Introduction to Chemical Reactions
Every moment of every day, chemical reactions are transforming the world around you. From the bread rising in your oven to the battery powering your phone, reactions break old bonds and create new substances. But how do we know a chemical change has happened? And where does everything go?
Before You Begin
You have a piece of paper and a match. You can either rip the paper into smaller pieces or burn the paper.
Write down your answers before reading on:
- What is the same about the paper before and after ripping? What is different?
- What is the same about the paper before and after burning? What is different?
- Which process do you think is a physical change, and which is a chemical change? Why?
Know
- The difference between a physical change and a chemical change
- The five signs that indicate a chemical reaction has occurred
- The law of conservation of mass
Understand
- Why burning is a chemical reaction but ripping is not
- How mass is conserved even when substances appear to disappear
- That chemical reactions create new substances with new properties
Can Do
- Identify whether a change is physical or chemical
- Recognise the signs of a chemical reaction in everyday situations
- Predict whether mass is conserved in a given change
Chemical Change vs Physical Change
Not all changes create new substances
Physical Chemical Change
The critical question is always: Has a new substance been formed?
In a physical change, the substance stays the same — only its shape, size or state changes. Rip paper: it is still paper, just smaller. Melt ice: it is still water, just liquid. These changes are usually reversible.
In a chemical change (chemical reaction), new substances with different properties are formed. Burn paper: you get ash, carbon dioxide and water vapour — none of which are paper. Cook an egg: the clear, runny white becomes solid and white. These changes are usually not reversible.
Common examples
| Physical changes | Chemical changes |
|---|---|
| Melting ice | Burning wood |
| Dissolving sugar in water | Rusting iron |
| Cutting hair | Baking a cake |
| Boiling water | Digesting food |
| Crushing a can | Exploding fireworks |
Signs of a Chemical Reaction
How to spot when chemistry is happening
Chemical reactions often give themselves away. Here are the five key signs:
- Colour change: A substance changes colour without dye being added. Example: iron rusting from silver-grey to reddish-brown.
- Gas production: Bubbles form that are not from boiling. Example: baking soda and vinegar producing CO₂ bubbles.
- Temperature change: The reaction gets hot (exothermic) or cold (endothermic) without heating or cooling being applied. Example: hand warmers getting hot when activated.
- Precipitate formation: A solid appears when two solutions are mixed. Example: mixing silver nitrate and sodium chloride forms a white solid.
- Light or sound: Some reactions glow, spark or explode. Example: fireworks producing light and sound.
Conservation of Mass
Nothing is lost, nothing is created
The law of conservation of mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. The atoms are simply rearranged — no atoms are created or destroyed.
This seems surprising when you burn a log and are left with a small pile of ash. Where did the mass go? It became gases — mainly carbon dioxide and water vapour — that drifted away into the air. If you burned the log in a sealed container and measured everything, the total mass would stay exactly the same.
Common Misconceptions
"If something disappears, mass is lost." No — the law of conservation of mass says mass is never lost in a chemical reaction. If a solid seems to vanish, it has likely become a gas that escaped into the air.
"Boiling water is a chemical reaction because bubbles form." No — boiling is a physical change (liquid to gas). The bubbles are water vapour, not a new substance.
Bushfires: Chemistry at Scale
Bushfires are one of Australia's most destructive natural hazards — and they are essentially massive chemical reactions. When eucalyptus leaves and bark burn, cellulose and other organic compounds react with oxygen from the air to produce carbon dioxide, water vapour, ash and enormous amounts of heat.
Australia's fire management practices, including Aboriginal cultural burning, are based on understanding how these combustion reactions work. Cool burns consume fine fuel without reaching the high temperatures that kill mature trees. This is chemistry applied at landscape scale.
✍ Copy Into Your Books
▾Physical vs Chemical
- Physical: same substance, reversible
- Chemical: new substance, not reversible
5 Signs of Reaction
- Colour change
- Gas production
- Temperature change
- Precipitate forms
- Light or sound
Conservation of Mass
- Mass of reactants = mass of products
- Atoms rearranged, not created/destroyed
Classify the Change
Sign Spotter
Test Your Understanding
1. Which of the following is a chemical change?
2. A white solid forms when two clear solutions are mixed. What sign of a chemical reaction is this?
3. According to the law of conservation of mass, what happens to the total mass during a chemical reaction?
4. A student burns 5 g of magnesium ribbon in a sealed container. After the reaction, the container and its contents weigh 8.3 g. How much oxygen was used?
5. Which observation would be the BEST evidence that a chemical reaction has occurred?
Short Answer Questions
1. Explain the difference between a physical change and a chemical change. Use one example of each in your answer. 4 MARKS
2. A student places 12 g of baking soda into a flask and adds 50 g of vinegar. After the reaction stops, the total mass in the flask is 58 g. The student concludes that mass was lost. Is the student correct? Explain your answer using the law of conservation of mass. 4 MARKS
3. Describe how Aboriginal cultural burning practices demonstrate an understanding of combustion chemistry. How does this knowledge help manage the Australian landscape? 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Would you now add anything about conservation of mass?
- Can you think of another everyday example of a chemical reaction?
Answers
▾MCQ 1
B — Wood burning is a chemical change because new substances (ash, carbon dioxide, water vapour) are formed with different properties from wood.
MCQ 2
C — A solid forming from a solution is a precipitate, which is a clear sign of a chemical reaction.
MCQ 3
A — The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
MCQ 4
D — 8.3 g (product) - 5 g (magnesium) = 3.3 g of oxygen used. The container was sealed, so all products were captured.
MCQ 5
B — Multiple signs together (colour change, gas production, temperature increase) provide the strongest evidence of a chemical reaction. Any single sign could have a non-chemical explanation.
Short Answer 1
Model answer: A physical change is a change in the form or appearance of a substance without forming a new substance. For example, melting ice is a physical change because the water is still H₂O, just in a different state. A chemical change is a process where new substances with different properties are formed. For example, burning paper is a chemical change because it produces ash, carbon dioxide and water vapour — substances with very different properties from paper.
Short Answer 2
Model answer: The student is incorrect. The law of conservation of mass states that the total mass of reactants equals the total mass of products in a chemical reaction. The "missing" 4 g (12 + 50 = 62 g reactants; 58 g measured) was carbon dioxide gas that escaped from the flask into the air. If the reaction had been done in a sealed container, the total mass would have remained 62 g. This shows that mass is conserved even when substances appear to disappear.
Short Answer 3
Model answer: Aboriginal cultural burning demonstrates understanding that combustion requires fuel, oxygen and heat, and that controlling these factors controls the reaction. Cool burns use low-intensity fires that consume fine fuel (leaves, bark) without reaching the high temperatures that kill mature trees. This is because less fuel means less heat released. This knowledge helps manage the landscape by reducing fuel loads, preventing catastrophic hot fires, promoting biodiversity and maintaining healthy ecosystems.
Boss Battle
Test your knowledge in a rapid-fire quiz battle. Defeat the boss by answering questions correctly!
Mark lesson as complete
Tick when you have finished all activities and checked your answers.