Oxides

Oxide questions combine classification with evidence. Avoid the overgeneralisation “metal oxide + water always forms an alkali”; many metal oxides are insoluble.

1. Definition

A. Oxide

An oxide is a compound that contains oxygen and one other element.

2. Key Ideas

• Oxides are classified by how they react with acids and alkalis:
  • acidic oxide: reacts with alkalis, but not acids, in the acid–base comparison (and often forms acids in water)
  • basic oxide: reacts with acids, but not alkalis, in the acid–base comparison
  • amphoteric oxide: reacts with both acids and alkalis
  • neutral oxide: reacts with neither acids nor alkalis (under normal lab conditions)
• Most non-metal oxides are acidic. Most metal oxides are basic. Learn the common amphoteric examples, but use reaction evidence when it is provided.
• Avoid overclaims:
  • Not all non-metal oxides form acids with water (some do not react, e.g. SiO₂).
  • Not all metal oxides react with water (many only react with acids, e.g. CuO).

3. Detailed Explanations

A. Acidic oxides (usually non-metal oxides)

An acidic oxide is an oxide that reacts with alkalis to form a salt (and often water). Many acidic oxides also form acids in water.

Reaction with an alkali shows acidic behaviour: CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l) SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)

These equations assume enough sodium hydroxide to form the carbonate or sulfite named. The oxide reacting with an alkali provides the classification evidence; merely finding oxygen in its formula does not.

Useful examples:

B. Basic oxides (usually metal oxides)

A basic oxide is an oxide that reacts with acids to form a salt and water (a neutralisation reaction).

Example: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

Some basic oxides react with water to form hydroxides. Their solutions are alkaline: Na₂O(s) + H₂O(l) → 2NaOH(aq) CaO(s) + H₂O(l) → Ca(OH)₂(s)

Calcium hydroxide is sparingly soluble. Some dissolves to make alkaline limewater, while excess solid can remain. Copper(II) oxide does not react with water to give an alkaline solution, but still neutralises acids: water solubility is not the test for basic behaviour.

Examples of basic oxides used in this course:

C. Amphoteric oxides (react with acids and alkalis)

An amphoteric oxide reacts with both acids and alkalis, forming salts.

Common amphoteric oxides to recognise:

With acids: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) PbO(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + H₂O(l)

Compare both reactions for zinc oxide

Use separate samples in the acid and alkali tests. White zinc oxide is insoluble in water, but reacts with dilute acid to give a colourless zinc-salt solution. It also reacts with a suitable strong sodium hydroxide solution to form soluble zincate species.

Both results establish amphoteric behaviour. One acid reaction alone would not distinguish zinc oxide from a basic oxide such as copper(II) oxide. The detailed zincate formula is not required for this classification; use a product formula if the question supplies one.

D. Neutral oxides (no acidic/basic behaviour)

A neutral oxide does not show acidic or basic properties (it does not react with acids or alkalis under normal conditions).

Examples (common at O-Level):

• water, H₂O
• carbon monoxide, CO
• nitrogen monoxide, NO

E. Summary table (classification by reactions)

4. Common Mistakes

• Writing “all non-metal oxides form acids with water” (false; some do not react).
• Writing “all metal oxides react with water” (false; many only react with acids).
• Using the wrong classification test: classification is about reactions, not about “metal vs non-metal” as an absolute rule.
• Leaving oxide reaction equations unbalanced.
• Forgetting that amphoteric oxides must react with both acids and alkalis.

5. Exam Tips

• If an oxide dissolves in water and the solution turns universal indicator purple, the solution is alkaline. A basic oxide such as Na₂O has reacted with water to form a soluble hydroxide; the oxide and resulting alkali are different substances.
• For an acidic oxide reacting with an alkali, use the products supplied and then balance the equation carefully.

6. Worked Examples

Modelled example 1

Classify from reactions

Core

Problem

Oxide X reacts with dilute hydrochloric acid to form salt and water but does not react with aqueous sodium hydroxide. Classify X.

Study the worked solution

1. Use the acid reaction

   Method

   Recognise neutralisation with acid.

   Reason

   An oxide that reacts with acid to form salt and water shows basic behaviour.

   Working

   X + acid → salt + water.

2. Use the non-reaction boundary

   Method

   Note that X does not react with alkali.

   Reason

   This rules out amphoteric behaviour.

