OxidesCambridge IGCSE Chemistry: Revision notes
Section 1
How Are Oxides Classified?
An oxide is a compound formed between an element and oxygen. Oxides are classified according to how they behave with acids and bases, which is closely linked to whether the element is a metal or a non-metal.
- Acidic oxides: typically formed by non-metals; they react with bases (e.g. metal oxides/alkalis) but not with acids
- Basic oxides: typically formed by metals; they react with acids but not with bases
- Amphoteric oxides: react with both acids and bases (see below)
This links directly to the trend across the periodic table: elements on the left (metals) tend to form basic oxides, while elements on the right (non-metals) tend to form acidic oxides.
Sulfur dioxide (SO2) and carbon dioxide (CO2) are acidic oxides. Copper(II) oxide (CuO) and calcium oxide (CaO) are basic oxides.
Section 2
What Are Amphoteric Oxides?
Some oxides do not fit neatly into 'acidic' or 'basic' — they can react with both.
- An amphoteric oxide reacts with both acids and bases to produce a salt and water
- This is different from a normal basic oxide, which only reacts with acids, or a normal acidic oxide, which only reacts with bases
- Two amphoteric oxides you need to know are aluminium oxide (Al2O3) and zinc oxide (ZnO)
Don't assume all metal oxides are simply 'basic' — aluminium oxide and zinc oxide are important exceptions that also react with alkalis.
Section 3
How Do Bases Relate to Oxides?
Bases are the class of substances that includes many metal oxides.
- A base is defined as an oxide or hydroxide of a metal
- An alkali is simply a base that is soluble in water
- All alkalis are bases, but not all bases are alkalis, since many metal oxides and hydroxides are insoluble
- Bases react with acids in a neutralisation reaction to form a salt and water
When a question asks you to identify a base, remember it can be either an oxide or a hydroxide of a metal — not just hydroxides.
Section 4
How Do Bases React With Acids and Ammonium Salts?
The characteristic reactions of bases help confirm whether a substance is behaving as a base:
- Base + acid → salt + water (a neutralisation reaction)
- Base + ammonium salt → salt + water + ammonia gas (heating a base with an ammonium salt releases ammonia, which can be identified by its pungent smell and by turning damp red litmus paper blue)
These reactions are used practically to test whether an unknown substance is a base, and to prepare salts.
Heating calcium oxide with ammonium chloride produces calcium chloride, water and ammonia gas, which can be detected with damp red litmus paper turning blue.
Must Know
- An oxide is a compound of an element with oxygen
- Acidic oxides (typically non-metal, e.g. SO2, CO2) react with bases; basic oxides (typically metal, e.g. CuO, CaO) react with acids
- Amphoteric oxides (Al2O3 and ZnO) react with both acids and bases to form a salt and water
- A base is an oxide or hydroxide of a metal; an alkali is a base that is soluble in water
- Base + acid → salt + water (neutralisation)
- Base + ammonium salt (heated) → salt + water + ammonia gas
That's the notes covered.
Carry on to the next subtopic.