Zinc oxide is said to be amphoteric because it

Step 1: Analyze the question The question asks why zinc oxide (ZnO) is described as amphoteric. So we need the meaning of "amphoteric".

Step 2: The concept An amphoteric substance is one that can behave both as an acid and as a base. In practice, it reacts with both acids and bases to form salt and water.

Common amphoteric oxides and hydroxides to remember: ZnO, Al₂O₃, PbO, and their hydroxides Zn(OH)₂, Al(OH)₃, Pb(OH)₂.

Step 3: Show the reactions

• With an acid (ZnO acts as a base): ZnO + 2HCl → ZnCl₂ + H₂O

• With a base (ZnO acts as an acid): ZnO + 2NaOH + H₂O → Na₂[Zn(OH)₄] (sodium zincate) or simply: ZnO + 2NaOH → Na₂ZnO₂ + H₂O

Step 4: Evaluate the options

• A. Forms an acid salt: wrong. An acid salt comes from partial replacement of the hydrogen in an acid (e.g. NaHSO₄). It has nothing to do with amphoterism.
• B. Is an insoluble base: wrong. ZnO is insoluble in water, but being insoluble is not what makes it amphoteric, and it is not purely basic.
• C. Forms a double salt: wrong. A double salt (e.g. alum) is a different concept.
• D. Reacts with a base or an acid: correct. This is exactly the definition of amphoteric behaviour.

Tip: "Amphi" means "both". Amphoteric = reacts with both acids and bases. When you see ZnO, Al₂O₃ or PbO in an exam, think "amphoteric" straight away.