To tell if SO3 (Sulfur trioxide) forms an acidic, basic (alkaline), or neutral solution in water we need to recognize the SO3 is a non-metal oxide (a non-metal bonded to Oxygen). Non-metal oxides tend to react with water to form basic solutions. For example:
SO3 + H2O = H2SO4.
H2SO4 is considered a strong acid and would result in an acidic solution.
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