Determination of density and viscosity of cosmetic products MCQs With Answer

Determination of density and viscosity of cosmetic products MCQs With Answer

This quiz collection is designed for M.Pharm students studying Herbal and Cosmetic Analysis (MPA 204T). It focuses on the principles, instruments, measurements and quality-control considerations involved in determining density and viscosity of cosmetic formulations. Questions cover techniques such as pycnometry, hydrometry, oscillatory and rotational rheometry (Brookfield and cone-and-plate), capillary viscometry, shear-rate dependence, temperature effects, sample preparation, calibration and data interpretation. The set aims to deepen conceptual understanding and analytical reasoning needed to select appropriate methods, interpret rheological data for Newtonian and non-Newtonian products, and apply good laboratory practice in cosmetic product characterization.

Q1. What is the primary advantage of using a digital density meter (oscillating U-tube) over a traditional pycnometer for cosmetic density determination?

• Lower cost and no need for calibration
• Higher throughput with automated temperature control and high precision
• Ability to directly measure viscosity and surface tension simultaneously
• Requires less operator skill because it uses visual reading like hydrometers

Correct Answer: Higher throughput with automated temperature control and high precision

Q2. In pycnometer density determination, which factor is most critical to control to ensure accurate specific gravity of emulsions?

• The color of the emulsion
• The exact mass of the empty pycnometer and temperature during weighing
• The brand of the pycnometer supplier
• The ambient light in the room

Correct Answer: The exact mass of the empty pycnometer and temperature during weighing

Q3. Which rheological property is described by the slope of a shear stress versus shear rate curve for a Newtonian fluid?

• Yield stress
• Apparent viscosity
• Elastic modulus
• Thixotropy index

Correct Answer: Apparent viscosity

Q4. When measuring viscosity of a shear-thinning cosmetic gel using a rotational viscometer, which practice gives the most meaningful comparative data?

• Measuring at a single arbitrary spindle speed without reporting shear rate
• Measuring across a range of shear rates and reporting apparent viscosity versus shear rate
• Measuring only at the highest motor torque setting to ensure instrument stability
• Diluting the gel with solvent until Newtonian behavior is observed

Correct Answer: Measuring across a range of shear rates and reporting apparent viscosity versus shear rate

Q5. For a cosmetic lotion with entrapped air, what preparative step improves density and viscosity measurement accuracy?

• Heating the sample above 100 °C for 30 minutes
• Degassing the sample gently under vacuum or centrifugation to remove air bubbles
• Adding xanthan gum to stabilize bubbles
• Storing the sample in bright light to disperse air

Correct Answer: Degassing the sample gently under vacuum or centrifugation to remove air bubbles

Q6. Which unit is commonly used to report dynamic viscosity in cosmetic literature?

• Pascal (Pa)
• Celsius (°C)
• Centipoise (cP) or mPa·s
• Newton (N)

Correct Answer: Centipoise (cP) or mPa·s

Q7. Which statement best describes the behavior of thixotropic cosmetic formulations?

• Viscosity increases with time under constant shear
• Viscosity decreases under shear and recovers slowly when shear is removed
• Viscosity remains constant regardless of shear history
• Viscosity immediately returns to initial value when shear ceases

Correct Answer: Viscosity decreases under shear and recovers slowly when shear is removed

Q8. In cone-and-plate rheometry, why is precise gap setting between cone and plate important for cosmetic samples?

• Because gap setting changes sample color
• Because the shear rate is directly determined by the cone angle and gap, affecting calculated shear stress and viscosity
• Because a larger gap eliminates the need to control temperature
• Because the gap setting reduces the sample density

Correct Answer: Because the shear rate is directly determined by the cone angle and gap, affecting calculated shear stress and viscosity

Q9. Which of the following best explains why viscosity typically decreases with increasing temperature for cosmetic fluids?

• Increased temperature reduces intermolecular interactions and increases molecular mobility, lowering viscosity
• Increased temperature always increases hydrogen bonding, raising viscosity
• Temperature changes do not affect viscosity
• Higher temperature increases sample density, which increases viscosity

Correct Answer: Increased temperature reduces intermolecular interactions and increases molecular mobility, lowering viscosity

Q10. Which method is most appropriate to determine the density of a powder used in a cosmetic formulation?

