D. Only on pressure

Answer: B. Decreases with temperature

Explanation:

The solubility of most gases in liquids decreases with increasing temperature because gas molecules gain more kinetic energy and escape more easily from the liquid phase.

This relationship can be represented as:

Increase in Temperature⇒decrease in Gas Solubility in Liquids

Although gas solubility is also affected by pressure (described by Henry's Law), among the given options, the correct statement is that it decreases with temperature.

6. Henry’s law is related to:

A. Solubility of solids in liquids
B. Solubility of gases in liquids
C. Solubility of liquids in solids
D. Partition coefficient

Answer: B. Solubility of gases in liquids

Explanation:

Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid at constant temperature.

The law is expressed as:

C=kP

Where:

• C = Concentration (solubility) of gas in liquid

• k = Henry’s law constant

• P = Partial pressure of the gas

Thus, Henry’s law specifically explains the solubility of gases in liquids.

7. Raoult’s law applies to:

A. Dilute solutions
B. Suspensions only
C. Colloidal systems only
D. Emulsions only

Answer: A. Dilute ideal solutions

Explanation:

Raoult's law applies mainly to ideal and dilute solutions. It states that the partial vapor pressure of a component in a solution is directly proportional to its mole fraction.

The mathematical expression is:

P=P∘XP

Where:

• P = Vapor pressure of the component in solution

• P∘ = Vapor pressure of pure component

• X = Mole fraction of the component

Therefore, Raoult’s law is associated with dilute solutions and not suspensions, colloids, or emulsions.

8. Raoult’s law: The vapor pressure of a solution depends upon

A. Solute Mole fraction
B. Solvent Mole fraction
C. Solvent Density
D. Solvent Viscosity

Answer: B. Mole fraction of solvent

Explanation:

According to Raoult's Law, the vapor pressure of a solution is directly proportional to the mole fraction of the solvent in the solution.

It is represented by:

P=P∘Xsolvent​

Where:

• P = Vapor pressure of solution

• P∘ = Vapor pressure of pure solvent

• Xsolvent​ = Mole fraction of solvent

As solute concentration increases, solvent mole fraction decreases, causing vapor pressure to decrease.

9. Which solvents have the highest dielectric constant?

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

Answer: C. Water

Explanation:

The dielectric constant is a measure of a solvent’s ability to reduce electrostatic forces between charged particles. A higher dielectric constant indicates greater polarity and better ability to dissolve ionic substances.

Among the given solvents:

• Water: Very high dielectric constant (~78)

• Ether: Low

• Chloroform: Moderate to low

• Benzene: Very low

Therefore, water has the highest dielectric constant.

10. Solvent High dielectric constant

A. Reduction in ionization
B. Solute ionization enhances
C. Decreases polarity
D. Prevents dissolution

Answer: B. Enhances ionization of solute

Explanation:

A solvent with a high dielectric constant reduces electrostatic attraction between ions, thereby promoting ion separation and increasing solute ionization.

This effect can be summarized as

↑Dielectric Constant=↑Ionization

Therefore:

• High dielectric constant → greater polarity → enhanced ionization and dissolution of ionic compounds.

Incorrect options:

• A. Reduction in ionization → opposite effect.

• C. Decreases polarity → high dielectric constant actually indicates high polarity.

• D. Prevents dissolution → generally enhances dissolution of ionic substances.

11. Which interaction is the strongest determining factor for solubility?

A. Van der Waals interaction
B. Dipole-dipole interaction
C. Hydrogen bonding
D. London forces

Answer: C. Hydrogen bonding

Explanation:

Among the given intermolecular forces, hydrogen bonding is generally the strongest and has a major influence on solubility, especially in polar solvents like water.

Hydrogen bonding significantly enhances solubility when the solute and solvent can form hydrogen bonds with each other.

Strength order of interactions (general trend):

Hydrogen Bonding>Dipole-Dipole>Van der Waals/London Forces

Examples:

• Alcohols are soluble in water due to hydrogen bonding.

