Every firework burst is a cascade of chemical changes — the same elements recombining into new substances, releasing light and energy in the process. Red from strontium, green from barium, blue from copper. The chemistry of change is happening all around you, every moment.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
You've probably seen iron rust and a candle burn. Both involve a metal or carbon reacting with oxygen. Here's the challenge: are rusting and burning the same type of change, or different? And is melting ice the same kind of change as burning wood?
Before you read on — write down what you think makes a change "chemical" versus "physical." What's your gut rule?
Type your initial response below — you will revisit this at the end of the lesson.
Write your initial response in your book. You will revisit it at the end of the lesson.
📚 Core Content
You can't always see bonds breaking and forming — but chemical reactions leave observable clues. NSW HSC requires you to identify and explain five observable indicators.
Atoms are never created or destroyed in a chemical reaction — they are just reshuffled into new arrangements. This is the Law of Conservation of Mass, established by Antoine Lavoisier in the 1770s: the total mass of reactants equals the total mass of products in a closed system.
Consider the reaction of hydrogen gas with oxygen gas to form water:
Modelling this with coloured balls (atoms) and sticks (bonds) makes the rearrangement visible — the same atoms appear on both sides, just connected differently.
Fireworks are a real-time demonstration of chemical change — metal salts undergoing combustion reactions that produce new substances and release energy as light. Each colour corresponds to a different metal salt:
| Colour | Metal salt | Indicator of chemical change |
|---|---|---|
| Red | Strontium compounds | Colour change, gas evolved, temperature increase |
| Green | Barium compounds | Colour change, gas evolved, temperature increase |
| Blue | Copper compounds | Colour change, gas evolved, temperature increase |
| Yellow | Sodium compounds | Colour change, gas evolved, temperature increase |
🧮 Worked Examples
🧪 Activities
1 Student A says: "When I dissolved salt in water and the water turned clear, that was a chemical change because the salt disappeared." Identify the error and correct it.
2 Student B says: "When ammonium nitrate dissolved in water, the beaker got really cold, so that proves it was a chemical change." Identify the error and correct it.
3 Student C says: "Rusting and burning are different types of change — rusting is physical (it's just the surface changing colour) and burning is chemical." Identify the error.
Type your working below before revealing answers above:
Complete these problems in your workbook.
| Observation | Indicator of chemical change? | New substance responsible |
|---|---|---|
| Red flash of light and colour | Your answer | Your answer |
| Loud bang and gas released | Your answer | Your answer |
| Warmth felt at a distance | Your answer | Your answer |
| White smoke (SrO) remaining | Your answer | Your answer |
Complete the table and answer A and B below:
Complete the table and answer A and B in your workbook.
Earlier you were asked: Are rusting and burning the same type of change? Is melting ice the same kind of change as burning wood?
The key insight: rusting and burning are both chemical changes — both produce new substances (iron(III) oxide; carbon dioxide and water) with different properties from the reactants. Melting ice is a physical change — water is still water, just in a different state. The test is never about appearance; it's always about whether new substances with different chemical properties are formed.
Now revisit your initial response. What did you get right? What has changed in your thinking?
Look back at your initial response in your book. Annotate it with what you now understand differently.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Wrong: A temperature change always means a chemical change has occurred.
Right: Temperature changes occur in both physical and chemical changes. Dissolving NaOH in water is exothermic but physical. A chemical change requires formation of a new substance with different properties. The definitive test is whether new chemical species are produced.
5 random questions from a replayable lesson bank — feedback shown immediately
✍️ Short Answer
8. Distinguish between a physical change and a chemical change. In your answer, explain two observable indicators that can suggest a chemical change has occurred and provide one example of each indicator. 4 MARKS
Type your answer below:
Answer in your workbook.
9. A student adds marble chips (calcium carbonate, CaCO3) to a beaker of dilute hydrochloric acid (HCl). They observe: vigorous bubbling, the marble chips gradually disappearing, and the beaker becoming slightly warm. (a) Identify three indicators of chemical change present. (1 mark each) (b) Write a balanced chemical equation for this reaction. Include state symbols. (1 mark) 4 MARKS
Type your answer below:
Answer in your workbook.
10. Fireworks produce brilliant colours through the combustion of metal salts. When a red firework (strontium nitrate, Sr(NO3)2) is ignited, observers see a red flash, hear an explosion, feel heat, and notice white strontium oxide (SrO) smoke. (a) Identify all observable indicators of chemical change present and, for each, name the new substance responsible. (3 marks) (b) Explain why a student who says "the firework just changed colour so it must be physical" is incorrect. Use at least two pieces of evidence. (2 marks) 5 MARKS
Type your answer below:
Answer in your workbook.
