Reactivity of basic oxides with cold water

Steps:

• Classify oxides: A (MgO) basic, B (Al2O3) amphoteric, C (SiO2) acidic, D (Na2O) strongly basic.
• Recall alkali metal oxides like Na2O react exothermically with water to form hydroxides.
• Note alkaline earth oxides like MgO react slowly, requiring heat for noticeable reaction.
• Confirm amphoteric and acidic oxides are insoluble and unreactive in cold water.

Why D is correct:

• Na2O reacts vigorously with cold water per the equation Na2O + H2O → 2NaOH, producing a strong base solution.

Why the others are wrong:

• A. MgO reacts too slowly with cold water to form Mg(OH)2 significantly in one day.
• B. Al2O3 is insoluble and does not react with cold water.
• C. SiO2 is insoluble quartz and unreactive with cold water.

Final answer: D