Chemical Changes is a core part of GCSE Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
pH scaleRuns from 0 to 14 — measures acidity or alkalinity
AcidA solution with pH below 7 — releases H⁺ ions
Base / alkaliSolution with pH above 7 — releases OH⁻ ions
NeutralpH exactly 7, like pure water
Universal indicatorChanges colour through the rainbow with pH
Strong vs weak acidStrong fully ionises; weak only partly ionises
NeutralisationAcid + base reaction making salt and water
SaltIonic compound from an acid where H⁺ is replaced
Acid + baseAcid + base → salt + water
Acid + carbonateAcid + carbonate → salt + water + CO₂
Acid + metalAcid + metal → salt + hydrogen gas
Salt namingHCl → chloride; H₂SO₄ → sulfate; HNO₃ → nitrate
Reactivity seriesK Na Ca Mg Al C Zn Fe H Cu Ag Au — most to least
DisplacementMore reactive metal kicks out a less reactive one
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What is the difference between a strong acid and a concentrated acid?✗ Strong and concentrated mean the same thing for acids. ✓ A strong acid fully dissociates in water; a concentrated acid has a high amount of acid per volume — these are independent properties.
When an acid neutralises an alkali, what is the pH of the resulting solution?✗ Neutralisation always produces a solution with pH exactly 7 regardless of the acid and base used. ✓ Approximately 7 if equivalent amounts of strong acid and strong alkali react — but it varies with strength.
What is the relationship between two enantiomers?✗ Enantiomers have different boiling points and react completely differently with all reagents. ✓ They are non-superimposable mirror images that rotate plane-polarised light in opposite directions but have identical chemical and physical properties otherwise.
Why do metals conduct electricity?✗ Metal atoms themselves move to carry the current. ✓ Delocalised electrons are free to move through the lattice of positive ions, carrying charge.
What are the products of acid + metal carbonate?✗ Salt + water + hydrogen ✓ Salt + water + CO₂
Practise Chemical Changes — free games
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