This mini-lesson walks you through the whole of AQA Topic 4.4 — Chemical changes: the reactivity series, oxidation & reduction, extracting metals, the reactions of acids and salts, the pH scale, and electrolysis.
Work through each screen, answer the questions as you go (some are wordy, some are calculations or equations) and collect ⭐ stars. HT-only ideas are flagged in purple. Press Start when you're ready.
Oxidation & reduction · oxygen
Gaining and losing oxygen
Metals react with oxygen to make metal oxides. Because the metal gains oxygen, this is an oxidation reaction:
In terms of oxygen:OXidation = gain of OXygen; reduction = loss of oxygen. These two always happen together.
Quick check
Oxidised or reduced?
?In the reaction 2CuO + C → 2Cu + CO₂, what has happened to the copper oxide?
The reactivity series
Ordering metals by reactivity
The reactivity of a metal depends on its tendency to form positive ions. We arrange metals in a reactivity series, deduced from their reactions with water and dilute acids. The non-metals carbon and hydrogen are included for comparison:
AQA's reactivity series: K, Na, Li, Ca, Mg, then (carbon), then Zn, Fe, then (hydrogen), then Cu.
Why it matters: a more reactive metal displaces a less reactive metal from its compound — and a metal's position relative to carbon and hydrogen decides how it is extracted and how it reacts with acid.
Predict it
Will it displace?
A more reactive metal displaces a less reactive one. Tap the correct outcome each time.
Extraction of metals
Reduction with carbon vs electrolysis
Most metals are found in the Earth as compounds (ores) and must be extracted. How we do it depends on the metal's position in the reactivity series, compared with carbon:
Less reactive than carbon (e.g. zinc, iron, copper) → can be extracted by reduction with carbon. The metal oxide loses oxygen to the carbon.
More reactive than carbon (e.g. potassium, sodium, calcium, magnesium, aluminium) → carbon can't do it, so we use electrolysis.
2Fe₂O₃ + 3C → 4Fe + 3CO₂iron(III) oxide is reduced by carbon — iron loses oxygen, carbon gains it
Gold and other very unreactive metals are found as the metal itself (native), so they need no chemical extraction at all.
Quick check
How would you extract it?
?Aluminium sits above carbon in the reactivity series. How must aluminium be extracted from its oxide?
Higher tier only
Redox in terms of electrons
At Higher tier, oxidation and reduction are defined by electrons, not just oxygen. Remember OIL RIG:
OIL RIGOxidation Is Loss of electrons · Reduction Is Gain of electrons
In a displacement reaction the more reactive metal loses electrons (oxidised); the less reactive metal ion gains electrons (reduced). The half equations show this:
Mg → Mg²⁺ + 2e⁻magnesium is oxidised — it loses 2 electrons
Cu²⁺ + 2e⁻ → Cucopper ions are reduced — they gain 2 electrons
Watch out (OIL RIG): it feels backwards, but losing electrons is oxidation and gaining electrons is reduction. The electrons must balance on both sides of the full ionic equation.
Higher · Quick check
Loss or gain of electrons?
?In the half equation Zn → Zn²⁺ + 2e⁻, the zinc is…
Reactions of acids · metals
Acid + metal → salt + hydrogen
Acids react with some metals (e.g. magnesium, zinc, iron) to make a salt and hydrogen gas:
acid + metal → salt + hydrogene.g. Mg + 2HCl → MgCl₂ + H₂
The salt's name comes from the acid: hydrochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate.
(HT) This is a redox reaction: the metal atoms lose electrons (oxidised) and the hydrogen ions gain electrons (reduced) to form H₂.
Name the salt
What salt forms?
?Zinc reacts with sulfuric acid. Name the salt produced.
Neutralisation & salts
Acid + base → salt + water
Acids are neutralised by bases. The base can be a metal oxide, a metal hydroxide (an alkali if soluble) or a metal carbonate:
Acid + base → salt + water. With a carbonate you also get carbon dioxide.
Watch out (naming): the salt's second word comes from the acid (chloride / sulfate / nitrate); the first word comes from the metal in the base.
Balance it
Balance the neutralisation
1Balance: __ HCl + Ca(OH)₂ → CaCl₂ + 2H₂O. What number goes in front of HCl?
× HCl
Hint: there are 2 Cl on the right (CaCl₂) and 2 OH in Ca(OH)₂ to neutralise.
Required practical 1
Making a pure, dry soluble salt
A soluble salt can be made by reacting an acid with an insoluble base (a metal, metal oxide, hydroxide or carbonate). The classic example is copper sulfate from sulfuric acid + copper oxide:
Warm the dilute acid (Bunsen burner), then add the insoluble solid a little at a time.
Keep adding until no more reacts (excess solid sits unreacted) — this ensures all the acid is used up.
Filter off the excess solid to leave a salt solution.
Crystallise: evaporate some water (water bath/electric heater), then leave to cool so crystals form. Pat dry.
