In a compatibility test, what should you observe to indicate compatibility?

• A. Temperature change
• B. Change in color
• C. Separation, precipitation, or gelation
• D. Increase in viscosity

Answer: (C)

Compatibility testing checks whether two formulations mix and stay uniformly combined without physical changes. If they are compatible, the mixture should remain evenly mixed and show no separation, clouding, precipitate, or gel formation over the observation period. Among common signs, separation, precipitation, or gelation actually indicate incompatibility, not compatibility. Temperature changes or color changes can occur for reasons unrelated to compatibility, and a simple increase in viscosity isn’t a definitive indicator by itself. So, to indicate compatibility, you’d observe the opposite of those signs: the mixture stays uniform with no separation, no precipitation, and no gel formation.