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ChemistryChemistry586 views·Updated 14 Jul 2026·6 pages

GCSE Chemistry Unit 4: Chemical Changes Study Guide

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Dae 🏎🪩@d_h708

Chemical changes are happening all around you - from the...

1
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 1

Reactivity of Metals

Ever wondered why some metals rust quickly whilst others stay shiny for years? It's all about reactivity - how eager metal atoms are to form positive ions when they react.

Metal oxides form when metals react with oxygen in oxidation reactions. Reactive metals like magnesium burn brilliantly in oxygen, whilst less reactive ones like iron just slowly tarnish without flames.

The reactivity series ranks metals from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, copper. Remember it with: "Please Stop Calling My Aunty Zebra In The Class." Metals above hydrogen react with acids producing hydrogen gas - the more reactive, the more vigorous the fizzing.

Displacement reactions happen when a more reactive metal kicks out a less reactive one from its compound. Pop some magnesium into copper sulphate solution and watch the copper form as the magnesium takes its place. Try copper in zinc sulphate though, and nothing happens - copper isn't reactive enough to displace zinc.

Key Point: Only metals above hydrogen in the reactivity series will react with acids to produce hydrogen gas.

2
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 2

Oxidation, Reduction and Acid Reactions

Here's where chemistry gets exciting - oxidation means losing electrons, whilst reduction means gaining them. Remember "OIL RIG": Oxidation Is Loss, Reduction Is Gain.

When zinc displaces copper from copper sulphate, zinc loses electrons (oxidised) whilst copper ions gain electrons (reduced). You can write these as half equations: Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.

Acids react with metals above hydrogen to produce salts and hydrogen gas. Whether you get chlorides, nitrates, or sulphates depends on which acid you use - hydrochloric acid makes chlorides, nitric acid makes nitrates, sulphuric acid makes sulphates.

Neutralisation happens when acids meet bases or alkalis, producing salts and water. The key reaction is always H⁺ + OH⁻ → H₂O. Metal carbonates add extra excitement by producing carbon dioxide too - that's why limestone fizzes in acid rain.

Remember: All acid-metal reactions are redox reactions where the metal gets oxidised and hydrogen gets reduced.

3
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 3

pH Scale and Acid Strength

The pH scale from 0-14 tells you how acidic or alkaline something is. Pure water sits neutrally at pH 7, acids are below 7, and alkalis are above 7. Universal indicator changes colour to show you exactly where your solution sits.

Strong acids like hydrochloric, nitric, and sulphuric acids completely break apart in water, releasing all their hydrogen ions. Weak acids like ethanoic acid (vinegar) only partially ionise - less than 1% of molecules actually release their hydrogen ions.

Don't confuse strong with concentrated! Concentrated just means lots of acid molecules per unit volume. You can have dilute strong acids and concentrated weak acids. When you dilute an acid 10 times, its pH increases by one unit.

Making soluble salts is straightforward - add solid base to acid until no more dissolves, filter off excess, then crystallise your salt solution. Just avoid metals that are too reactive (sodium) or too unreactive (copper).

Top Tip: As pH decreases by one unit, hydrogen ion concentration increases by a factor of 10 - that's why pH 2 is 10 times more acidic than pH 3.

4
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 4

Titrations

Titrations let you find unknown concentrations by measuring exactly how much acid neutralises a known amount of alkali. You'll use a burette for the alkali, a pipette for precise acid measurement, and an indicator to spot the exact neutralisation point.

The method is all about precision - add alkali drop by drop near the end point, swirl constantly, and repeat until you get concordant results (within 0.1 cm³ of each other). Only average your concordant titres, never the dodgy first attempt.

Titration calculations follow three simple steps: calculate moles of the known substance, use the balanced equation to find moles of the unknown, then work out concentration. If 25 cm³ of HCl neutralises 22.4 cm³ of 2 mol/dm³ NaOH, you've got 0.0448 moles of NaOH, so 0.0448 moles of HCl too (1:1 ratio), giving 1.79 mol/dm³ HCl.

Remember your equipment - pipettes measure one fixed volume accurately, burettes measure variable volumes. Getting these mixed up in exams costs marks, so practise identifying them from diagrams.

Exam Success: Always convert cm³ to dm³ by dividing by 1000 before using the formula: moles = concentration × volume.

