Salam Academy
Sign inCreate account
Learn0620Papers 2, 4 · ExtendedAcids, bases and saltsOxides
Acids, bases and salts · Subtopic 2 of 3

Oxides

Metal oxides are usually basic: they react with acids to form salts. Non-metal oxides are usually acidic: they react with bases, and soluble ones give acidic solutions. Amphoteric oxides (Al2O3\mathrm{Al_2O_3}, ZnO\mathrm{ZnO}) react with both acids and bases. Neutral oxides (CO, NO, N2O\mathrm{N_2O}) react with neither.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 25 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Classify oxides as acidic, basic, amphoteric or neutral.
  • Predict the reactions of oxides.
Pause and think

Why does burning sulfur-containing coal cause acid rain, while burning magnesium never does?

Section 2 of 14

Core idea

Start hereAcid from the sky

Rain falling over a coal power station can be as acidic as vinegar. The culprit is an invisible oxide of sulfur. Whether an oxide is acidic or basic depends on whether its element is a metal.

The big idea

Metal oxides are basic, non-metal oxides acidic. A few (ZnO, Al2O3\mathrm{Al_2O_3}) are amphoteric; a few (CO, NO) are neutral.

1. Classification

Classifying an oxide

Test with an acid and with an alkali.

Worked exampleSulfur dioxide and lime

Power stations remove SO2\mathrm{SO_2} from flue gases with calcium oxide.

Goal
Explain and write an equation.
Strategy
Acidic oxide + basic oxide → salt.
  1. SO2 acidic, CaO basic\mathrm{SO_2}\ \text{acidic},\ \mathrm{CaO}\ \text{basic}
    Non-metal vs metal oxide.
  2. CaO+SO2→CaSO3\mathrm{CaO+SO_2\rightarrow CaSO_3}
    They neutralise to form calcium sulfite.
AnswerCaO+SO2→CaSO3\mathrm{CaO+SO_2\rightarrow CaSO_3}
✓ CheckAtoms balance ✓.
Edge caseInsoluble but still acidic SiO2\mathrm{SiO_2} doesn't dissolve in water, but it reacts with hot concentrated alkali, so it is classed as acidic.
Pause and think

Why can't you tell whether CuO is basic just by adding it to water?

Summary

  • Metal → basic. Non-metal → acidic.
  • ZnO, Al2O3\mathrm{Al_2O_3} amphoteric.
  • CO, NO neutral.
Key words
Amphoteric
Reacts with both acids and bases.
Basic oxide
A metal oxide that neutralises acids.
Acidic oxide
A non-metal oxide that reacts with bases.
HistoryLavoisier's mistake. Lavoisier named oxygen from Greek words meaning "acid-maker", because he thought all acids contained it. Hydrochloric acid, HCl, proved him wrong.
Fun factLimelight. Heating calcium oxide (lime) makes it glow brilliant white. Theatres used it as stage lighting in the 1800s, giving us the phrase "in the limelight".
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in