Why is "Saying amphoteric reacts only with acids" a common mistake in Oxides?

Why it happens: Recognising it as a metal oxide. How to avoid it: Amphoteric reacts with BOTH acids AND alkalis. Source: 0620/42 — recurring

Why is "Calling the non-metal oxide basic" a common mistake in Oxides?

Why it happens: Default thinking. How to avoid it: Non-metal oxides → ACIDIC (or neutral). Metal oxides → BASIC (or amphoteric).

Why is "Forgetting water as a neutral oxide" a common mistake in Oxides?

Why it happens: Not thinking of H_2O as 'an oxide'. How to avoid it: H_2O IS technically a neutral oxide of hydrogen.

Acids, Bases and SaltsCambridge IGCSE Chemistry 0620 Extended

Oxides

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Oxides — Cambridge IGCSE 0620 Chemistry Extended (2026)

Acidic, basic, neutral and amphoteric oxides. The classification depends on whether the oxide reacts with acids, bases, or both.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

9.2 — Classify oxides as acidic, basic, amphoteric or neutral.
9.2 — Describe reactions of common oxides with acids and bases.

Acidic oxides

Non-metal oxides. React with bases to make salt + water. Many dissolve in water to make acids.

Acidic oxides are usually formed by NON-METALS combining with oxygen.

Examples and the acids they form in water:

$\text{CO}_{2} + \text{H}_{2}\text{O} \rightleftharpoons \text{H}_{2}\text{CO}_{3}$ (carbonic acid).
$\text{SO}_{2} + \text{H}_{2}\text{O} \to \text{H}_{2}\text{SO}_{3}$ (sulfurous acid).
$\text{SO}_{3} + \text{H}_{2}\text{O} \to \text{H}_{2}\text{SO}_{4}$ (sulfuric acid).
$\text{NO}_{2}$ : dissolves in water to give a mixture of HNO₂ and HNO₃.

Reaction with bases. Acidic oxides neutralise bases: $\text{CO}_{2} + 2\text{NaOH} \to \text{Na}_{2}\text{CO}_{3} + \text{H}_{2}\text{O}.$

Acid rain. SO₂ from burning fossil fuels (sulfur impurity) reacts with O₂ and water in the atmosphere to make H₂SO₃ / H₂SO₄ → acid rain. Damages buildings (limestone), kills aquatic life, harms forests.

Worked qualitative. Why is sulfur dioxide a pollutant? It's an acidic oxide → forms acid rain → environmental damage.

Classify an oxide by what it reacts with: acids, bases, both, or neither.

Acidic oxide + base → salt + water.
Acidic oxide + water → acid.
Made by non-metals (often).
SO₂ contributes to acid rain.

Basic oxides

Metal oxides. React with acids to make salt + water. Soluble ones dissolve in water → alkalis.

Basic oxides are formed by METALS combining with oxygen.

Reaction with acids: $\text{MgO} + 2\text{HCl} \to \text{MgCl}_{2} + \text{H}_{2}\text{O}.$ $\text{CuO} + \text{H}_{2}\text{SO}_{4} \to \text{CuSO}_{4} + \text{H}_{2}\text{O}.$

Soluble basic oxides + water → alkali.

$\text{Na}_{2}\text{O} + \text{H}_{2}\text{O} \to 2\text{NaOH}$ .
$\text{CaO} + \text{H}_{2}\text{O} \to \text{Ca(OH)}_{2}$ .

(Most metal oxides are insoluble; only Group 1 and some Group 2 oxides are soluble.)

Worked qualitative. A student adds black copper(II) oxide powder to dilute sulfuric acid. The black solid disappears; the solution turns blue. Why? CuO reacts with H₂SO₄ to give CuSO₄ (blue solution).

Basic oxide + acid → salt + water.
Soluble metal oxide + water → alkali.
Most metal oxides are insoluble in water.
Examples: MgO, CuO, Na₂O.

Amphoteric and neutral oxides

Amphoteric: reacts with BOTH acids AND bases. Neutral: reacts with NEITHER.

Amphoteric oxides. React with BOTH acids AND bases.

Examples: $\text{Al}_{2}\text{O}_{3}$ , $\text{ZnO}$ .

Reactions of $\text{ZnO}$ :

With acid: $\text{ZnO} + 2\text{HCl} \to \text{ZnCl}_{2} + \text{H}_{2}\text{O}$ (acts as a base).
With alkali: $\text{ZnO} + 2\text{NaOH} + \text{H}_{2}\text{O} \to \text{Na}_{2}[\text{Zn(OH)}_{4}]$ (acts as an acid; forms a zincate complex). Sometimes simplified to $\text{ZnO} + 2\text{NaOH} \to \text{Na}_{2}\text{ZnO}_{2} + \text{H}_{2}\text{O}$ (sodium zincate).

Reactions of $\text{Al}_{2}\text{O}_{3}$ : Similar — reacts with both HCl and NaOH.

Cambridge tip. "Amphoteric" comes from Greek for "both". It's a property that some metalloid-region elements have because their oxides have a balance of properties.

Neutral oxides. Don't react with acids OR bases. Often non-metal oxides where the oxidation state is unusual.

