Relative humidity MCQs With Answer

Understanding relative humidity is essential for B. Pharm students preparing for exams and practicals. This set of Relative humidity MCQs With Answer focuses on humidity definitions, measurement techniques, psychrometric calculations, dew point, saturation vapor pressure, and effects on pharmaceutical stability, formulation, packaging, and storage. These targeted multiple-choice questions cover instruments (sling psychrometer, capacitive and resistive hygrometers), humidity control in manufacturing, equilibrium moisture content, and regulatory considerations — helping you master calculation-based problems and real-world applications in drug storage and quality assurance. Each question includes clear options and answers to reinforce learning and assess readiness for university exams and professional practice. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the correct definition of relative humidity (RH)?

• The amount of water vapor present per unit volume of air
• The ratio of water vapor pressure to saturation vapor pressure multiplied by 100
• The absolute mass of water in a given mass of dry air
• The temperature at which air becomes saturated with water vapor

Correct Answer: The ratio of water vapor pressure to saturation vapor pressure multiplied by 100

Q2. Which instrument directly measures dew point?

• Sling psychrometer
• Dew point hygrometer
• Resistive humidity sensor

Correct Answer: Dew point hygrometer

Q3. If the actual vapor pressure is 12 mmHg and the saturation vapor pressure at that temperature is 24 mmHg, what is RH?

• 50%
• 2%
• 200%
• 0.5%

Correct Answer: 50%

Q4. Which humidity parameter is most directly relevant to microbial growth in pharmaceutical products?

• Absolute humidity
• Relative humidity
• Dry bulb temperature
• Wind speed

Correct Answer: Relative humidity

Q5. Which of the following best describes absolute humidity?

• Mass of water vapor per unit mass of dry air
• Mass of water vapor per unit volume of air
• The percentage ratio of actual to saturated vapor pressure
• Temperature at condensation

Correct Answer: Mass of water vapor per unit volume of air

Q6. Which device measures humidity using electrical capacitance changes as moisture content varies?

• Thermistor
• Capacitive hygrometer
• Sling psychrometer
• Dew point mirror

Correct Answer: Capacitive hygrometer

Q7. On a psychrometric chart, which two properties are primarily plotted to determine RH?

• Dry bulb temperature and wet bulb temperature
• Wet bulb temperature and dew point
• Dry bulb temperature and relative pressure
• Dew point and atmospheric pressure

Correct Answer: Dry bulb temperature and wet bulb temperature

Q8. Which equation represents relative humidity when using partial pressures?

• RH = (actual vapor pressure / saturation vapor pressure) × 100
• RH = (saturation vapor pressure / actual vapor pressure) × 100
• RH = (absolute humidity × 100) / dry bulb temperature
• RH = wet bulb depression × 100

Correct Answer: RH = (actual vapor pressure / saturation vapor pressure) × 100

Q9. Which factor increases the saturation vapor pressure of air?

• Lowering temperature
• Increasing temperature
• Decreasing atmospheric pressure
• Removing water vapor

Correct Answer: Increasing temperature

Q10. For pharmaceutical storage, which RH range is generally considered safe for many solid oral dosage forms?

• Below 10% RH
• 30–50% RH
• 70–90% RH
• 90–100% RH

Correct Answer: 30–50% RH

Q11. What is the wet bulb depression used for in psychrometry?

• To measure absolute humidity directly
• To estimate relative humidity from dry and wet bulb temperatures
• To calculate atmospheric pressure
• To determine solubility of drugs

Correct Answer: To estimate relative humidity from dry and wet bulb temperatures

Q12. A sling psychrometer uses which principle to estimate RH?

• Change in electrical resistance
• Evaporative cooling from a wet-bulb thermometer
• Reflection of infrared radiation
• Mass change due to moisture sorption

Correct Answer: Evaporative cooling from a wet-bulb thermometer

Q13. Which term describes the moisture content at which a hygroscopic powder neither gains nor loses moisture in a given RH?

• Equilibrium moisture content (EMC)
• Absolute humidity
• Wet bulb temperature
• Critical micelle concentration

Correct Answer: Equilibrium moisture content (EMC)

Q14. How does high RH generally affect tablet physical stability?

• Decreases disintegration time consistently
• Can cause swelling, softening, or capping and reduce stability
• Eliminates polymorphic transformations
• Has no impact on tablets

Correct Answer: Can cause swelling, softening, or capping and reduce stability

Q15. Which packaging material provides the best barrier against moisture ingress for hygroscopic drugs?

• Perforated polyethylene
• Aluminum foil laminate
• Cloth wrap
• Paperboard

Correct Answer: Aluminum foil laminate

Q16. What is the primary purpose of desiccants in pharmaceutical packaging?

• To add moisture to the container
• To adsorb or absorb moisture and maintain low RH
• To increase oxygen levels
• To control temperature only

Correct Answer: To adsorb or absorb moisture and maintain low RH

Q17. Which of the following best describes dew point?

