Predict Period 3 oxide reactions with water
Write the specified Period 3 oxide–water reactions and infer whether resulting solutions are acidic, neutral or alkaline.
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 type of solution forms when Na₂O reacts with water?
What you will learn
- Write oxide–water equations for the specified Period 3 oxides
- Identify oxides with no reaction in water
- Predict likely solution pH qualitatively
Metal oxides form basic solutions or remain unreacted
Na₂O reacts with water: Na₂O(s) + H₂O(l) → 2NaOH(aq), giving an alkaline solution. MgO reacts slowly and only slightly with water: MgO(s) + H₂O(l) → Mg(OH)₂(s/aq); the sparingly soluble hydroxide gives a weakly alkaline suspension/solution.
Al₂O₃ and SiO₂ do not react appreciably with water under the syllabus conditions. Their presence does not make the water strongly alkaline or acidic; an unreacted solid is not the same as an oxide producing ions in solution.
Non-metal oxides form acidic solutions
P₄O₁₀ reacts with water: P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq). SO₂ dissolves to give an acidic solution, often represented as SO₂(aq) + H₂O(l) ⇌ H₂SO₃(aq). SO₃ reacts to form sulfuric acid: SO₃(g) + H₂O(l) → H₂SO₄(aq).
The likely pH direction is more reliable than an exact pH value without concentration data: Na₂O-derived solution is alkaline; P₄O₁₀, SO₂ and SO₃ solutions are acidic. Avoid assigning a numerical pH when no concentration is given.
Pause and explain
Which Period 3 oxide is expected not to react appreciably with water in this syllabus set?
Worked example
Predict the likely pH class when P₄O₁₀ reacts with water and write the equation.
Show the worked solution
- P₄O₁₀ forms phosphoric acid in water.
- Balance four phosphorus atoms by making four H₃PO₄.
- Balance H and O with six H₂O.
Answer P₄O₁₀ + 6H₂O → 4H₃PO₄; the solution is acidic (pH below 7 in the stated reference).
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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 the reaction of Na₂O with water and classify the resulting solution.
Give me a hint
Form aqueous sodium hydroxide and balance sodium atoms.
Compare my reasoning
- One Na₂O has two Na atoms.
- Use 2NaOH as product.
- NaOH makes an alkaline solution.
Na₂O(s) + H₂O(l) → 2NaOH(aq); the solution is alkaline.
Look for these in your work
- Equation balanced
- Product identified
- pH class correct
State what happens when Al₂O₃ and SiO₂ are added to water under the syllabus conditions.
Give me a hint
Distinguish an insoluble solid from an oxide that reacts to form acid or alkali.
Compare my reasoning
- Neither oxide reacts appreciably with water.
- They remain as solids rather than producing a substantial new solution species.
- Do not assign an acid or alkaline pH from these oxide additions alone.
Al₂O₃ and SiO₂ do not react appreciably with water; do not infer a substantial pH change from adding them.
Look for these in your work
- Both oxides named
- No-reaction behavior stated
- pH limitation noted
Compare the likely pH direction for SO₂(aq) and SO₃ added to water. Include representative equations.
Give me a hint
Both form acidic solutions, but one is represented by an equilibrium and one by formation of sulfuric acid.
Compare my reasoning
- SO₂ + H₂O ⇌ H₂SO₃ represents an acidic solution.
- SO₃ + H₂O → H₂SO₄ forms an acidic solution.
- Both are below neutral in the stated reference; no exact pH follows without concentration.
Both solutions are acidic: SO₂ + H₂O ⇌ H₂SO₃ and SO₃ + H₂O → H₂SO₄. Exact pH needs concentration data.
Look for these in your work
- Equations distinguished
- Acidity direction correct
- No unsupported numerical pH
Common mistakes
- Claiming every oxide reacts with water
- Calling an insoluble solid's suspension a solution of known pH
- Writing a numeric pH without concentration
- Confusing a reaction with water with acid/base reaction against another reagent
What can you explain now?
What type of solution forms when Na₂O reacts with water?
Compare with the explanation
Alkaline
Na₂O forms sodium hydroxide, which produces OH⁻ in water.
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Make it stick
Which Period 3 oxides react with water, which do not, and what pH direction follows?
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