Propellants used in aerosols are critical components that deliver liquid, semi-solid, or powder formulations from containers as sprays, foams, or mists. For B.Pharm students this topic covers classification (CFCs, HCFCs, HFCs, hydrocarbons like propane/butane/isobutane, and dimethyl ether), key physicochemical properties (vapor pressure, boiling point, density), formulation interactions (solubility, cosolvents, emulsions, suspensions), safety and flammability, regulatory issues (Montreal Protocol, phase-out) and device compatibility (valves, metering systems). Understanding propellant selection affects product performance, stability, patient safety, environmental impact and practical quality control considerations. Key keywords: propellants, aerosols, hydrocarbon propellants, dimethyl ether, HFCs, vapor pressure, metering valve. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which of the following best defines a propellant used in pharmaceutical aerosols?
- A substance that acts only as a solvent for the drug
- A gas or liquefied gas that expels and atomizes the formulation from the container
- An inert powder used to stabilize suspensions
- A type of surfactant that reduces surface tension
Correct Answer: A gas or liquefied gas that expels and atomizes the formulation from the container
Q2. Which propellant class was largely phased out due to ozone depletion concerns?
- Hydrocarbons (propane, butane)
- Hydrofluorocarbons (HFCs)
- Chlorofluorocarbons (CFCs)
- Dimethyl ether (DME)
Correct Answer: Chlorofluorocarbons (CFCs)
Q3. Dimethyl ether (DME) is often used in aerosols because it:
- Is non-flammable and extremely inert
- Provides both propellant action and solvent properties
- Has a very high boiling point making it stable in containers
- Is a particulate suspending agent
Correct Answer: Provides both propellant action and solvent properties
Q4. Which property most directly determines the vapor pressure delivered by a liquefied propellant at a given temperature?
- Viscosity
- Boiling point and vapor–liquid equilibrium
- Surface tension
- pH of the formulation
Correct Answer: Boiling point and vapor–liquid equilibrium
Q5. A metering valve in a metered-dose inhaler (MDI) is primarily designed to:
- Filter particles during actuation
- Deliver a reproducible volume of formulation per actuation
- Heat the formulation before expulsion
- Change the chemical composition of the propellant
Correct Answer: Deliver a reproducible volume of formulation per actuation
Q6. Which of the following propellants is most associated with flammability concerns in domestic aerosols?
- HFC-134a
- Propane/butane/isobutane (hydrocarbons)
- CFC-12
- Nitrogen (compressed gas)
Correct Answer: Propane/butane/isobutane (hydrocarbons)
Q7. In aerosol formulation, a propellant that is miscible with the formulation liquid will most likely:
- Form a separate headspace gas only
- Act as a cosolvent and alter drug solubility
- Increase the viscosity of the formulation significantly
- Prevent atomization entirely
Correct Answer: Act as a cosolvent and alter drug solubility
Q8. Which regulatory treaty is primarily responsible for the global phase-out of CFC propellants?
- Kyoto Protocol
- Montreal Protocol
- Geneva Convention
- Paris Agreement
Correct Answer: Montreal Protocol
Q9. For suspension aerosols, which property of the propellant influences suspension stability the most?
- Color
- Density relative to dispersed particles
- Optical refractive index
- Surface pH
Correct Answer: Density relative to dispersed particles
Q10. Which filling method uses refrigeration to liquefy propellant before adding it to the container?
- Pressure filling
- Cold filling
- Vacuum filling
- Spray filling
Correct Answer: Cold filling
Q11. Which test parameter assesses the consistency of dose delivered over repeated actuations?
- Leakage test
- Dose uniformity/delivered dose per actuation
- pH stability
- Viscosity profile
Correct Answer: Dose uniformity/delivered dose per actuation
Q12. When selecting an aerosol propellant for an aqueous formulation, a formulators main concern is:
- Propellant color matching
- Miscibility and potential to form emulsions or cosolvent effects
- The propellant’s nutritional value
- The propellant’s taste
Correct Answer: Miscibility and potential to form emulsions or cosolvent effects
Q13. Which type of propellant remains entirely gaseous at room temperature unless compressed?
- Liquefied gas propellants (e.g., hydrocarbons, HFCs)
- Compressed gases such as nitrogen or carbon dioxide
- High boiling point chlorinated solvents
- Dimethyl sulfoxide
Correct Answer: Compressed gases such as nitrogen or carbon dioxide
Q14. HFCs replaced CFCs because HFCs:
- Have higher ozone depletion potential
- Are non-volatile liquids under all conditions
- Have negligible ozone depletion potential though may have global warming potential
- Are solid at room temperature
Correct Answer: Have negligible ozone depletion potential though may have global warming potential
Q15. Which property of a propellant primarily affects spray droplet size from an aerosol nozzle?