5
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 5

Electrolysis Basics

Electrolysis uses electricity to break apart ionic compounds when they're molten or dissolved. Think of it as forcing chemical reactions that wouldn't normally happen - you're literally pulling compounds apart with electrical power.

When you pass current through molten salts, positive ions head to the negative electrode (cathode) where they gain electrons and become metal atoms. Meanwhile, negative ions move to the positive electrode (anode) where they lose electrons to form non-metals.

Aluminium extraction showcases electrolysis perfectly. Aluminium oxide mixed with cryolite gets electrolysed - aluminium forms at the cathode, oxygen at the anode. The cryolite lowers the melting point, saving energy costs, but you still need massive amounts of electricity making aluminium expensive.

The carbon anodes keep getting eaten away because they react with the oxygen produced, forming carbon dioxide. That's why aluminium smelters constantly replace their anodes - it's an ongoing cost of the process.

Remember OIL RIG: At the anode, oxidation happens (ions lose electrons). At the cathode, reduction happens (ions gain electrons).

6
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 6

Electrolysing Solutions

Aqueous solutions make electrolysis trickier because water adds H⁺ and OH⁻ ions to the mix. Now you've got competition - which ions actually get discharged at each electrode?

At the negative electrode (cathode), the less reactive element wins. If you electrolyse sodium chloride solution, hydrogen forms instead of sodium because sodium is too reactive. But with copper chloride solution, copper metal forms because copper is less reactive than hydrogen.

At the positive electrode (anode), oxygen usually forms from OH⁻ ions - unless halide ions are present. Then you get the halogen instead. Electrolyse sodium chloride and you'll get chlorine gas, not oxygen.

Watch for the visual clues in the required practical. If you electrolyse copper sulphate solution, the cathode gains mass as copper deposits on it, whilst the blue colour fades as copper ions get removed from solution. Meanwhile, oxygen bubbles form at the anode.

Quick Check: Reactive metals above hydrogen in the reactivity series won't form at the cathode in aqueous solutions - hydrogen will form instead.

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ChemistryChemistry586 views·Updated 14 Jul 2026·6 pages

GCSE Chemistry Unit 4: Chemical Changes Study Guide

user profile picture
Dae 🏎🪩@d_h708

Chemical changes are happening all around you - from the rust on a bike to the fizzing when you drop antacid in water. Understanding how metals react, acids work, and electricity can break apart compounds gives you the tools to...

1
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 1

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Reactivity of Metals

Ever wondered why some metals rust quickly whilst others stay shiny for years? It's all about reactivity - how eager metal atoms are to form positive ions when they react.

Metal oxides form when metals react with oxygen in oxidation reactions. Reactive metals like magnesium burn brilliantly in oxygen, whilst less reactive ones like iron just slowly tarnish without flames.

The reactivity series ranks metals from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, copper. Remember it with: "Please Stop Calling My Aunty Zebra In The Class." Metals above hydrogen react with acids producing hydrogen gas - the more reactive, the more vigorous the fizzing.

Displacement reactions happen when a more reactive metal kicks out a less reactive one from its compound. Pop some magnesium into copper sulphate solution and watch the copper form as the magnesium takes its place. Try copper in zinc sulphate though, and nothing happens - copper isn't reactive enough to displace zinc.

Key Point: Only metals above hydrogen in the reactivity series will react with acids to produce hydrogen gas.

2
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 2

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Oxidation, Reduction and Acid Reactions

Here's where chemistry gets exciting - oxidation means losing electrons, whilst reduction means gaining them. Remember "OIL RIG": Oxidation Is Loss, Reduction Is Gain.

When zinc displaces copper from copper sulphate, zinc loses electrons (oxidised) whilst copper ions gain electrons (reduced). You can write these as half equations: Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.

Acids react with metals above hydrogen to produce salts and hydrogen gas. Whether you get chlorides, nitrates, or sulphates depends on which acid you use - hydrochloric acid makes chlorides, nitric acid makes nitrates, sulphuric acid makes sulphates.

Neutralisation happens when acids meet bases or alkalis, producing salts and water. The key reaction is always H⁺ + OH⁻ → H₂O. Metal carbonates add extra excitement by producing carbon dioxide too - that's why limestone fizzes in acid rain.

Remember: All acid-metal reactions are redox reactions where the metal gets oxidised and hydrogen gets reduced.

