Non-Metal Oxide – IGCSE Chemistry Definition

A non-metal oxide is a compound formed between a non-metal element and oxygen. Most non-metal oxides are classified as acidic oxides because they dissolve in water to form acids or react with bases to form a salt and water. A few non-metal oxides are neutral, meaning they do not react with either acids or bases.

Acidic non-metal oxides

Common acidic oxides include carbon dioxide (CO₂), sulfur dioxide (SO₂), sulfur trioxide (SO₃), and phosphorus(V) oxide (P₄O₁₀). When these dissolve in water, they form acids.

CO₂ + H₂O → H₂CO₃ (carbonic acid) SO₂ + H₂O → H₂SO₃ (sulfurous acid) SO₃ + H₂O → H₂SO₄ (sulfuric acid)

Acidic oxides also react with bases to form a salt and water. For example: CO₂ + 2NaOH → Na₂CO₃ + H₂O.

Neutral non-metal oxides

Water (H₂O) and carbon monoxide (CO) are neutral oxides. They do not react with acids or bases under normal conditions. Water is itself the solvent in which acids and bases operate, and CO is a molecular gas that does not ionise.

Link to acid rain

Sulfur dioxide and nitrogen oxides are acidic oxides released by burning fossil fuels and in vehicle engines. When they dissolve in rainwater, they form sulfurous acid, sulfuric acid, and nitric acid, causing acid rain. This link between non-metal oxides and environmental damage is a common exam topic.

Exam context

Paper 2 and Paper 4 questions test oxide classification. Candidates must identify whether an oxide is acidic, basic, amphoteric, or neutral based on the element involved. A common question asks candidates to write the equation for the reaction of a named non-metal oxide with water or with a base.

Worked exam question

Sulfur dioxide dissolves in rainwater. (a) Write the equation for this reaction. (b) Classify sulfur dioxide as an oxide. (c) Explain why this causes environmental problems. (3 marks)

(a) SO₂ + H₂O → H₂SO₃ [1]. (b) Acidic oxide [1]. (c) The sulfurous acid formed lowers the pH of rainwater, creating acid rain. Acid rain damages limestone buildings, kills fish in lakes, and harms trees and plants [1].

Common mistakes

Candidates sometimes classify CO as an acidic oxide. Carbon monoxide is a neutral oxide; it does not dissolve in water to form an acid. Carbon dioxide is the acidic one. Another error is saying non-metal oxides are “basic,” which reverses the rule. Non-metal oxides are acidic; metal oxides are basic.