- Vapor pressure and energy available for atomization
- Color index
- Solubility in water only
- pH
Correct Answer: Vapor pressure and energy available for atomization
Q16. Which compatibility issue is most critical between propellant and valve elastomers?
- Elastomer color change only
- Swelling, softening or degradation of elastomer leading to leakage or valve failure
- Increase in elastomer tensile strength
- Enhanced biocompatibility of elastomer
Correct Answer: Swelling, softening or degradation of elastomer leading to leakage or valve failure
Q17. In a two-phase aerosol (liquid + vapor), at equilibrium the vapor pressure is independent of:
- Temperature
- Amount of liquid present (provided liquid remains)
- Propellant identity
- Container material
Correct Answer: Amount of liquid present (provided liquid remains)
Q18. Which propellant is also a good solvent for many organic substances and can dissolve oils and resins in formulations?
- Nitrogen
- Carbon dioxide
- Dimethyl ether (DME)
- Water vapor
Correct Answer: Dimethyl ether (DME)
Q19. What is the main safety concern when formulating with hydrocarbon propellants in pharmaceutical products?
- They are corrosive to metal containers
- High flammability and explosion risk during manufacture and use
- They always cause skin sensitization
- They are radioactive
Correct Answer: High flammability and explosion risk during manufacture and use
Q20. Which analytical test measures the internal pressure of an aerosol container as a function of temperature?
- pH titration
- Pressure–temperature (P–T) profile or vapor pressure determination
- Optical rotation
- Viscosity index
Correct Answer: Pressure–temperature (P–T) profile or vapor pressure determination
Q21. A formulary note: adding ethanol to an HFA aerosol formulation typically acts as:
- An antimicrobial preservative only
- A cosolvent that modifies solubility and vapor–liquid equilibria
- A propellant replacement
- A polymerization initiator
Correct Answer: A cosolvent that modifies solubility and vapor–liquid equilibria
Q22. In a pressurized metered-dose inhaler, which factor helps achieve fine particle fraction suitable for pulmonary delivery?
- Low vapor pressure propellant only
- Optimized nozzle, formulation volatility, and propellant selection to control atomization
- High container wall thickness
- Opaque container color
Correct Answer: Optimized nozzle, formulation volatility, and propellant selection to control atomization
Q23. Which method is commonly used to check for leakage in filled aerosol containers?
- Thermal decomposition
- Leak detection using pressure decay or bubble emission tests
- pH measurement
- UV-visible spectroscopy
Correct Answer: Leak detection using pressure decay or bubble emission tests
Q24. If a propellant has a very low boiling point relative to ambient temperature, it will generally:
- Remain completely liquid at room temperature
- Require compression or refrigeration to be liquefied in the container
- Be non-volatile and non-atomizable
- Always be non-flammable
Correct Answer: Require compression or refrigeration to be liquefied in the container
Q25. Which aerosol propellant is least likely to contribute to ozone depletion but may have global warming potential?
- CFCs
- HCFCs
- HFCs
- Chlorine gas
Correct Answer: HFCs
Q26. For a suspension aerosol, what formulation strategy improves dose uniformity of suspended drug particles?
- Maximizing particle aggregation
- Matching propellant and vehicle density to particle density and using appropriate wetting agents
- Avoiding any surfactants
- Using very large particle sizes (>1000 µm)
Correct Answer: Matching propellant and vehicle density to particle density and using appropriate wetting agents
Q27. Which phenomenon explains why vapor pressure of a propellant increases sharply with temperature?
- Henry’s law only
- Clausius–Clapeyron relationship describing vapor–liquid equilibrium
- Beer–Lambert law
- Raoult’s law for ideal solids
Correct Answer: Clausius–Clapeyron relationship describing vapor–liquid equilibrium
Q28. Which propellant type is commonly used when product non-flammability is required (e.g., certain topical pharmaceuticals)?
- Hydrocarbon blend
- Dimethyl ether (DME)
- Compressed nitrogen or carbon dioxide
- Butane
Correct Answer: Compressed nitrogen or carbon dioxide
Q29. Which factor is NOT a direct consideration when choosing a propellant for a nasal spray formulation?
- Compatibility with nasal mucosa and safety
- Vapor pressure and droplet size distribution
- Regulatory approval and environmental impact
- The propellant’s ability to change the active pharmaceutical ingredient’s chemical structure into a different API
Correct Answer: The propellant’s ability to change the active pharmaceutical ingredient’s chemical structure into a different API
Q30. During quality control, the “actuation force” of an aerosol valve is measured because it affects:
- The chemical potency of the drug
- User ergonomics, dose reproducibility and patient compliance
- The container’s optical clarity
- The propellant’s molecular weight
Correct Answer: User ergonomics, dose reproducibility and patient compliance
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.
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