9.2.4—Reactions/equations: the acid / base behaviour
- Syllabus
- 9701–2028–2029
- Objective
- 9.2.4
- Level
- AS
Period 3 oxide and hydroxide behaviour changes from basic to acidic
Across Period 3, oxides and hydroxides generally change from basic on the left to acidic on the right as bonding becomes more covalent and the central atom attracts oxygen more strongly.
Basic oxides react with acids; acidic oxides react with bases or water to form acids. Amphoteric substances such as Al₂O₃ can react in both directions.
MgO reacts with HCl, while SO₃ reacts with water to form H₂SO₄. Al₂O₃ reacts with both acids and strong bases.
The trend is not simply “metal to non-metal”. Silicon dioxide is a giant covalent solid and its reaction behaviour must be considered separately from molecular oxides.
ConceptA-Level CAIE Chemistry AS