• Sugars dissolve readily in water because of extensive hydrogen bonding.

12. “Like dissolves like,” it shows:

A. Polar solutes are best in polar solvents
B. All solutes equally soluble
C. Solubility does not depend on polarity
D. Nonpolar solutes dissolve best in polar solvents

Answer: A. Polar solutes dissolve best in polar solvents

Explanation:

The principle "like dissolves like” means that substances with similar polarity dissolve well in each other.

• Polar solutes dissolve best in polar solvents (e.g., salt or sugar in water).

• Nonpolar solutes dissolve best in nonpolar solvents (e.g., iodine in benzene).

This concept is based on similarity in intermolecular forces.

Polar solute→Polar solvent

Nonpolar solute→Nonpolar solvent

13. The definition of partition coefficient is

A. Solubility Ratio in two miscible solvents
B. Concentration Ratio in Two Immiscible Solvents
C. Ratio of densities of solvents
D. Ratio of vapor pressures

Answer: B. Ratio of concentrations in two immiscible solvents

Explanation:

The partition coefficient is defined as the ratio of concentrations of a solute distributed between two immiscible solvents at equilibrium and constant temperature.

It is expressed as:

K=C1/C2

Where:

• K = Partition coefficient

• C1​ = Concentration of solute in first solvent

• C2​ = Concentration of solute in the second solvent

Example: Distribution of iodine between water and carbon tetrachloride.

14. A drug with a high partition coefficient is:

A. Insoluble in oils
B. Lipophilic
C. Highly ionized
D. Hydrophilic

Answer: B. Lipophilic

Explanation:

A high partition coefficient indicates that the drug preferentially dissolves in the organic (lipid/oil) phase rather than the aqueous phase. Therefore, such drugs are considered lipophilic.

This relationship is represented as:

↑Partition Coefficient = ↑Lipophilicity

Key points:

• Lipophilic drugs cross biological membranes more easily.

• Hydrophilic or highly ionized drugs generally have lower partition coefficients.

15. The partition coefficient predicts:

A. Drug Stability
B. Drug degradation
C. Drug dosage form
D. Drug absorption across membranes

Answer: D. Drug absorption through membranes

Explanation:

The Partition Coefficient is an important parameter used to predict a drug’s ability to cross biological membranes.

A higher partition coefficient indicates greater lipid solubility, which generally enhances membrane permeation and absorption.

This relationship can be represented as:

↑Partition Coefficient⇒↑Membrane Permeability

Therefore, the partition coefficient is widely used in pharmaceutics and medicinal chemistry to estimate:

• Drug absorption

• Membrane transport

• Bioavailability

• Distribution into tissues

16. The unit of partition coefficient is:

A. atm
B. mg/mL
C. Dimensionless
D. mol/L

Answer: C. Dimensionless

Explanation:

The partition coefficient is the ratio of concentrations of a solute in two immiscible solvents.

Since both concentrations have the same units, they cancel each other.

K=C1 / C2

Therefore, the partition coefficient has no unit and is dimensionless.

17. Which method is used to determine the partition coefficient?

A. Filtration
B. Titration
C. Distillation
D. Shake flask method

Answer: B. Shake flask method

Explanation:

The shake flask method is the standard method used to determine the partition coefficient of a solute between two immiscible solvents.

In this method:

1. The solute is added to two immiscible solvents.

2. The mixture is shaken until equilibrium is reached.

3. The concentrations in each layer are measured.

4. The partition coefficient is calculated.

The formula used is:

K = Corganic / Caqueous​​

Therefore, the correct answer is the shake flask method.

18. What is the pH-partition hypothesis?

A. Drug crystallization
B. Drug degradation
C. Drug Metabolism
D. Drug absorption through biological membranes

Answer: D. Drug absorption through biological membranes

Explanation:

The pH-partition hypothesis states that drugs are absorbed across biological membranes mainly in their unionized (lipid-soluble) form.

