GCSE Chemistry Chemical Changes Notes
Chemical changes rearrange atoms into new substances. This topic covers acids, the reactivity series, salt preparation and electrolysis.
Key concepts
- Acid
- A source of H⁺ ions in solution. Reacts with metals, bases and carbonates to form salts.
- Neutralisation
- Acid + base → salt + water: H⁺ + OH⁻ → H₂O.
- Reactivity series
- Ranking of metals by tendency to lose electrons: K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au.
- Displacement
- A more reactive metal takes the place of a less reactive metal in a compound.
- Acid + metal → salt + hydrogen (fizzing produces a gas that pops).
- Acid + carbonate → salt + water + carbon dioxide (limewater turns milky).
- Metals more reactive than carbon are extracted by electrolysis; less reactive by heating with carbon.
Worked example
Write a word and balanced symbol equation for the reaction of magnesium with dilute hydrochloric acid.
- 1Reactants: magnesium + hydrochloric acid.
- 2Products: salt (magnesium chloride) + hydrogen.
- 3Symbols: Mg + HCl → MgCl₂ + H₂.
- 4Balance: Mg + 2HCl → MgCl₂ + H₂.
Answer: Mg + 2HCl → MgCl₂ + H₂
Common mistakes
✗ Forgetting to balance charges in the salt formula.
✓ MgCl₂ not MgCl — Mg²⁺ needs two Cl⁻ ions.
✗ Saying gold is very reactive because it looks shiny.
✓ Gold is very unreactive — that is why it stays shiny.
✗ Mixing up oxidation and reduction.
✓ OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain.
Quick check
Try these, then reveal the answers.
Name the salt from sulfuric acid + sodium hydroxide.Reveal answer
Sodium sulfate
Which is more reactive, zinc or copper?Reveal answer
Zinc
State the neutralisation ionic equation.Reveal answer
H⁺ + OH⁻ → H₂O
Exam tips
- Learn the reactivity series in order — several marks depend on it.
- Give observations, not conclusions: 'colourless gas that pops with a lit splint', not just 'hydrogen'.
- Show state symbols in symbol equations when marks are awarded for them.
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Frequently asked questions
How do I predict the salt from an acid + base reaction?+
Take the metal from the base and the acid's anion: HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate.
Why can carbon extract iron but not aluminium?+
Iron is below carbon in the reactivity series so carbon can displace it; aluminium is above carbon and must be extracted by electrolysis.
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