3
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 3

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pH Scale and Acid Strength

The pH scale from 0-14 tells you how acidic or alkaline something is. Pure water sits neutrally at pH 7, acids are below 7, and alkalis are above 7. Universal indicator changes colour to show you exactly where your solution sits.

Strong acids like hydrochloric, nitric, and sulphuric acids completely break apart in water, releasing all their hydrogen ions. Weak acids like ethanoic acid (vinegar) only partially ionise - less than 1% of molecules actually release their hydrogen ions.

Don't confuse strong with concentrated! Concentrated just means lots of acid molecules per unit volume. You can have dilute strong acids and concentrated weak acids. When you dilute an acid 10 times, its pH increases by one unit.

Making soluble salts is straightforward - add solid base to acid until no more dissolves, filter off excess, then crystallise your salt solution. Just avoid metals that are too reactive (sodium) or too unreactive (copper).

Top Tip: As pH decreases by one unit, hydrogen ion concentration increases by a factor of 10 - that's why pH 2 is 10 times more acidic than pH 3.

4
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 4

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Titrations

Titrations let you find unknown concentrations by measuring exactly how much acid neutralises a known amount of alkali. You'll use a burette for the alkali, a pipette for precise acid measurement, and an indicator to spot the exact neutralisation point.

The method is all about precision - add alkali drop by drop near the end point, swirl constantly, and repeat until you get concordant results (within 0.1 cm³ of each other). Only average your concordant titres, never the dodgy first attempt.

Titration calculations follow three simple steps: calculate moles of the known substance, use the balanced equation to find moles of the unknown, then work out concentration. If 25 cm³ of HCl neutralises 22.4 cm³ of 2 mol/dm³ NaOH, you've got 0.0448 moles of NaOH, so 0.0448 moles of HCl too (1:1 ratio), giving 1.79 mol/dm³ HCl.

Remember your equipment - pipettes measure one fixed volume accurately, burettes measure variable volumes. Getting these mixed up in exams costs marks, so practise identifying them from diagrams.

Exam Success: Always convert cm³ to dm³ by dividing by 1000 before using the formula: moles = concentration × volume.

5
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 5

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Electrolysis Basics

Electrolysis uses electricity to break apart ionic compounds when they're molten or dissolved. Think of it as forcing chemical reactions that wouldn't normally happen - you're literally pulling compounds apart with electrical power.

When you pass current through molten salts, positive ions head to the negative electrode (cathode) where they gain electrons and become metal atoms. Meanwhile, negative ions move to the positive electrode (anode) where they lose electrons to form non-metals.

Aluminium extraction showcases electrolysis perfectly. Aluminium oxide mixed with cryolite gets electrolysed - aluminium forms at the cathode, oxygen at the anode. The cryolite lowers the melting point, saving energy costs, but you still need massive amounts of electricity making aluminium expensive.

The carbon anodes keep getting eaten away because they react with the oxygen produced, forming carbon dioxide. That's why aluminium smelters constantly replace their anodes - it's an ongoing cost of the process.

Remember OIL RIG: At the anode, oxidation happens (ions lose electrons). At the cathode, reduction happens (ions gain electrons).

6
of 6
Chemistry Unit 4 (chemical changes) summary notes GCSE  – page 6

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Electrolysing Solutions

Aqueous solutions make electrolysis trickier because water adds H⁺ and OH⁻ ions to the mix. Now you've got competition - which ions actually get discharged at each electrode?

At the negative electrode (cathode), the less reactive element wins. If you electrolyse sodium chloride solution, hydrogen forms instead of sodium because sodium is too reactive. But with copper chloride solution, copper metal forms because copper is less reactive than hydrogen.

At the positive electrode (anode), oxygen usually forms from OH⁻ ions - unless halide ions are present. Then you get the halogen instead. Electrolyse sodium chloride and you'll get chlorine gas, not oxygen.

Watch for the visual clues in the required practical. If you electrolyse copper sulphate solution, the cathode gains mass as copper deposits on it, whilst the blue colour fades as copper ions get removed from solution. Meanwhile, oxygen bubbles form at the anode.

Quick Check: Reactive metals above hydrogen in the reactivity series won't form at the cathode in aqueous solutions - hydrogen will form instead.

We thought you’d never ask...

Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.

You can download the app from Google Play Store and Apple App Store.

That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.

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