Overview
What you’ll learn
- State that a chemical change produces one or more new substances.
- Write simple word equations and explain that atoms are rearranged while mass is conserved.
- Describe combustion, thermal decomposition, oxidation and neutralisation.
- Describe the reactions of acids with alkalis, and the reactions of acids with metals and carbonates.
- Use litmus, Universal Indicator and a natural indicator to tell acids from alkalis.
- Test hydrogen with a lighted splint, oxygen with a glowing splint, and carbon dioxide with limewater.
In 30 seconds
- 1A chemical change produces new substances. A physical change, such as melting or dissolving, does not.
- 2In a reaction, atoms are rearranged. They are not created or destroyed, so mass is conserved in a closed container.
- 3Use word equations to name reactants and products. Counting atoms in a symbol equation is not required yet.
- 4Combustion is burning in oxygen. Thermal decomposition is breaking a compound down by heat. Oxidation includes rusting and the use of oxygen in respiration.
- 5An acid and an alkali react in neutralisation to form a salt and water.
- 6Litmus turns red in acid and blue in alkali. Universal Indicator shows a range of colours. Some plant extracts are natural indicators.
- 7Hydrogen pops with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky.
Notes · 1 of 6
Chemical change and physical change
Look for a new substance, not just a new shape or temperature.
In a chemical change, one or more new substances form. Burning, rusting and neutralisation are chemical changes. The products have different properties from the reactants.
Melting ice, boiling water, dissolving sugar and bending a wire are physical changes. The substance is still the same. Many physical changes are easy to reverse. Many chemical changes are not.
| Clue | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Example | Ice melting | Wood burning |
| Easy to reverse? | Often yes | Often no |
Notes · 2 of 6
Word equations and conservation of mass
Name the substances. The atoms you start with are the atoms you finish with.
A word equation names the reactants on the left and the products on the right. Examples: magnesium + oxygen → magnesium oxide; hydrochloric acid + sodium hydroxide → sodium chloride + water; calcium carbonate → calcium oxide + carbon dioxide. Count the names, not the atoms in a formula.
During the reaction the atoms are rearranged into new substances. No atoms appear or disappear. If the container is closed, the total mass stays the same. If a gas escapes into the room, the mass of what is left in the beaker can fall, but the missing mass is in the gas.
Notes · 3 of 6
Combustion, decomposition and oxidation
Three named types of chemical change appear often in questions.
- Combustion: a substance burns in oxygen and gives out heat and light. Fuel + oxygen → carbon dioxide + water, for many fuels.
- Thermal decomposition: a compound breaks down when it is heated. Heating calcium carbonate gives calcium oxide and carbon dioxide.
- Oxidation: a substance gains oxygen. Rusting is slow oxidation of iron in oxygen and water. Respiration is oxidation of food in living cells.
Paint, oil or a plastic coat slows rusting because it keeps air and water away from the iron. The coat does not change the iron into a new element.
Notes · 4 of 6
Acids, alkalis and neutralisation
Acids and alkalis are chemical opposites. Together they can make a neutral solution.
Acids taste sour and are corrosive when concentrated. Alkalis feel soapy and are also corrosive when concentrated. Never taste a laboratory chemical. A neutral solution is neither acidic nor alkaline.
Neutralisation: acid + alkali → salt + water. Some metals react with acid to give a salt and hydrogen: magnesium + hydrochloric acid → magnesium chloride + hydrogen. A carbonate gives a salt, water and carbon dioxide: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. You do not need a reactivity series.
Notes · 5 of 6
Tests for hydrogen, oxygen and carbon dioxide
The test matches the gas the reaction is expected to make.
| Gas | Test | Result | One reaction that can make it |
|---|---|---|---|
| Hydrogen | Lighted splint at the mouth of the tube | A squeaky pop | Acid + some metals |
| Oxygen | Glowing splint | The splint relights | Some reactions that release oxygen, including photosynthesis in plants |
| Carbon dioxide | Bubble the gas through limewater | Limewater turns milky | Acid + carbonate, or burning a fuel |
A positive test is evidence of that gas. It does not, by itself, name every other product. Write the rest of the word equation from the reactants you were given.
