Acidic and basic oxides: connect behaviour with elements
Classify the required examples through acid–base reactions while keeping the metallic trend qualified.
Section 7.2 · Oxides
Core and Extended · Chemistry 0620, 2026–2028. Amphoteric exceptions follow in an Extended lesson.
Before you begin
- Identify metals and non-metals in formulas.
- Describe acid–base neutralisation products.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which element in CO₂ is non-metallic?
What you will learn
- Classify SO₂ and CO₂ as acidic, and CuO and CaO as basic oxides.
- Connect the examples with non-metallic and metallic character without treating the trend as universal.
Classify by reaction, not oxygen content alone
Carbon dioxide and sulfur dioxide are acidic oxides in the required examples. They are oxides of non-metallic elements and react with suitable bases to form salts and water. For instance, CO₂+2NaOH→Na₂CO₃+H₂O describes carbon dioxide reacting with excess sodium hydroxide in the stated carbonate-forming comparison. The ratio and conditions matter; a different amount of alkali can give a different carbonate species.
Copper(II) oxide and calcium oxide are basic oxides of metals. They react with acids to form salts and water: CuO+2HCl→CuCl₂+H₂O and CaO+2HCl→CaCl₂+H₂O. Copper(II) oxide is insoluble in water but remains basic; solubility does not decide whether an oxide can neutralise an acid.
Use the trend without erasing exceptions
Metallic character often accompanies basic oxide behaviour in these examples, while non-metallic character accompanies acidic oxide behaviour. State both the example and its reaction evidence. Oxygen in a formula does not make a substance acidic, and a metal oxide is not automatically an alkali. Calcium oxide reacts with water to give calcium hydroxide; CuO does not simply give an alkaline aqueous solution by dissolving.
The trend has exceptions. Some metal oxides react with both acids and bases and are amphoteric, as taught in the Extended sequence. Some other oxides are neutral rather than fitting the acidic/basic pair. Do not use this lesson's four examples to assert that every non-metal oxide is acidic or every metal oxide has only basic reactions.
Pause and explain
Why can insoluble CuO still be classified as basic?
Worked example
Classify CO₂ and CuO, giving one reaction equation as evidence for each.
Show the worked solution
- CO₂ is an acidic oxide of the non-metal carbon.
- CO₂+2NaOH→Na₂CO₃+H₂O demonstrates reaction with a suitable base under the stated conditions.
- CuO is a basic oxide of copper; CuO+2HCl→CuCl₂+H₂O demonstrates its reaction with acid.
Answer CO₂ is acidic; CuO is basic, supported by their reactions with base and acid respectively.
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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.
Classify SO₂ and CuO and relate each to the element bonded with oxygen.
Give me a hint
Match non-metal and metal with the required examples.
Compare my reasoning
- Sulfur is non-metallic, and SO₂ is an acidic oxide.
- Copper is metallic, and CuO is a basic oxide.
- Their reactions with bases and acids respectively support the categories.
SO₂: acidic/non-metal oxide. CuO: basic/metal oxide.
Look for these in your work
- Both examples are classified correctly.
- Character and reaction evidence are linked.
Write and balance calcium oxide with hydrochloric acid, and explain the oxide category.
Give me a hint
A basic oxide and acid form salt and water.
Compare my reasoning
- The salt is calcium chloride, CaCl₂.
- Balance CaO+2HCl→CaCl₂+H₂O.
- Reaction with acid to form salt and water supports CaO being a basic oxide.
CaO+2HCl→CaCl₂+H₂O; CaO is basic.
Look for these in your work
- The equation conserves atoms.
- The category is justified through behaviour.
A student says CO₂ is basic because it contains oxygen, and CuO is neutral because it is insoluble. Correct both classification rules.
Give me a hint
The important evidence is which acid–base reactions the oxide undergoes.
Compare my reasoning
- CO₂ is acidic in the required examples; suitable alkali neutralises it to form a salt and water.
- CuO is basic and neutralises acids despite being insoluble in water.
- Oxygen content and water solubility alone do not classify oxide character.
CO₂ is acidic and CuO basic; reaction behaviour replaces both incorrect rules.
Look for these in your work
- Both misconceptions are corrected.
- The explanation uses acid–base evidence.
Common mistakes
- Classifying every oxygen-containing compound as acidic.
- Calling insoluble CuO neutral because it does not dissolve to make an alkali.
- Treating the metallic/non-metallic trend as an exception-free law.
What can you explain now?
Classify SO₂ and CaO using the required oxide categories.
Compare with the explanation
SO₂ is acidic; CaO is basic.
Sulfur is non-metallic and calcium metallic; these required examples fit the typical oxide-character trend.
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Make it stick
Recall all four required oxide classifications. Give a reaction-based reason and one limitation of the metallic/non-metallic trend.
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.