• Temperature at which air becomes saturated and condensation begins
• Temperature at which freezing occurs
• Temperature measured by a wet bulb
• Absolute humidity multiplied by pressure

Correct Answer: Temperature at which air becomes saturated and condensation begins

Q18. If RH is constant, increasing temperature will typically cause the air to:

• Become saturated immediately
• Have lower relative humidity because saturation vapor pressure increases
• Increase absolute humidity without any change
• Decrease saturation vapor pressure

Correct Answer: Have lower relative humidity because saturation vapor pressure increases

Q19. Which humidity sensor type has the fastest response time and is widely used in cleanroom monitoring?

• Gravimetric moisture sampler
• Capacitive sensor
• Classic hair hygrometer
• Sling psychrometer

Correct Answer: Capacitive sensor

Q20. In stability studies, why is controlling RH important for solid dosage forms?

• RH affects only color and not potency
• RH can influence drug degradation, polymorphism, and microbial growth
• RH is irrelevant if temperature is controlled
• RH only matters for parenteral products

Correct Answer: RH can influence drug degradation, polymorphism, and microbial growth

Q21. Which regulatory guideline emphasizes control of humidity during pharmaceutical manufacturing and storage?

• ICH Q1A
• ICH Q7
• ICH Q10
• FDA Guidance on Glass Vials

Correct Answer: ICH Q7

Q22. What is the common unit for expressing saturation vapor pressure in pharmaceutical calculations?

• Percent by weight
• mmHg or kPa
• mg/cm2
• Degrees Celsius

Correct Answer: mmHg or kPa

Q23. Which process will lower RH inside a closed chamber without changing temperature?

• Adding moisture source
• Using a desiccant to remove water vapor
• Increasing air pressure
• Introducing humid air

Correct Answer: Using a desiccant to remove water vapor

Q24. Which formulation is most likely to be negatively affected by elevated RH?

• Non-hygroscopic tablets
• Hygroscopic powders and efflorescent salts
• Enteric-coated tablets stored in sealed aluminum blisters
• Parenteral solutions in glass ampoules

Correct Answer: Hygroscopic powders and efflorescent salts

Q25. Which of the following best describes how RH is measured by a resistive humidity sensor?

• By measuring color change of a dye
• By measuring change in electrical resistance of a hygroscopic film
• By measuring dew formation on a cooled surface
• By calculating from dry bulb temperature only

Correct Answer: By measuring change in electrical resistance of a hygroscopic film

Q26. A sealed storage area shows RH of 80% at 25°C. What is likely to happen to anhydrous salts stored there?

• They will remain unchanged
• They may absorb moisture and convert to hydrates
• They will sublimate
• They will become more soluble in air

Correct Answer: They may absorb moisture and convert to hydrates

Q27. Which mathematical relationship helps convert between vapor pressure and dew point approximately?

• Antoine equation or empirical vapor pressure-temperature relationships
• Henderson-Hasselbalch equation
• Arrhenius equation
• Raoult’s law for ideal solutions only

Correct Answer: Antoine equation or empirical vapor pressure-temperature relationships

Q28. What is the likely effect of low RH on static-sensitive pharmaceutical processes?

• Increased risk of static charge buildup leading to powder handling issues
• Reduced electrostatic effects
• Improved microbial growth
• Enhanced tablet dissolution

Correct Answer: Increased risk of static charge buildup leading to powder handling issues

Q29. Which term denotes the mass of water vapor per unit mass of dry air?

• Absolute humidity
• Specific humidity (or mixing ratio)
• Relative humidity
• Vapor pressure deficit

Correct Answer: Specific humidity (or mixing ratio)

Q30. Which humidity-related parameter is most useful when assessing comfort and packaging performance together?

• Dew point alone
• Relative humidity combined with temperature
• Absolute pressure only
• Wind chill factor

Correct Answer: Relative humidity combined with temperature

Q31. For a hygroscopic excipient, what is the significance of its isotherm curve?

• It describes moisture uptake as a function of RH at constant temperature
• It shows how pH affects dissolution
• It gives melting point vs temperature
• It describes microbial growth vs time

Correct Answer: It describes moisture uptake as a function of RH at constant temperature

Q32. Which condition is most likely to cause caking of powdered pharmaceuticals?

• Low RH with very low temperature
• Exposure to high RH leading to partial dissolution and recrystallization
• Storage under inert dry gas
• Continuous agitation in low humidity

Correct Answer: Exposure to high RH leading to partial dissolution and recrystallization

Q33. Which of the following is the best method to calibrate a capacitive hygrometer in the laboratory?

• Using saturated salt solutions that create known RH environments
• Measuring only temperature with a thermometer
• Comparing to visual inspection of condensation
• Using a barometer

Correct Answer: Using saturated salt solutions that create known RH environments

Q34. Which saturated salt solution approximates 75% RH at 25°C, often used for calibration?