$\text{CO}$ — carbon monoxide.
$\text{NO}$ — nitric oxide.
$\text{H}_{2}\text{O}$ — water (debatable; doesn't classically react as an acid or base, but is amphoteric in the modern sense).

Amphoteric: reacts with acid AND base. Al₂O₃, ZnO.
Neutral: reacts with NEITHER. CO, NO.
Cambridge spelling: 'amphoteric'.

How it’s examined

Oxide classification appears most years on Paper 2 (3-4 marks: classify a list, predict reactions) and Paper 4 (4-6 marks: explain amphoteric behaviour). Examiner reports flag students forgetting amphoteric oxides exist — they default to either basic or acidic.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (9.2); 0620/22 May/Jun 2024 — Q10 (oxide classification); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Oxides

Step-by-step solutions to past-paper-style questions on oxides, written exactly the way a tutor would explain them at the board.

1Classify oxides
Extended• Adapted from 0620/42 May/Jun 2024 Q9• classify
Question
Classify each as acidic, basic, amphoteric or neutral: $\text{CO}_{2}$ , $\text{Na}_{2}\text{O}$ , $\text{Al}_{2}\text{O}_{3}$ , $\text{CO}$ .
Step-by-step solution
1. Step 1
  $\text{CO}_{2}$ — non-metal oxide → acidic.
2. Step 2
  $\text{Na}_{2}\text{O}$ — Group I metal oxide → basic.
3. Step 3
  $\text{Al}_{2}\text{O}_{3}$ — reacts with both acids AND alkalis → amphoteric.
4. Step 4
  CO — does not react with acids or alkalis → neutral.
Answer
$\text{CO}_{2}$ : acidic. $\text{Na}_{2}\text{O}$ : basic. $\text{Al}_{2}\text{O}_{3}$ : amphoteric. CO: neutral.
2Acidic oxide + alkali
Extended• acidic
Question
Write the equation for $\text{SO}_{2}$ reacting with sodium hydroxide solution.
Step-by-step solution
1. Step 1
  Acidic oxide reacts with alkali to give salt + water.
2. Step 2
  $\text{SO}_{2}$ → sodium sulfite.
  $\text{SO}_{2} + 2\text{NaOH} \to \text{Na}_{2}\text{SO}_{3} + \text{H}_{2}\text{O}$
Answer
$\text{SO}_{2} + 2\text{NaOH} \to \text{Na}_{2}\text{SO}_{3} + \text{H}_{2}\text{O}$
3Amphoteric oxide reactions
Extended• amphoteric
Question
Write equations for $\text{Al}_{2}\text{O}_{3}$ reacting with (a) HCl and (b) NaOH.
Step-by-step solution
1. Step 1
  (a) With acid (acts as a base):
  $\text{Al}_{2}\text{O}_{3} + 6\text{HCl} \to 2\text{AlCl}_{3} + 3\text{H}_{2}\text{O}$
2. Step 2
  (b) With alkali (acts as an acid):
  $\text{Al}_{2}\text{O}_{3} + 2\text{NaOH} + 3\text{H}_{2}\text{O} \to 2\text{NaAl(OH)}_{4}$
Answer
Reacts with both acids and alkalis.
Examiner tip
'Amphoteric' is the keyword the mark scheme expects.

Key Definitions and Keywords — Oxides

Definitions to memorise and the exact keywords mark schemes credit for oxides answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Acidic oxide
Examiner keyword
Non-metal oxide that reacts with bases/alkalis to form salt + water. e.g. $\text{CO}_{2}$ , $\text{SO}_{2}$ .
Basic oxide
Examiner keyword
Metal oxide that reacts with acids to form salt + water. e.g. $\text{Na}_{2}\text{O}$ , $\text{MgO}$ .
Amphoteric oxide
Examiner keyword
Reacts with BOTH acids AND bases to form salt + water. e.g. $\text{Al}_{2}\text{O}_{3}$ , $\text{ZnO}$ .
Neutral oxide
Examiner keyword
Reacts with neither acids nor bases. e.g. CO, $\text{H}_{2}\text{O}$ , NO.

Common Mistakes and Misconceptions — Oxides

The traps other students keep falling into on oxides questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying amphoteric reacts only with acids
0620/42 — recurring
Why it happens
Recognising it as a metal oxide.
How to avoid it
Amphoteric reacts with BOTH acids AND alkalis.
✕Calling the non-metal oxide basic
Why it happens
Default thinking.
How to avoid it
Non-metal oxides → ACIDIC (or neutral). Metal oxides → BASIC (or amphoteric).
✕Forgetting water as a neutral oxide
Why it happens
Not thinking of $\text{H}_{2}\text{O}$ as 'an oxide'.
How to avoid it
$\text{H}_{2}\text{O}$ IS technically a neutral oxide of hydrogen.

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Oxides

Short Study Notes in the form of Slides

Short Study Notes — Oxides

Detailed Notes

What you’ll learn

How it’s examined

1Classify oxides

2Acidic oxide + alkali

3Amphoteric oxide reactions

Acidic oxide

Basic oxide

Amphoteric oxide

Neutral oxide

✕Saying amphoteric reacts only with acids

✕Calling the non-metal oxide basic

✕Forgetting water as a neutral oxide

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