Classify Period 3 oxides and hydroxides as acidic, basic or amphoteric
Use reaction equations to distinguish basic, acidic and amphoteric Period 3 oxides and hydroxides.
Section 9 · The Periodic Table: chemical periodicity
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Period 3 trends are qualitative unless supplied data are given. Equations show the stated products and conditions; observations and pH depend on concentration and experimental conditions. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, balanced equations and ionic charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does amphoteric mean for an oxide?
What you will learn
- Classify the specified oxides by acid/base behavior
- Write representative neutralisation equations
- Explain amphoteric behavior of aluminium oxide and hydroxide
Basic oxides react with acids
Na₂O and MgO are basic oxides. For example, Na₂O + 2HCl → 2NaCl + H₂O and MgO + 2HCl → MgCl₂ + H₂O. NaOH is a strong soluble alkali; Mg(OH)₂ is a base but is sparingly soluble in water.
A basic oxide consumes acid to form a salt and water. A hydroxide containing OH groups is not automatically soluble: distinguish chemical basicity from how much dissolves.
Aluminium compounds are amphoteric; non-metal oxides are acidic
Al₂O₃ and Al(OH)₃ are amphoteric: they react with acids and with aqueous sodium hydroxide. Ionic equations include Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O; Al₂O₃ + 2OH⁻ + 3H₂O → 2[Al(OH)₄]⁻; Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O; Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻.
P₄O₁₀, SO₂ and SO₃ are acidic oxides. Representative reactions with sodium hydroxide are P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O; SO₂ + 2NaOH → Na₂SO₃ + H₂O; SO₃ + 2NaOH → Na₂SO₄ + H₂O. Conditions and product forms matter; use the equation and reagent stated in the question.
Pause and explain
Which Period 3 oxide is amphoteric in the syllabus set?
Worked example
Show that Al₂O₃ is amphoteric using one acid and one alkali equation.
Show the worked solution
- With acid: Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O.
- With aqueous hydroxide: Al₂O₃ + 2OH⁻ + 3H₂O → 2[Al(OH)₄]⁻.
- Reaction with both acid and base establishes amphoteric behavior.
Answer Al₂O₃ reacts with both H⁺ and OH⁻, so it is amphoteric.
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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.
Write an equation for MgO reacting with hydrochloric acid and classify MgO.
Give me a hint
A basic oxide neutralises acid to form a salt and water.
Compare my reasoning
- Mg²⁺ combines with two Cl⁻ to form MgCl₂.
- O²⁻ and 2H⁺ form H₂O.
- MgO + 2HCl → MgCl₂ + H₂O.
MgO + 2HCl → MgCl₂ + H₂O; MgO is a basic oxide.
Look for these in your work
- Salt formula correct
- Equation balanced
- Classification justified
Write the ionic reaction of Al(OH)₃ with excess aqueous OH⁻ and identify the behavior shown.
Give me a hint
Use the soluble tetrahydroxoaluminate ion under alkaline aqueous conditions.
Compare my reasoning
- Al(OH)₃ reacts with one OH⁻.
- The product ion is [Al(OH)₄]⁻.
- Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻.
Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻; this is its reaction with base and supports amphoteric behavior.
Look for these in your work
- Aqueous hydroxide specified
- Atoms and charge balanced
- Base reaction interpreted
A learner labels SO₃ as a base because it contains oxygen. Correct the classification with a reaction.
Give me a hint
Use its reaction with water or sodium hydroxide to identify the acidic behavior.
Compare my reasoning
- SO₃ forms sulfuric acid with water.
- It also reacts with NaOH to form sulfate and water.
- SO₃ + 2NaOH → Na₂SO₄ + H₂O.
SO₃ is an acidic oxide: it forms H₂SO₄ with water and is neutralised by NaOH to form Na₂SO₄ and H₂O.
Look for these in your work
- Classification corrected
- Evidence supplied
- Balanced equation included
Common mistakes
- Calling Al₂O₃ only basic because it reacts with acid
- Calling Mg(OH)₂ a strong soluble alkali
- Confusing acidic oxide with a dissolved acid
- Writing an aluminium aluminate equation without stating aqueous alkaline conditions
What can you explain now?
What does amphoteric mean for an oxide?
Compare with the explanation
It reacts with both acids and bases
Both acid and alkali behavior are required.
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Make it stick
Which reaction evidence distinguishes basic, acidic and amphoteric Period 3 oxides?
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.