Basic oxides raise pH by forming hydroxides in water

Steps:

• Classify oxides: basic oxides form bases with water, acidic form acids, amphoteric or neutral do not significantly alter pH.
• Test MgO: reacts as MgO + H2O → Mg(OH)2, a base that increases OH- ions.
• Test others: Al2O3 insoluble and amphoteric, SiO2 neutral and insoluble, SO2 forms H2SO3 acid.
• Conclude: only basic oxide increases pH.

Why A is correct:

• MgO is a basic oxide; it reacts with water to form magnesium hydroxide (Mg(OH)2), which dissociates to release OH- ions, raising pH per Arrhenius base definition.

Why the others are wrong:

• B. Al2O3 is amphoteric and insoluble in water, forming no significant base or acid to change pH.
• C. SiO2 is neutral and insoluble, reacting minimally with water without pH increase.
• D. SO2 is an acidic oxide; it dissolves to form sulfurous acid (H2SO3), releasing H+ ions and decreasing pH.

Final answer: A