Which metal will dissolve in aqueous sodium hydroxide?

Analyze the question: We need a metal that reacts with (dissolves in) aqueous NaOH. This points to an amphoteric metal, one that reacts with both acids and strong bases.

Concept: Amphoteric metals and their oxides/hydroxides include aluminium, zinc, lead, and tin. Reaction with a strong alkali gives a soluble complex salt and hydrogen gas.

Checking the options:

• A. Aluminium: Dissolves in NaOH. This is the classic example in Nigerian exams. 2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂ (sodium aluminate, or tetrahydroxoaluminate)
• B. Copper: Not amphoteric. It does not react with NaOH.
• C. Iron: Does not dissolve in NaOH, since Fe(OH)₂ and Fe(OH)₃ are insoluble basic hydroxides.
• D. "Sikeer": This is probably a typo for "silver". Silver does not react with NaOH either.
• E. Tin: Tin is actually also amphoteric. It dissolves in hot, concentrated NaOH to form sodium stannate, Sn + 2NaOH + 4H₂O → Na₂[Sn(OH)₆] + 2H₂. However, this needs hot, concentrated alkali, and it is not the standard textbook answer.

Aluminium is the best answer because it readily dissolves in aqueous NaOH, even at ordinary conditions, and is the standard example in the syllabus. Tin is technically amphoteric, but it reacts only under harsher conditions, so it is not the intended answer.

Answer: A (aluminium)

Exam tip: Remember the amphoteric metals with "Al Zn Pb Sn" (think "All Zebras Php Sn..."), but when only one is offered as a simple choice, aluminium and zinc are the usual NECO/WAEC/JAMB picks. Also, when a question says "aqueous NaOH" without "hot" or "concentrated," pick aluminium.