Chemical Changes
What's covered
Key facts
Acid + metal carbonate → salt + water + carbon dioxide.
During electrolysis, negative ions (anions) move towards the anode and positive ions (cations) move towards the cathode.
Aluminium is deposited at the cathode because Al³⁺ ions are attracted to the negative electrode.
In brine electrolysis, chlorine gas is produced at the anode.
In a redox reaction, the oxidising agent is itself reduced, and the reducing agent is itself oxidised.
Copper sulfate crystals are prepared in school by adding excess copper(II) oxide to warm dilute sulfuric acid, filtering, then crystallising.
Copper cannot displace iron from iron sulfate because copper is less reactive than iron.
Acid + reactive metal → salt + hydrogen gas.
At the anode (positive electrode), ions lose electrons — this is called oxidation.
Aluminium must be extracted by electrolysis (not reduction with carbon) because it is above carbon in the reactivity series, so carbon cannot displace it.
Sample questions
A taste of the 90 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
What ions do acids release in water?
- •Chloride ions (Cl⁻)
- ✓Hydrogen ions (H⁺)
- •Hydroxide ions (OH⁻)
- •Sodium ions (Na⁺)
During electrolysis, what happens at the cathode?
- •Negative ions lose electrons and are oxidised
- ✓Positive ions (cations) gain electrons and are reduced
- •Positive ions lose electrons and are oxidised
- •The compound decomposes by thermal decomposition
What are the cathode and anode half-equations for electrolysis of aluminium oxide?
- •Cathode: 2O²⁻ → O₂ + 4e⁻. Anode: Al³⁺ + 3e⁻ → Al.
- •Cathode: Al → Al³⁺ + 3e⁻. Anode: O₂ + 4e⁻ → 2O²⁻.
- ✓Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻.
- •Cathode: Al³⁺ + e⁻ → Al⁺. Anode: O²⁻ → O + 2e⁻.
What are the three products of the electrolysis of brine (concentrated sodium chloride solution)?
- ✓Chlorine gas (at anode), hydrogen gas (at cathode), and sodium hydroxide solution
- •Oxygen gas (at anode), hydrogen gas (at cathode), and sodium chloride solution (unchanged)
- •Sodium gas (at cathode), chlorine gas (at anode), and water vapour (produced)
- •Sodium metal (at cathode), chlorine gas (at anode), and water (left over)
What is oxidation in terms of electrons?
- •Gain of electrons
- •Gain of neutrons
- ✓Loss of electrons
- •Loss of protons
Describe how to prepare pure, dry copper sulfate crystals from copper oxide and dilute sulfuric acid.
- ✓Add excess CuO to warm dilute H₂SO₄; stir, filter, evaporate
- •Add H₂SO₄ to copper chloride solution and evaporate until dry crystals form
- •Electrolyse copper sulfate solution with copper electrodes and collect crystals
- •Mix copper powder with dilute H₂SO₄ in a boiling tube; crystals form on cooling
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