The degree of ionization depends on:

• pH of the medium

• pKa of the drug

Unionized drugs are more lipophilic and cross membranes more easily.

This concept can be summarized as

Unionized Drug⇒↑Membrane Absorption

Therefore, the pH-partition hypothesis explains drug absorption through biological membranes.

19. The best absorption medium for Weak acidic drugs is

A. Neutral medium
B. Alkaline medium
C. Acidic medium
D. Highly buffered medium

Answer: C. Acidic medium

Explanation:

According to the pH-Partition Hypothesis, weak acidic drugs are absorbed best in an acidic medium because they remain largely unionized in acidic pH, making them more lipid-soluble and able to cross biological membranes easily.

For weak acids:

Weak Acid in Acidic pH⇒Unionized Form⇒↑Absorption

Example:

• Aspirin is better absorbed in the stomach’s acidic environment.

20. The solubility of weak basic drugs is highest in:

A. Acidic pH
B. Basic pH
C. Neutral pH
D. Nonpolar solvent

Answer: A. Acidic pH

Explanation:

Weak basic drugs become more ionized and soluble in an acidic medium because they accept protons (H+).

This can be represented as:

Weak Base + H⁺ = Ionized Form = ↑Solubility

Therefore:

• Acidic pH → increases ionization → increases aqueous solubility of weak bases.

Example:

• Alkaloids and many basic drugs show greater solubility in acidic solutions.

21. The Henderson–Hasselbalch equation determines

A. Refractive index
B. Solubility-pH relationship
C. Surface tension
D. Vapor pressure

Answer: B. Solubility-pH relationship

Explanation:

The Henderson–Hasselbalch equation is used to determine the relationship between

• pH of the medium

• pKa of the drug

• Degree of ionization

It helps explain the solubility and absorption behavior of weak acids and weak bases.

For weak acids:

pH = pKa + log(A/HA)

Since ionized forms are generally more water-soluble, this equation is important in understanding the

22. Critical solution temperature means

A. The temperature above which the two liquids are in solution form
B. Temperature below which all solids are in solution form
C. Solvent Melting Point
D. Solvent Boiling Point

Answer: A. Temperature above which two liquids are completely miscible

Explanation:

Critical solution temperature is the temperature at which two partially miscible liquids become completely miscible in all proportions.

Thus, above (or in some systems below) this temperature, the liquids form a single homogeneous solution.

This concept is commonly represented as

At CST = Complete Miscibility of Two Liquids

Example:

• Phenol and water show an upper critical solution temperature.

23. The lowest critical solution temperature is shown by which system:

A. Phenol-water
B. Nicotine-water
C. Methanol-water
D. Acetone-water

Answer: B. Nicotine-water system

Explanation:

The Lower Critical Solution Temperature (LCST) is the temperature below which two partially miscible liquids become completely miscible.

The nicotine–water system exhibits a lower critical solution temperature.

In contrast:

• Phenol–water shows an upper critical solution temperature (UCST).

• Methanol–water and acetone–water are completely miscible under ordinary conditions.

Conceptually:

Nicotine-Water⇒Lower Critical Solution Temperature (LCST)

24. A solute distribution between benzene and water with concentrations of 16 g/L and 4 g/L, respectively. The partition coefficient is

A. 0.20
B. 2.5
C. 4
D. 5.5

Answer: C. 4

Using the partition coefficient formula:

K=C1 / C2​

Given:

• Concentration in benzene = 16 g/L

• Concentration in water = 4 g/L

K = 16 / 4 = 4

27. If the pKa of a weak acid is 5 and the pH is 3, the drug will be predominantly:

A. Insoluble
B. Ionized
C. Unionized
D. Completely degraded

Answer: A. Unionized

Explanation:

For a weak acid, when the pH is lower than the pKa, the drug remains predominantly in the unionized form.