Why excess solid? Using more base than the acid can dissolve guarantees the acid is fully neutralised — and the leftover solid is easy to filter out.
Quick check
Getting a pure salt
?When making copper sulfate, why is excess copper oxide added to the warm acid?
The pH scale & neutralisation
Acids, alkalis and pH
Acids produce hydrogen ions, H⁺ in water; alkalis contain hydroxide ions, OH⁻. The pH scale (0–14) measures acidity, read with universal indicator or a pH probe:
Universal indicator: red strong acid → green neutral → purple strong alkali.
In neutralisation between an acid and an alkali, H⁺ and OH⁻ combine to make water:
H⁺ + OH⁻ → H₂Othe ionic equation for neutralisation
Watch out (pH): pH 7 is neutral. A lower pH means more acidic (not less!). pH above 7 is alkaline.
Quick check
Reading the pH scale
?Solution A has pH 2 and solution B has pH 5. Which statement is correct?
Higher tier only
Strong vs weak acids
This is about how much an acid ionises in water — not the same as concentration:
A strong acid is completely ionised in solution (e.g. hydrochloric, nitric, sulfuric).
A weak acid is only partially ionised (e.g. ethanoic, citric, carbonic).
stronger acid → lower pHat the same concentration, more H⁺ ions means a lower pH
As the pH falls by 1 unit, the H⁺ concentration rises by a factor of 10. So pH 2 has ten times the H⁺ of pH 3.
Don't confuse two pairs:dilute/concentrated = amount of acid in a volume; weak/strong = degree of ionisation. A weak acid can still be concentrated.
Higher · Quick check
Strong, weak, dilute?
?Two solutions have the same concentration. One is hydrochloric acid (pH 1), one is ethanoic acid (pH 3). Why is the hydrochloric acid's pH lower?
Electrolysis · the process
Splitting compounds with electricity
When an ionic compound is melted or dissolved, its ions are free to move — it becomes an electrolyte. Passing a current makes the ions move to the electrodes, where they are discharged as elements:
Positive ions → cathode (–); negative ions → anode (+). Molten lead bromide gives lead at the cathode and bromine at the anode.
Watch out (cathode): at the cathode, positive ions gain electrons (reduction). "Cat-ions go to the cat-hode." At the anode, negative ions lose electrons (oxidation).
Aqueous solutions & aluminium
Rules for the products
With aqueous solutions, water also provides H⁺ and OH⁻ ions, so there is competition. The rules (inert electrodes):
Cathode (–):hydrogen is produced unless the metal is less reactive than hydrogen (e.g. copper) — then the metal is deposited.
Anode (+):oxygen is produced unless a halide (Cl⁻, Br⁻, I⁻) is present — then the halogen forms.
Reactive metals like aluminium are extracted by electrolysis of the molten oxide:
aluminium oxide dissolved in molten cryolitecryolite lowers the melting point, saving energy
The carbon anode burns away: oxygen made at the positive electrode reacts with the hot carbon to form CO₂, so the anode must be replaced regularly.
Sort it
Cathode or anode?
Electrolysis of aqueous copper chloride and sodium sulfate. Tap a product, then the electrode where it forms.
➖ Cathode
➕ Anode
Higher tier only
Half equations at the electrodes
At Higher tier you write half equations for each electrode. At the cathode ions gain electrons (reduction); at the anode ions lose electrons (oxidation):
Pb²⁺ + 2e⁻ → Pb · 2Br⁻ → Br₂ + 2e⁻lead is reduced at the cathode; bromide is oxidised at the anode
Check the balance: atoms and charge must balance. In 2H⁺ + 2e⁻ → H₂ the 2 positive charges are cancelled by 2 electrons, giving neutral H₂.
Higher · Balance it
Complete the half equation
2Complete the cathode half equation: Al³⁺ + __ e⁻ → Al. How many electrons are gained?
electrons
Hint: the charge on Al³⁺ is 3+, so it needs 3 electrons to become a neutral Al atom.
Required practical · copper sulfate
Electrolysis of copper sulfate
In the required practical you electrolyse aqueous copper(II) sulfate with inert (carbon) electrodes and observe what forms:
Cathode (–): a coating of copper appears — copper is less reactive than hydrogen, so the metal is deposited.
Anode (+): bubbles of oxygen are released (no halide is present).
Cathode: Cu²⁺ + 2e⁻ → Cucopper ions are reduced and deposited on the electrode
Investigation idea: changing the current or concentration changes the mass of copper deposited or the volume of gas collected — a good hypothesis to test.
Quick check
Products at the electrodes
?Aqueous copper sulfate is electrolysed with inert electrodes. What forms at the cathode?
Recap
The key ideas to know
Reactivity series: K Na Li Ca Mg (C) Zn Fe (H) Cu — more reactive displaces less reactive
Redox (oxygen): oxidation = gain O · reduction = loss O
Redox (electrons, HT): OIL RIG — Oxidation Is Loss, Reduction Is Gain