Which of the following oxides, when added to water, produces a solution with a pH greater than 7?
Show the answer and explanation
Correct answer
C. CaO
Principle or equation
Metal oxides are basic and react with water to form metal hydroxides, producing alkaline solutions. Non-metal oxides are acidic and form acids.
Why this answer is correct
CaO (calcium oxide) is a basic oxide. It reacts with water: CaO + H₂O → Ca(OH)₂, which is a strong base, giving a pH > 7. The other oxides are non-metal oxides: SO₂ forms sulfurous acid, CO₂ forms carbonic acid, P₂O₅ forms phosphoric acid, all acidic (pH < 7).
Example
Na₂O + H₂O → 2NaOH, pH > 7.
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