Using the Henderson–Hasselbalch Equation:

pH=pKa+log⁡([A−] / [HA])

Given:

• pKa = 5pK_a = 5pKa​=5

• pH = 3pH = 3pH=3

Since pH < pKa

• [HA](unionized form) predominates.

Therefore, the weak acid will be mainly unionized and more lipid-soluble.

28. Which method is preferred to improve the solubility of a poorly water-soluble drug?

A. Salt removal
B. Increasing temperature
C. Increasing crystallinity
D. Particle size reduction

Answer: A. Particle size reduction

Explanation:

Reducing particle size increases the surface area of the drug exposed to the solvent, thereby enhancing the rate of dissolution and improving apparent solubility behavior of poorly water-soluble drugs.

This relationship is expressed conceptually as:

↓Particle Size = ↑Surface Area = ↑Dissolution

Why are other options incorrect?

• A. Salt removal → may reduce solubility.

• B. Increasing temperature → not universally effective for all drugs.

• C. Increasing crystallinity → generally decreases solubility because crystalline forms are more stable and less soluble.

29. Which cosolvent enhances the solubility of hydrophobic drugs?

A. Water
B. Ethanol
C. Benzene
D. Petroleum ether

Answer: B. Ethanol

Explanation:

Ethanol is widely used as a cosolvent to enhance the solubility of hydrophobic (poorly water-soluble) drugs.

Cosolvency works by reducing the polarity of the solvent system, thereby improving the dissolution of nonpolar or slightly polar drugs.

Conceptually:

Cosolvent Addition = ↑Solubility of Hydrophobic Drugs

Examples of pharmaceutical cosolvents:

• Ethanol

• Propylene glycol

• PEG 400

Why others are incorrect:

• Water → poor solvent for many hydrophobic drugs.

• Benzene and petroleum ether → not preferred pharmaceutical cosolvents due to toxicity and incompatibility.

30. The salting-out phenomenon causes

A. Increasing the solubility of a nonelectrolyte
B. Decreasing solubility of a nonelectrolyte
C. Complete ionization
D. Suspension Formation

Answer: B. Decrease in solubility of nonelectrolyte

Explanation:

The salting-out phenomenon occurs when the addition of a salt (electrolyte) decreases the solubility of a nonelectrolyte in water.

This happens because:

• Salt ions strongly attract water molecules.

• Fewer water molecules remain available to solvate the nonelectrolyte.

• As a result, the nonelectrolyte precipitates or becomes less soluble.

Conceptually:

↑Salt Concentration = ↓Solubility of Nonelectrolyte

31. Which factor most favors membrane permeability?

A. Viscosity
B. Surface tension
C. Refractive index
D. Partition coefficient

Answer: D. Partition coefficient

Explanation:

The partition coefficient is one of the most important factors influencing membrane permeability.

A higher partition coefficient indicates greater lipid solubility, allowing the drug to pass more easily through the lipid-rich biological membranes.

This relationship is expressed as:

Increased Partition Coefficient = Increased Membrane Permeability

Therefore, drugs with appropriate lipophilicity generally show better absorption across membranes.

32. Which one is an ideal solution?

A. Ethanol & water
B. Phenol & water
C. Benzene & toluene
D. Acetone & chloroform

Answer: A. Benzene & toluene

Explanation:

An ideal solution is a solution that obeys Raoult's Law over the entire concentration range.

Ideal solutions are formed when:

• Intermolecular attractions between unlike molecules are nearly equal to those between like molecules.

• No heat or volume change occurs on mixing.

The benzene–toluene system behaves nearly ideally because both are nonpolar aromatic hydrocarbons with similar molecular structures.

Conceptually:

Ideal Solution = ΔHmix ≈ 0, ΔVmix ≈ 0

Why others are incorrect:

• Ethanol & water → strong hydrogen bonding; non-ideal.

• Phenol & water → partial miscibility; non-ideal.

• Acetone & chloroform → specific intermolecular interactions; non-ideal.