Notes · 6 of 6
Indicators
Indicators change colour. They do not take part as the main reactant in the word equation.
| Indicator | Acid | Neutral | Alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Universal Indicator | Red (strong) to orange or yellow | Green | Blue to purple (strong) |
| Red cabbage juice | Red or pink | Purple | Green or yellow |
Litmus tells you acid or alkali. Universal Indicator gives a better idea of how acidic or alkaline a solution is. A plant extract such as red cabbage is a natural indicator.
Revise
Key terms
The words examiners expect you to use precisely.
- Chemical change
- A change that produces one or more new substances.
- Physical change
- A change in which no new substance is formed, such as melting or dissolving.
- Reactant
- A substance that is used up in a chemical reaction.
- Product
- A new substance formed in a chemical reaction.
- Word equation
- An equation that names the reactants and products in words.
- Combustion
- Burning in oxygen, a chemical change that releases heat and light.
Show 7 more key termsShow fewer key terms
- Thermal decomposition
- Breaking a compound into simpler substances by heating.
- Oxidation
- A reaction in which a substance gains oxygen, such as rusting.
- Neutralisation
- The reaction of an acid with an alkali to form a salt and water.
- Indicator
- A substance that changes colour in acids and alkalis.
- Test for hydrogen
- A lighted splint at the mouth of the tube gives a squeaky pop.
- Test for oxygen
- A glowing splint relights.
- Test for carbon dioxide
- The gas turns limewater milky.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Calling melting or dissolving a chemical change.
The substance is still the same. No new substance forms.
Instead: Melting and dissolving are physical changes.
Mistake: Saying atoms are destroyed when a candle seems to disappear.
The wax turns into gases. The atoms are still there.
Instead: Atoms are rearranged. Mass is conserved if the gases are included.
Show 5 more mistakesShow fewer mistakes
Mistake: Writing a balanced symbol equation when the question asks for a word equation.
Balancing is not required, and a wrong formula loses the point.
Instead: Name the reactants and products in words.
Mistake: Saying acids turn red litmus blue.
That is the alkali result.
Instead: Acid turns blue litmus red. Alkali turns red litmus blue.
Mistake: Calling rusting a physical change because the object is still iron-coloured.
Rust is a new substance with different properties.
Instead: Rusting is oxidation, a chemical change.
Mistake: Saying a gas leaving a beaker means mass was destroyed.
The mass left the beaker with the gas.
Instead: Total mass is conserved. An open beaker can lose mass if a gas escapes.
Mistake: Using the pop test for carbon dioxide.
A squeaky pop is the test for hydrogen.
Instead: Hydrogen pops with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Identify a chemical change
Which change produces a new substance?
Ready for more practice on Chapter 3: Chemical Changes?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 37 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 3 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 3.
Mark scheme
PROChapter 3 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What is a chemical change in Sec 2 Science?
A change that produces one or more new substances. Burning, rusting and neutralisation are chemical changes. Melting and dissolving are physical changes.
How do I write a chemical equation for this chapter?
Write a word equation that names the reactants and the products. For example, magnesium + oxygen → magnesium oxide. Then use the gas tests if a question asks you to identify hydrogen, oxygen or carbon dioxide.
What is the difference between combustion and oxidation?
Combustion is burning in oxygen, usually with a flame. Oxidation is the wider idea of gaining oxygen. Rusting and respiration are oxidation. Combustion is one kind of oxidation.
What colours should I learn for indicators?
Litmus is red in acid and blue in alkali. Universal Indicator is red in strong acid, green when neutral, and purple in strong alkali.
Is chemical changes in the Sec 2 EOY?
Most schools that follow the usual G2/G3 scheme teach this chapter in Sec 2 and include it in the Sec 2 end-of-year paper. A few schools move a topic across the two years, so check your school’s assessment outline.
Up next
Chapter 4: Interactions within Ecosystems