   Working

   No reaction with NaOH(aq).

3. Classify

   Working

   Oxide X is basic.

Guided practice 2

Amphoteric logic

About 4 min

Problem

Oxide Y reacts with dilute nitric acid and aqueous sodium hydroxide, forming salts in both cases. Classify Y.

Match the reaction pattern

Oxide type

AmphotericBasic onlyNeutral

Check my attempt

Hints

Hint 1: two directions

The defining word for reacting with both acids and alkalis is amphoteric.

View solution step by step

1. Combine both observations

   Method

   Use reactions with acid and alkali together.

   Reason

   Neither reaction alone establishes the dual behaviour.

   Working

   Y reacts with both categories.

2. Classify

   Working

   Oxide Y is amphoteric.

Common misconception 3

Fix the overclaim

Find and correct the mistake

Learner claim

“All non-metal oxides form acids in water.” Explain why this is wrong and give a counterexample.

Test the absolute word

Counterexample

SiO2CO2SO2

Best classification basis

Reactions with acids/alkalis/waterElement name only

Check my attempt

View solution step by step

1. Provide the counterexample

   Method

   Use SiO₂.

   Reason

   It is a non-metal oxide that does not react with water to form an acid.

   Working

   SiO₂ + H₂O: no direct acid-forming reaction.

2. Correct the rule

   Method

   Classify oxides using reaction evidence.

   Reason

   Metal/non-metal character is a useful tendency, not an absolute test.

   Working

   Justify the oxide type from its reactions.

Examiner practice 4

Balance the equation (sodium oxide)

2 marks

Examination question

For a reaction whose specified product is sodium oxide, balance Na + O₂ → Na₂O and include state symbols. [2 marks]

Change coefficients only

Write the balanced equation

View solution step by step

1. Balance oxygen

   1 mark

   Method

   Place 2 before Na₂O.

   Reason

   One O₂ molecule supplies two oxygen atoms.

   Working

   Na + O₂ → 2Na₂O.

2. Balance sodium and state symbols

   1 mark

   Method

   Place 4 before Na and add the state symbols.

   Reason

   Two formula units contain four sodium atoms.

   Working

   4Na(s) + O₂(g) → 2Na₂O(s).

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark coefficients and complete state-labelled equation.

Mark points

1. 1 mark: Uses 4:1:2 coefficients.
2. 1 mark: Includes Na(s), O2(g) and Na2O(s).

Calculate my mark

Challenge 5

Acidic oxide + alkali

Minimal support

Equation transfer

Write a balanced equation for CO₂ reacting with aqueous sodium hydroxide to form sodium carbonate and water.

Balance the named products

Balanced equation

CO2 + 2NaOH → Na2CO3 + H2OCO2 + NaOH → Na2CO3 + H2OCO2 + O2 → CO3

Check my attempt

Hints

Hint 1: balance sodium

The product Na₂CO₃ contains two sodium atoms.

View solution step by step

1. Write the product skeleton

   Method

   Use sodium carbonate and water as supplied.

   Reason

   An acidic oxide reacts with alkali to form salt and water.

   Working

   CO₂ + NaOH → Na₂CO₃ + H₂O.

2. Balance and add states

   Method

   Place 2 before sodium hydroxide.

   Reason

   This supplies two Na and makes H and O counts balance.

   Working

   CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l).

Next step

Try Mind Stretcher 1 independently.

7. Mind Stretchers

Mind stretcher 1: Identify the oxide from dataExtension

Question: A white oxide reacts with water to give a clear solution. The solution turns universal indicator purple and reacts with dilute hydrochloric acid to form a salt and water. Suggest a possible oxide.

Show answer

A possible answer is sodium oxide, Na₂O. It forms NaOH(aq) in water (alkaline solution) and neutralises acids.

Mind stretcher 2: Classify from two testsExtension

Question: Oxide Z does not react with dilute hydrochloric acid but reacts with aqueous sodium hydroxide to form a salt and water. Classify Z and justify your answer from both observations.

Show answer

Z is an acidic oxide. Its reaction with an alkali shows acidic behaviour, while its lack of reaction with dilute acid rules out basic and amphoteric behaviour under the tested conditions.

Try independently: Oxide Q leaves water neutral. Does that prove Q is a neutral oxide? Explain how you would obtain the evidence needed for classification.

8. Quiz