33. Positive deviation in Raoult’s law occurs

A. If the solute-solvent interaction is stronger
B. If the solute-solvent interaction is weaker
C. If the solution is ideal
D. If no intermolecular forces exist

Answer: B. Solute-solvent interaction is weaker

Explanation:

Positive deviation from Raoult's Law occurs when the attraction between unlike molecules (solute–solvent) is weaker than the attraction between like molecules.

As a result:

• Molecules escape more easily into the vapor phase.

• Vapor pressure becomes higher than predicted by Raoult’s law.

Conceptually:

Weaker Solute-Solvent Interaction = Increase in Vapor Pressure

Examples:

• The ethanol–cyclohexane system shows positive deviation.

34. Negative deviation in Raoult’s law indicates

A. No vapor pressure
B. Complete immiscibility
C. Weak intermolecular attraction
D. Strong intermolecular attraction

Answer: D. Strong intermolecular attraction

Explanation:

Negative Deviation from Raoult's Law occurs when the attraction between unlike molecules (solute–solvent) is stronger than that between like molecules.

Because of stronger intermolecular attraction:

• Molecules escape less readily into the vapor phase.

• Vapor pressure becomes lower than predicted by Raoult's law.

Conceptually:

Stronger Solute-Solvent Attraction = Increase in Vapor Pressure

Example:

• The acetone–chloroform system shows negative deviation due to hydrogen bonding.

35. The most favorable solvent for an ionic compound is

A. Benzene
B. Carbon tetrachloride
C. Alcohol
D. Water

Answer: D. Water

Explanation:

Dielectric constant and polarity are major factors determining the solubility of ionic compounds.

Water is the most favorable solvent for ionic compounds because

• It has a high dielectric constant.

• It is highly polar.

• It stabilizes ions through hydration.

This can be summarized as:

High Polarity + High Dielectric Constant = Increase in Solubility of Ionic Compounds

Why others are incorrect:

• Benzene and carbon tetrachloride are nonpolar solvents.

• Alcohol is less polar than water and generally less effective for ionic compounds.

36. The solubility product applies to:

A. Emulsions
B. Sparingly soluble salts
C. Polar solutions
D. Gases

Answer: B. Sparingly soluble salts

Explanation:

The solubility product (Ksp) applies to sparingly soluble salts that establish equilibrium between the undissolved solid and its ions in solution.

Example:

AB ⇌ A⁺ + B⁻

The solubility product expression is:

Ksp = [A⁺][B−]

Thus, Ksp helps determine the following:

• Solubility of sparingly soluble salts

• Precipitation conditions

• Common ion effect.

37. Which one is TRUE related to pH and solubility?

A. Weak acids are highly soluble at low pH
B. Weak acids are highly soluble at high pH
C. Weak bases are insoluble in acids
D. pH never affects solubility

Answer: B. Weak acids are more soluble at high pH

Weak acids ionize more in alkaline (high pH) media:

HA ⇌ H⁺ + A

At high pH, the concentration of H⁺ decreases, shifting the equilibrium toward ionization. The ionized form is more water-soluble.

Examples:

• Aspirin

• Barbiturates

Similarly:

• Weak bases become more soluble at low pH (acidic medium).

Therefore, pH significantly affects the solubility of ionizable drugs.

38. The distribution coefficient differs from the partition coefficient because

A. It includes unionized species
B. It excludes ionized species
C. It is temperature-independent
D. It is always constant

Answer: A. Includes ionized species

Explanation:

• Partition coefficient (P) refers to the distribution of only the unionized form of a solute between two immiscible phases (commonly oil and water).

• Distribution coefficient (D) refers to the distribution of both ionized and unionized forms of the solute at a specific pH.

Therefore, the distribution coefficient depends on pH, while the partition coefficient is generally measured for the non-ionized species only.

39. The use of critical solution temperature in pharmacy is in

A. Ointments Preparation
B. Identification of purity and composition of liquids
C. Distillation
D. Drying

Answer: B. Identification of purity and composition of liquids

Explanation:

The Critical Solution Temperature (CST) is the temperature at which two partially miscible liquids become completely miscible in all proportions.

