Why is caffeine only soluble in water that is strongly acidic?
Is that true? I think the solubility of caffeine is about 180 mg/L in water @ 100 Celsius. I guess it depends upon your definition of solubility. At any rate, look at the structure of caffeine. There is a N in one of the heterocyclic rings that has a pair of electrons that are unused. That makes caffeine a base. The pKa for caffeine is about 10.5 or so and Kb = about 1 x 10-4--not exactly a strong base but stronger than NH3 at 10^-5.