Which of the following oxides, when added to water, produces 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; they react with water to form acids, lowering pH. Metal oxides are basic and form alkaline solutions.
Why this answer is correct
SO3 is a non-metal oxide. It reacts with water to form sulfuric acid (H2SO4), which is a strong acid, giving a solution with pH < 7. Na2O, CaO, and MgO are metal oxides that form bases (NaOH, Ca(OH)2, Mg(OH)2) and produce basic solutions (pH > 7).
Example
CO2 dissolves in water to form carbonic acid (H2CO3), giving a slightly acidic solution (pH ~5.6).
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