In pharmacy and physical chemistry, CST is mainly used for:

• determining the purity of liquids,

• studying the composition of binary liquid mixtures,

• analyzing miscibility behavior of solvents.

Impurities generally alter the CST value, making it useful for purity assessment.

40. Which one for solubility is CORRECT?

A. Gases are highly soluble at high temperatures.
B. Solubility does not depend upon molecular structure
C. Hydrogen bonding increases aqueous solubility
D. Nonpolar solutes are soluble in water

Answer: C. Hydrogen bonding can increase aqueous solubility

Explanation:

Hydrogen bonding enhances the interaction between a solute and water, thereby increasing aqueous solubility.

Examples:

• Alcohols

• Sugars

• Amines

Why the other options are incorrect:

• A. Gas solubility generally decreases with increasing temperature.

• B. Molecular structure strongly affects solubility.

• D. Nonpolar solutes are usually poorly soluble in water (“like dissolves like”).

41. A weakly acidic drug shows pH-dependent solubility. Which equation describes this statement?

A. Henderson-Hasselbalch equation
B. Noyes-Whitney equation
C. Clausius-Clapeyron equation
D. Arrhenius equation

Answer: C. Henderson-Hasselbalch equation

Explanation:

The Henderson–Hasselbalch equation explains the relationship between pH, pKa, and ionization of weak acids and bases, which directly affects their solubility.

For a weak acid:

pH = pKa + log⁡[A−] / [HA]

At higher pH, weak acids become more ionized.

• The ionized form is more water-soluble.

Other options:

• B. Noyes–Whitney equation → dissolution rate

• C. Clausius–Clapeyron equation → vapor pressure/temperature relation

• D. Arrhenius equation → effect of temperature on reaction rate

42. Which one determines deviation from ideal behavior?

A. Interfacial tension
B. Intermolecular forces
C. Boiling point only
D. Molecular weight only

Answer: B. Intermolecular forces

Explanation:

Deviation from ideal behavior in solutions mainly occurs due to differences in intermolecular forces between:

• solute–solute,

• solvent–solvent, and

• solute–solvent molecules.

• If intermolecular attractions are similar → solution behaves ideally.

• If attractions are stronger or weaker → positive or negative deviation from Raoult’s Law occurs.

Other options are not the primary determining factors:

• A. Interfacial tension is related but not the main cause.

• C. Boiling point alone does not determine deviation.

• D. Molecular weight alone is insufficient.

43. The partition coefficient of iodine between carbon tetrachloride and water is high because iodine is:

A. Highly polar
B. Nonpolar
C. Complete ionization
D. Strong hydration

Answer: B. Nonpolar

Explanation:

Iodine is a nonpolar substance and therefore dissolves better in nonpolar solvents such as carbon tetrachloride.

According to the principle,

“Like dissolves like."

• Nonpolar solutes prefer nonpolar solvents.

• Polar solutes prefer polar solvents like water.

Hence, iodine has a high partition coefficient in favor of carbon tetrachloride over water.

44. At equilibrium, the chemical potential of a solute in two immiscible phases is:

A. Zero
B. Infinite
C. Equal
D. None of the above

Answer: C. Equal

Explanation:

At equilibrium, a solute distributed between two immiscible phases has the same chemical potential in both phases.

This equality of chemical potential is the basis of

• distribution law,

• partition coefficient, and

• phase equilibrium concepts.

When the chemical potentials become equal, there is no net transfer of solute between the two phases.

45. Which one increases dissolution rate without changing equilibrium solubility?

A. Size reduction
B. Micronization
C. Cosolvency
D. pH adjustment

Answer: B. Micronization

Explanation:

Micronization reduces particle size, thereby increasing the surface area of the drug particles. This enhances the dissolution rate according to the Noyes–Whitney equation, but it does not change the equilibrium solubility of the drug.