1. Student A: Error — used "disappeared" as evidence without checking whether a new substance formed. Correction: Dissolving salt is physical; NaCl dissociates to Na⁺ and Cl⁻ but remains chemically the same; recoverable by evaporation.
2. Student B: Error — used temperature change alone as proof of chemical change. Correction: Temperature change is an indicator, not a defining criterion. Dissolving NH₄NO₃ is an endothermic physical change — no new substance is formed.
3. Student C: Error — misclassified rusting as physical because "only the surface changed colour." Correction: Rusting produces Fe₂O₃, a new substance — this is a chemical change. Colour change at the surface is evidence of a new substance, not evidence of a physical change.
Row 1: Colour change — Yes, indicator of chemical change. New substance: excited electrons in strontium atoms releasing energy as red light (strontium oxide SrO formed)
Row 2: Gas evolved (explosion/bang) — Yes, indicator. New substances: CO₂ and NO₂ gases from decomposition of nitrate
Row 3: Temperature change (warmth felt) — Yes, indicator. Exothermic combustion releasing heat energy
Row 4: New product SrO visible as white smoke — Yes, indicator. SrO is a new solid substance with different properties from Sr(NO₃)₂
Question A: The ignition is a chemical change because new substances are formed (SrO, CO₂, NO₂) with different properties from the original strontium nitrate. Multiple indicators confirm this — gas evolved, colour change, temperature increase, and new solid product. The original substance is consumed and cannot be recovered.
Question B: The original Sr(NO₃)₂ is white powder; the product SrO is also white but is a chemically different substance. SrO has a different melting point, different reactivity with water, and different chemical formula. The fact that the product looks similar in colour does not mean it is the same substance — its chemical identity has changed, confirming a chemical change.
1. D — The defining criterion for chemical change is the production of a new substance with different chemical properties. Temperature, colour, and bubbles are indicators but not the definition.
2. B — The copper sulfate was recovered unchanged; no new substance was produced. Applying heat to evaporate is a physical process here.
3. C — 2Mg + O₂ → 2MgO: left side has 2 Mg and 2 O; right side has 2 Mg and 2 O. Balanced. Option B has 2 O left and 1 O right (unbalanced). Option A uses O atoms not O₂ molecules.
4. A — Three indicators: bubbles (gas evolved = H₂), zinc disappears (solid consumed, chemical reaction), test tube warms (exothermic temperature change). The solution remaining colourless is not an indicator of change.
5. B — Dissolving NaCl is physical (NaCl unchanged, recoverable). Burning candle wax produces CO₂ and H₂O — chemical change.
6. C (Band 5 — Evaluate) — New substances NaOH(aq) and H₂(g) are formed. 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). This is fundamentally different from NaCl dissolving — the sodium atoms are consumed and incorporated into new compounds.
7. A (Band 6 — Create) — The strongest evidence combines: (1) conservation of mass check (sealed container), (2) testing chemical properties of the product — MgO has a different melting point (~2852°C vs Mg ~650°C), reacts differently with acid, and has different reactivity. Option B documents only one indicator; C documents only temperature; D attempts to prove the substance is unchanged (which would disprove chemical change).
Q8 (4 marks): A physical change alters the form or appearance of a substance without producing a new substance — the chemical identity remains the same [1]. A chemical change produces one or more new substances with different chemical properties — bonds are broken and formed [1]. Indicator 1 — gas evolved: bubbles/fizzing indicate a new gaseous product has formed; e.g. adding acid to marble chips produces CO₂ gas [1]. Indicator 2 — colour change: a new colour appears because the new substance absorbs different wavelengths of light; e.g. copper turning green as it oxidises to form copper carbonate [1]. Accept any two of the five indicators with valid examples.
Q9 (4 marks): (a) Indicator 1: gas evolved — vigorous bubbling indicates CO₂ gas produced [1]. Indicator 2: solid disappearing — marble chips consumed, indicating reaction forming new substances [1]. Indicator 3: temperature change — beaker becomes warm, indicating exothermic reaction [1]. (b) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1 — must be balanced with state symbols].
Q10 (5 marks): (a) Indicator 1: colour change (red flash) — new substance: excited strontium atoms emitting red light as electrons fall back; SrO formed [1]. Indicator 2: gas evolved (explosion/bang) — new substances: CO₂ and NO₂ gases from decomposition of NO₃⁻ [1]. Indicator 3: temperature change (heat felt) — exothermic combustion releases energy as heat [1]. (b) The student is incorrect because: first, a colour change is an indicator of chemical change (evidence a new substance is forming), not evidence of physical change [1]; second, the formation of white SrO smoke confirms a new substance with different chemical properties from the original Sr(NO₃)₂ has been produced — a defining criterion for chemical change [1].
Climb platforms, hit checkpoints, and answer questions on Physical & Chemical Change. Quick recall from lessons 1–1.
Tick when you've finished all activities and checked your answers.