Metal Oxide – IGCSE Chemistry Definition
IGCSE Chemistry definition of metal oxide: a compound of a metal and oxygen that acts as a base, reacting with acids to form a salt and water.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
A metal oxide is a compound composed of a metal element and oxygen. Most metal oxides are classified as basic oxides because they react with acids to produce a salt and water. Some metal oxides are amphoteric, meaning they can react with both acids and bases. Metal oxides are an important class of bases in IGCSE Chemistry.
Basic metal oxides
Most metal oxides are basic. When a basic oxide reacts with an acid, the products are a salt and water. This is a neutralisation reaction. For example, copper(II) oxide reacts with sulfuric acid to form copper(II) sulfate and water: CuO + H₂SO₄ → CuSO₄ + H₂O.
Soluble metal oxides (mainly Group I and some Group II oxides) dissolve in water to form alkaline solutions. Sodium oxide dissolves in water to form sodium hydroxide: Na₂O + H₂O → 2NaOH. These are also basic oxides.
Amphoteric metal oxides
A few metal oxides are amphoteric. Aluminium oxide (Al₂O₃) and zinc oxide (ZnO) can react with both acids and bases. For example, ZnO reacts with HCl to form ZnCl₂ + H₂O, and also reacts with NaOH to form Na₂ZnO₂ + H₂O. This dual reactivity sets them apart from purely basic oxides.
Exam context
Paper 2 and Paper 4 questions ask candidates to classify oxides as acidic, basic, amphoteric, or neutral. Knowing that metal oxides are usually basic is a key rule. Candidates may also be asked to write equations for the reaction of a metal oxide with an acid.
Worked exam question
Magnesium oxide reacts with dilute hydrochloric acid. (a) Classify magnesium oxide. (b) Write the balanced equation. (c) Name the salt formed. (3 marks)
(a) Basic oxide [1]. (b) MgO + 2HCl → MgCl₂ + H₂O [1]. (c) Magnesium chloride [1].
Common mistakes
Candidates sometimes confuse metal oxides with non-metal oxides. The type of element bonded to oxygen determines the classification. Metal + oxygen = basic (or amphoteric). Non-metal + oxygen = acidic (or neutral). Writing “metallic oxide” instead of “metal oxide” is acceptable but less precise.
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