The dissolution relationship is described by:

dC/dt = DA(Cs − C) / h​

Where increasing surface area, A, increases dissolution rate.

Why others are incorrect:

• A. Size reduction is a broad term, but micronization specifically refers to very fine particle size reduction used for this purpose.

• C. pH adjustment can increase equilibrium solubility of ionizable drugs.

• D. Cosolvency increases solubility by improving solvent properties.

46. Which one exhibits both upper and lower CST?

A. Water-nicotinic acid
B. Nicotine-water
C. Water-ethanol
D. Triethylamine-water

Answer: A.

Explanation:

The Critical Solution The temperature system of nicotine and water shows both:

• Upper Critical Solution Temperature (UCST)

• Lower Critical Solution Temperature (LCST)

This means:

• The liquids are completely miscible below the LCST.

• partially miscible between LCST and UCST,

• and again completely miscible above the UCST.

Other systems:

• Water–ethanol → completely miscible at ordinary temperatures.

• Triethylamine–water → mainly shows LCST.

• Water–Nicotinic acid does not exhibit both CSTs.

47. The poor aqueous solubility of drugs is due to:

A. High dielectric constant
B. High crystallinity and lipophilicity
C. Complete ionization
D. Low molecular weight

Answer: B. High crystallinity and lipophilicity

Explanation:

Poor aqueous solubility commonly occurs when a drug has:

• High crystallinity → strong crystal lattice energy makes dissolution difficult.

• High lipophilicity → drug prefers nonpolar environments rather than water.

These properties reduce interaction with water molecules and decrease solubility.

Other options:

• A. High dielectric constant generally favors the solubility of ionic/polar substances.

• C. Complete ionization usually increases aqueous solubility.

• D. Low molecular weight often improves solubility rather than reducing it.

48. Which one is the best indicator of lipophilicity?

A. Melting point
B. Surface tension
C. Log P
D. pKa

Answer: C. Log P

Explanation:

The partition coefficient expressed as Log P is the best indicator of the lipophilicity of a drug.

It represents the distribution of a unionized drug between

• a nonpolar phase (usually octanol)

• and water.

The relationship is

log⁡P = log⁡(Coctanol / Cwater)

• Higher Log P → more lipophilic drug

• Lower Log P → more hydrophilic drug

Other options:

• The melting point relates to crystal properties.

• Surface tension relates to liquid surface behavior.

• pKa indicates ionization tendency, not lipophilicity directly.

49. Which one is the correct statement regarding Raoult’s law?

A. Vapor pressure increases with an increase in solute concentration
B. Vapor pressure decreases with an increase in solute concentration
C. Vapor pressure remains constant
D. Vapor pressure depends only on temperature

Answer: A. Vapor pressure decreases with an increase in solute concentration

Explanation:

According to Raoult’s Law, the vapor pressure of a solvent in an ideal solution is directly proportional to the mole fraction of the solvent.

For a nonvolatile solute:

P = XsolventP0

As solute concentration increases:

• mole fraction of solvent decreases,

• Therefore, vapor pressure decreases.

Hence, adding more solute lowers the vapor pressure of the solution.

50. Which dosage form depends on solubility enhancement techniques?

A. Gargles
B. Injectables
C. Syrups
D. Oral solid dosage forms of poorly soluble drugs

Answer: D. Oral solid dosage forms of poorly soluble drugs

Explanation:

Poorly water-soluble drugs often show low bioavailability when administered orally. Therefore, solubility enhancement techniques such as

• particle size reduction,

• solid dispersion,

• complexation,

• use of surfactants,

• micronization, and

• nanotechnology

are mainly applied in oral solid dosage forms (tablets and capsules) to improve dissolution and absorption.

• Gargles are aqueous solutions used locally.

• Injectables usually require complete solubility or special formulation methods.

• Syrups already contain dissolved drug substances.

Hence, oral solid dosage forms of poorly soluble drugs rely most on solubility enhancement techniques.

Dr Alok Singh