Amphoteric oxides: reactions on both sides
Recognise aluminium oxide and zinc oxide by their reactions with acids and bases.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Explain acidic and basic oxide behaviour.
- Distinguish an oxide's reaction pattern from its solubility.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What products belong to a basic oxide reacting with an acid?
What you will learn
- Define amphoteric oxides through reactions with both acids and bases to form salt and water.
- Classify Al₂O₃ and ZnO and distinguish amphoteric from a neutral or universal metal-oxide rule.
Both reactions are the defining evidence
An amphoteric oxide reacts with acids and with bases to produce salts and water in the syllabus description. Aluminium oxide, Al₂O₃, and zinc oxide, ZnO, are the two required examples. Reaction with acid demonstrates one side of the behaviour; reaction with a suitable strong base under appropriate conditions demonstrates the other. One acid reaction alone does not distinguish amphoteric ZnO from an ordinary basic oxide.
For acid examples, ZnO+2HCl→ZnCl₂+H₂O and Al₂O₃+6HCl→2AlCl₃+3H₂O conserve atoms. With suitable bases, salts of zinc-containing or aluminium-containing species form. The precise salt formula can depend on conditions and representation; the syllabus objective here is the amphoteric definition and these oxide identities, rather than memorising one formula as universal for every aqueous and heated alkali system.
Amphoteric does not mean neutral
A neutral oxide does not show the same acid–base salt-and-water behaviour, whereas an amphoteric oxide can respond on both sides. Neither label means automatically safe to handle. Aluminium and zinc are metals, but their oxides demonstrate why the simple metal-oxide/basic trend has limits. Aluminium oxide's insolubility in water also does not prevent its amphoteric reaction behaviour.
When an unfamiliar oxide is tested in a supplied question, identify separate evidence for reaction with acid and reaction with base. State the observed or supplied products and the conditions rather than guessing a category from a metal name alone. Practical reaction conditions and preparation are teacher-supervised; students can classify and explain supplied evidence without performing the tests at home.
Pause and explain
Which evidence distinguishes an amphoteric oxide most clearly?
Worked example
An oxide X reacts with hydrochloric acid and, separately, with a suitable strong base to form salts and water. Classify X and give the two required named examples.
Show the worked solution
- Reaction with acid alone could support basic or amphoteric behaviour.
- Reaction with base as well provides the second part of the amphoteric definition.
- Al₂O₃ and ZnO are the required examples.
Answer X is amphoteric; aluminium oxide and zinc oxide are the named examples.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define amphoteric and identify Al₂O₃ and ZnO.
Give me a hint
Use both acid and base reactions in the definition.
Compare my reasoning
- An amphoteric oxide reacts with acids to form salt and water.
- It also reacts with suitable bases to form salt and water in the syllabus description.
- The required examples are aluminium oxide Al₂O₃ and zinc oxide ZnO.
Amphoteric: reacts with both acids and bases; Al₂O₃ and ZnO are examples.
Look for these in your work
- Both kinds of reaction appear.
- Names and formulas agree.
Balance aluminium oxide reacting with hydrochloric acid. Explain why that equation alone is not a complete demonstration of amphoteric behaviour.
Give me a hint
Two aluminium atoms form two AlCl₃ units.
Compare my reasoning
- Write Al₂O₃+HCl→AlCl₃+H₂O, keeping formulas fixed.
- Balance Al₂O₃+6HCl→2AlCl₃+3H₂O.
- This shows only acid-side behaviour; reaction with a suitable base is also needed as evidence of amphoteric character.
Al₂O₃+6HCl→2AlCl₃+3H₂O; acid reaction alone does not establish both behaviours.
Look for these in your work
- All atoms balance.
- The evidence limitation is explicit.
A supplied oxide reacts with acid but not with the stated base under suitable test conditions. Another reacts with both to form salts and water. Classify the second and assess whether the first has proven amphoteric behaviour.
Give me a hint
Require the two separate reactions rather than transferring the second oxide's result to the first.
Compare my reasoning
- The second oxide fits amphoteric behaviour because both acid and base reactions are supplied.
- The first's acid reaction alone has not demonstrated amphoteric behaviour.
- In this evidence set the first supports basic behaviour; avoid asserting a different untested base condition without information.
The second is amphoteric; the first has not shown amphoteric behaviour in the supplied comparison.
Look for these in your work
- Each substance's evidence is kept separate.
- The conclusion stays within supplied conditions.
Common mistakes
- Using one acid reaction as proof of amphoteric behaviour.
- Calling amphoteric the same as neutral.
- Assuming every metal oxide reacts only with acids.
- Treating one zincate or aluminate formula as independent of conditions.
What can you explain now?
Name the required amphoteric oxides and give their formulas.
Compare with the explanation
Aluminium oxide, Al₂O₃, and zinc oxide, ZnO.
Both have acid- and base-reacting behaviour under appropriate conditions, unlike a simple basic-only classification.
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Make it stick
Explain amphoteric using two reaction directions and the two required oxide formulas. State why neither metal identity nor one acid reaction suffices.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.