WAEC 2011 Chemistry · Question 20

Which of the following metallic oxides is amphoteric?

Options

A.AI2O3
B.Fe2O3
C.MgO
D.Na2O

Answer: A. AI2O3

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We need the metallic oxide that is amphoteric, meaning it can react with both acids and bases.

Step 2: The concept

Basic oxides (usually metals like Na, Mg, Fe) react with acids only to form salt and water.
Acidic oxides (non-metals like CO₂, SO₂) react with bases only.
Amphoteric oxides react with both acids and bases to form salts and water.

The common amphoteric oxides to memorise are Al₂O₃, ZnO, PbO (and SnO). Remember them as "All Zebras Play" or simply "Al, Zn, Pb".

Step 3: Check each option

A. Al₂O₃: It reacts with acids and with bases, so it is amphoteric. • With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O • With base: Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]
B. Fe₂O₃: Basic oxide. It reacts with acids only.
C. MgO: Basic oxide. It reacts with acids only.
D. Na₂O: Strongly basic oxide. It dissolves in water to give NaOH.

Step 4: Answer

Exam tip: In WASSCE and JAMB, whenever "amphoteric oxide" appears, look first for Al₂O₃, ZnO or PbO. Group 1 and 2 metal oxides (Na₂O, MgO, CaO) are basic.

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