Introduction to Chemical Reactions
In 2019–20, Australia's Black Summer bushfires burned 18.6 million hectares, but every atom of ash and gas obeyed one unbreakable rule.
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When you burn a piece of wood, it turns to ash and smoke. A solid log becomes a small pile of ash, where do you think the rest of the material went? Write down your best guess before you start.
You leave a bicycle outside and it slowly turns rusty. Your friend says this is just a physical change because the bike is "still there." Do you agree or disagree, and why? Think about what makes a chemical change different from a physical one.
● 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
Crack a glow stick and a cold blue light appears, no electricity, no flame, just two liquids mixing to make something completely new. A chemical reaction is a process in which one or more substances are converted into one or more different substances. The starting materials are called reactants; the substances formed are called products. Chemical reactions involve the breaking and forming of chemical bonds, which rearranges atoms into new configurations.
Chemical reactions are everywhere. When you digest food, cook a meal, burn fuel, or even breathe, chemical reactions are occurring. The key feature that distinguishes a chemical reaction from a physical change is that new substances with different chemical properties are formed.
Signs of a chemical reaction include: colour change, gas production (bubbling or fizzing), temperature change, precipitate formation (a solid forming from a solution), and light or sound emission. Not all chemical reactions show obvious signs, but these indicators help identify when a reaction has occurred.
When you mix baking soda (sodium hydrogen carbonate) and vinegar (acetic acid), a vigorous reaction produces carbon dioxide gas, water, and sodium acetate. The fizzing and bubbling are clear signs that a chemical reaction is occurring. The products have completely different properties from the reactants - sodium acetate is a salt, carbon dioxide is a gas, and water is a liquid. You cannot simply evaporate the mixture to get back the original baking soda and vinegar.
Australian chemistry: The chemical industry is one of Australia largest manufacturing sectors, contributing over $40 billion annually to the economy. Companies like Orica (explosives and fertilisers), Incitec Pivot (fertilisers), and CSIRO-developed technologies all depend on controlled chemical reactions. Understanding the difference between physical and chemical changes is fundamental to chemical engineering and process design.
Any change involving bubbles is a chemical reaction. This is false. Boiling water produces bubbles, but this is a physical change (liquid to gas), not a chemical reaction. The water molecules themselves do not change - only their arrangement and spacing change. To identify a chemical reaction, look for new substances with different properties, not just bubbles or changes of state.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
The law of conservation of mass states that in a chemical reaction, matter is neither created nor destroyed. The total mass of the reactants equals the total mass of the products. This law, first clearly stated by Antoine Lavoisier in 1789, is a cornerstone of chemistry.
At the atomic level, conservation of mass makes perfect sense. Chemical reactions involve rearranging atoms from reactant molecules into product molecules. No atoms are created or destroyed, so the total mass must remain constant.
However, this law can seem violated in open systems. If a reaction produces a gas that escapes into the air, the mass in the container appears to decrease. If a reaction consumes a gas from the air, the mass appears to increase. These apparent violations are explained by realising that the gas is part of the system. When reactions are performed in sealed containers, mass is always conserved.
Lavoisier classic experiment heated mercury oxide in a sealed container. The red mercury oxide decomposed into silvery mercury liquid and oxygen gas. The total mass of the container and its contents did not change, even though the appearance changed dramatically. When Lavoisier opened the container, oxygen rushed in and the mass increased - but this was because he added matter from outside, not because the reaction created mass.
Australian chemical manufacturing: BlueScope Steel operates blast furnaces at Port Kembla where iron ore is converted to steel through chemical reactions involving coke (carbon), limestone, and oxygen. Process engineers carefully measure mass inputs and outputs at every stage to ensure efficiency and comply with environmental regulations. Even at industrial scales involving thousands of tonnes, conservation of mass is strictly monitored.
Mass is lost when wood burns. This seems true because the ashes weigh much less than the original wood. But burning wood is a chemical reaction with oxygen from the air. The products are carbon dioxide gas, water vapour, and ash. If you burn wood in a sealed container and measure all products including the gases, the total mass equals the wood plus the oxygen consumed. The apparent mass loss occurs because the gaseous products escape into the air.
Chemical equations are the language of chemistry. They concisely describe what happens in a reaction using chemical formulas and symbols.
A simple equation looks like this:
2H2(g) + O2(g) -> 2H2O(l)
This tells us that two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of liquid water. The arrow ( -> ) means yields or produces. State symbols in parentheses show the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Balancing equations ensures that the number of atoms of each element is the same on both sides, satisfying conservation of mass. You balance by adding coefficients (numbers in front of formulas), never by changing subscripts within formulas, because that would change the substance identity.
The combustion of methane (natural gas) is written:
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)
Check the balance: Carbon: 1 on each side. Hydrogen: 4 on each side. Oxygen: 4 on each side. The equation is balanced. This tells us that one methane molecule needs two oxygen molecules to burn completely, producing one carbon dioxide molecule and two water molecules. In Australia, natural gas used for heating and cooking is mostly methane, so this reaction occurs in millions of homes daily.
Australian chemistry education: The Royal Australian Chemical Institute (RACI) promotes chemistry education and runs the Australian National Chemistry Quiz for school students. Understanding chemical equations is a foundational skill assessed in this quiz and in state science exams. Professional chemists in Australian universities and industries use equations daily to design reactions, calculate yields, and ensure safety.
You can balance equations by changing subscripts. This is false and dangerous. Subscripts define the chemical formula - they tell you how many atoms of each element are in one molecule. Changing H2O to H2O2 changes water into hydrogen peroxide, a completely different and much more reactive substance. Only coefficients can be changed to balance equations, because coefficients tell you how many molecules are involved, not what the molecules are.
Classify each change as physical or chemical.
Wrong: "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.
Right: Mass is always conserved. A solid that appears to vanish has likely become a gas, if you collected all the gases produced, total mass would equal the starting mass.
Wrong: "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.
Right: Boiling is a physical change, water molecules gain enough energy to escape as gas, but no new substance is formed. You can condense the steam back to liquid water unchanged.
Wrong: "All chemical reactions produce heat." No, many reactions absorb heat (endothermic), such as dissolving ammonium nitrate in water, which makes the container feel cold.
Right: Chemical reactions can either release heat (exothermic) or absorb heat (endothermic). Instant cold packs and photosynthesis are everyday examples of endothermic reactions.
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
At the start of this lesson, the hook asked you about a 1 kg log burning in a bushfire, the ash left behind weighs only 30–50 g, yet nothing truly disappears. Think back to what you guessed then.
Now that you know about conservation of mass and the difference between chemical and physical changes, where do you think that "missing" mass actually goes? How has your understanding of what happens during burning changed?
Q1. 1. Explain the difference between a physical change and a chemical change. Use one example of each in your answer. 4 MARKS
Q2. 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
Q3. 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?
Model answers (click to reveal)
Answers
▾MCQ 1
BWood burning is a chemical change because new substances (ash, carbon dioxide, water vapour) are formed with different properties from wood.
MCQ 2
CA solid forming from a solution is a precipitate, which is a clear sign of a chemical reaction.
MCQ 3
AThe 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
D8.3 g (product) - 5 g (magnesium) = 3.3 g of oxygen used. The container was sealed, so all products were captured.
MCQ 5
BMultiple 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.