• Oscillating U-tube density meter designed for liquids
• Pycnometry using liquid displacement (helium or liquid pycnometer) to measure true particle density
• Rotational viscometer
• Brookfield spindle method

Correct Answer: Pycnometry using liquid displacement (helium or liquid pycnometer) to measure true particle density

Q11. When calibrating a rotational viscometer, which step is essential to ensure accuracy for cosmetics testing?

• Calibrate using standard silicone oils of known viscosity at the same temperature and spindle geometry
• Only perform zero adjustment and skip standard fluids
• Calibrate at room temperature using water without accounting for temperature drift
• Use density standards instead of viscosity standards

Correct Answer: Calibrate using standard silicone oils of known viscosity at the same temperature and spindle geometry

Q12. Specific gravity of a cosmetic product is the ratio of its density to that of water at a reference temperature. What is a common reference temperature for reporting specific gravity?

• 0 °C
• 100 °C
• 20 °C or 25 °C commonly 20 °C
• Body temperature 37 °C only

Correct Answer: 20 °C or 25 °C commonly 20 °C

Q13. Which phenomenon must be considered when measuring viscosity of concentrated suspensions used in cosmetics?

• Only Newtonian flow rules apply
• Particle-particle interactions increase apparent viscosity and may cause yield stress or shear-thickening
• Suspensions always behave as ideal gases under shear
• Viscosity is independent of particle size distribution

Correct Answer: Particle-particle interactions increase apparent viscosity and may cause yield stress or shear-thickening

Q14. In capillary viscometry (e.g., Ostwald viscometer), what is the key measurement used to calculate viscosity of Newtonian cosmetic oils?

• Time for a fixed volume to flow between two marks under gravity
• Optical rotation of the fluid
• Electrical conductivity of the oil
• Mass lost during evaporation

Correct Answer: Time for a fixed volume to flow between two marks under gravity

Q15. For quality control, which density-related parameter is most useful to detect adulteration of an essential oil used in cosmetics?

• Refractive index only
• Specific gravity compared to authentic reference values
• Color under UV light only
• pH measurement

Correct Answer: Specific gravity compared to authentic reference values

Q16. Which rheological term describes the minimum stress needed to initiate flow in some cosmetic creams?

• Viscosity index
• Yield stress
• Shear modulus
• Surface tension

Correct Answer: Yield stress

Q17. What is the main limitation when using a hydrometer to measure density of cosmetic liquids?

• Hydrometers provide extremely high precision for viscous creams
• Hydrometer readings can be affected by surface tension, temperature, and are difficult for viscous or non-homogeneous samples
• Hydrometers are not affected by dissolved solids
• Hydrometers measure viscosity not density

Correct Answer: Hydrometer readings can be affected by surface tension, temperature, and are difficult for viscous or non-homogeneous samples

Q18. When interpreting rheological data for a cosmetic emulsion, why is it important to report the measurement temperature and pre-shear history?

• Because temperature and shear history significantly influence viscosity, structure, and apparent flow behavior of emulsions
• Because these parameters only affect color, not rheology
• Because pre-shear history determines chemical composition
• Because temperature has no effect if the sample is opaque

Correct Answer: Because temperature and shear history significantly influence viscosity, structure, and apparent flow behavior of emulsions

Q19. Which mathematical model is commonly used to describe shear-thinning behavior in cosmetic formulations?

• Newtonian model
• Power-law (Ostwald-de Waele) model
• Ideal gas law
• Beer’s law

Correct Answer: Power-law (Ostwald-de Waele) model

Q20. During a viscosity measurement, the instrument shows unexpected oscillations in torque for a gel sample. Which is the most plausible cause and corrective action?

• Cause: thermal degradation; Action: increase spindle speed indefinitely
• Cause: slip at the spindle/sample interface or sample instability; Action: use appropriate roughened geometry or add a vane spindle and ensure proper sample loading
• Cause: wrong density; Action: change density standards
• Cause: too low room humidity; Action: humidify the lab

Correct Answer: Cause: slip at the spindle/sample interface or sample instability; Action: use appropriate roughened geometry or add a vane spindle and ensure proper sample loading

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