Which of the following oxides, when added to water, forms a solution with a pH less than 7?
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Correct answer
C. SO3
Principle or equation
Non-metal oxides are typically acidic oxides; they react with water to form acids, lowering pH below 7.
Why this answer is correct
SO3 is a non-metal oxide that reacts with water to form sulfuric acid (H2SO4), an acid that gives a pH < 7. Na2O, CaO, and K2O are metal oxides that form basic solutions (pH > 7).
Example
CO2 dissolves in water to form carbonic acid, making the solution acidic.
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