• Potassium chloride (KCl)
• Sodium chloride (NaCl)
• Magnesium chloride (MgCl2)
• Sodium nitrate (NaNO3)

Correct Answer: Potassium chloride (KCl)

Q35. What is vapor pressure deficit (VPD) in context of pharmaceutical storage?

• Difference between saturation vapor pressure and actual vapor pressure
• Sum of absolute and relative humidity
• Temperature minus dew point
• Ratio of wet to dry bulb temperature

Correct Answer: Difference between saturation vapor pressure and actual vapor pressure

Q36. Which humidity condition promotes hydrolysis of moisture-sensitive drugs?

• Low RH with low temperature
• High RH with presence of liquid water or high water activity
• High atmospheric pressure only
• Vacuum conditions

Correct Answer: High RH with presence of liquid water or high water activity

Q37. How is equilibrium relative humidity (ERH) related to water activity (aw) of a solid?

• ERH (%) ≈ aw × 100
• ERH = aw + 100
• ERH is unrelated to aw
• ERH (%) ≈ aw / 100

Correct Answer: ERH (%) ≈ aw × 100

Q38. Which humidity control approach is commonly used in cleanrooms to maintain target RH?

• Using only desiccant boxes inside rooms
• Central HVAC systems with humidifiers and dehumidifiers
• Leaving doors open for natural ventilation
• Using ice blocks to cool air

Correct Answer: Central HVAC systems with humidifiers and dehumidifiers

Q39. What effect does RH have on the freeze-drying (lyophilization) process?

• RH during storage is irrelevant to frozen samples
• Residual moisture after lyophilization affects stability; process control prevents moisture pickup
• High RH improves sublimation rate
• Low RH prevents vitrification

Correct Answer: Residual moisture after lyophilization affects stability; process control prevents moisture pickup

Q40. A sample shows RH = 40% at 20°C. Which action is most appropriate for moisture-sensitive API storage?

• Store in open trays on bench
• Package in moisture barrier containers with desiccants
• Expose to ambient lab air overnight
• Increase room RH to above 70%

Correct Answer: Package in moisture barrier containers with desiccants

Q41. During a humidity challenge test, why are conditioned chambers used?

• To create controlled RH and temperature conditions for stability testing
• To sterilize products using humidity
• To measure pH changes
• To reduce oxygen concentration only

Correct Answer: To create controlled RH and temperature conditions for stability testing

Q42. Which phenomenon describes salts releasing water of crystallization at high RH?

• Efflorescence
• Hygroscopicity
• Deliquescence
• Lyophilization

Correct Answer: Deliquescence

Q43. Which describes efflorescence?

• Uptake of moisture and formation of a solution
• Release of water of crystallization to form a powdery surface
• Complete dissolution in atmospheric water
• Formation of an oil layer on tablets

Correct Answer: Release of water of crystallization to form a powdery surface

Q44. In RH calculation using partial pressures, which constant is needed for saturation vapor pressure at a given temperature?

• Gas constant for ideal gases only
• Temperature-dependent saturation vapor pressure value (from tables or equations)
• Avogadro’s number
• Boiling point of water at standard pressure

Correct Answer: Temperature-dependent saturation vapor pressure value (from tables or equations)

Q45. Which of the following is an example of a hygroscopic excipient commonly used in tablets?

• Microcrystalline cellulose (MCC) is strongly hygroscopic
• Lactose is often somewhat hygroscopic
• Sodium chloride is never hygroscopic
• Polyethylene glycol is completely inert to moisture

Correct Answer: Lactose is often somewhat hygroscopic

Q46. Which approach reduces RH fluctuations during transit of pharmaceuticals?

• Using breathable packaging only
• Using insulated, moisture-barrier packaging with desiccants or humidity indicator cards
• Shipping without any packaging
• Exposing products to outdoor air frequently

Correct Answer: Using insulated, moisture-barrier packaging with desiccants or humidity indicator cards

Q47. Which of the following best indicates desiccant exhaustion in a sealed package?

• Change in color of humidity indicator card
• Increase in package size
• Decrease in tablet weight only
• Formation of frost on the outside

Correct Answer: Change in color of humidity indicator card

Q48. Which lab practice helps minimize RH impact during weighing of moisture-sensitive powders?

• Weighing on open bench in high RH conditions
• Equilibrating samples in a desiccator and using a glovebox or dry cabinet for weighing
• Using wet gloves
• Leaving containers open to air during weighing

Correct Answer: Equilibrating samples in a desiccator and using a glovebox or dry cabinet for weighing

Q49. If saturation vapor pressure at 30°C is 31.8 mmHg and actual vapor pressure is 15.9 mmHg, RH is:

• 25%
• 50%
• 75%
• 100%

Correct Answer: 50%

Q50. Why is understanding RH crucial for B. Pharm students beyond exams?

• It only matters for meteorology students
• It is critical for formulation design, stability testing, packaging, and good manufacturing practice
• It replaces the need to learn dissolution testing
• It has no real-world applications

Correct Answer: It is critical for formulation design, stability testing, packaging, and good manufacturing practice

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com