Oxides
Acidic, basic, amphoteric and neutral oxides.
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Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 7.2.1Classify oxides as acidic, basic, amphoteric or neutral.
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Acidic, basic, amphoteric and neutral oxides
Oxides can be classified by how they react with acids and bases:
Basic oxides are typically metal oxides. They react with acids to form a salt and water, and do not react with bases. Example: copper(II) oxide + hydrochloric acid → copper(II) chloride + water.
Acidic oxides are typically non-metal oxides. They react with bases (alkalis) to form a salt and water, and many also dissolve in water to form an acidic solution. Example: carbon dioxide dissolves in water to form a weakly acidic solution (carbonic acid), and sulfur dioxide is responsible for acid rain.
Amphoteric oxides react with both acids and bases, forming a salt and water either way. The two IGCSE examples are aluminium oxide and zinc oxide.
Neutral oxides react with neither acids nor bases. Examples include carbon monoxide and water itself.
The general pattern
This classification follows the Periodic Table reasonably well: elements on the left (metals) tend to form basic oxides, elements on the right (non-metals) tend to form acidic oxides, and the amphoteric examples sit near the metal/non-metal boundary.
Think of it like this
An amphoteric oxide is like a person fluent in two languages who can hold a conversation with either group — it "speaks the language" of both acids and bases, reacting comfortably with either.
Common misconceptions
- Assuming all metal oxides are basic without exception. Aluminium oxide and zinc oxide, both metal oxides, are amphoteric rather than simply basic.
- Thinking a neutral oxide is the same as a basic or acidic oxide that simply "doesn't work very well". Neutral oxides genuinely do not react with either acids or bases.
- Believing an oxidising oxide reacting with an acid always gives the same products as it would with a base. For an amphoteric oxide, the identity of the salt formed depends on which one it reacts with.
In the exam
- To prove an oxide is amphoteric, an answer needs to show it reacting with BOTH an acid and a base, forming a salt and water each time — one reaction alone does not establish amphoteric behaviour.
- When asked to classify an unfamiliar oxide, check first whether it is formed from a metal or a non-metal element as your starting point.