Physical & Chemical Change
In 1871, the Great Chicago Fire burned for three days — but what killed most victims was not the flames, but suffocation in poorly ventilated buildings where smouldering timber produced CO rather than CO₂. Whether a fire produces harmless or lethal gases depends entirely on the type of chemical change occurring. The same fire, two completely different reactions.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Key facts
- The five observable indicators of chemical change
- Examples of physical and chemical changes
- The Law of Conservation of Mass
Concepts
- Why new substance formation defines chemical change
- How atoms are rearranged (not created/destroyed)
- Why chemical equations must be balanced
Skills
- Classify changes as physical or chemical with justification
- Identify multiple indicators of chemical change in a scenario
- Check whether a chemical equation is balanced
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.
The five observable indicators of chemical change are: colour change, gas evolved (bubbling/fizzing), precipitate formed, temperature change, and solid disappearing — name the specific new substance responsible for each indicator in HSC answers.
Pause — copy the highlighted definition into your book before moving on.
Quick check: Which of the following is the BEST evidence that a chemical change has occurred?
We just saw that chemical changes produce observable indicators because new substances form. That raises a question: if new substances appear, where do the atoms come from? This card answers it → atoms are rearranged, never created or destroyed — the Law of Conservation of Mass.
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.
The Law of Conservation of Mass: total mass of reactants = total mass of products — atoms are rearranged, never created or destroyed. Balance equations by changing coefficients only; changing a subscript creates a different substance entirely.
Add the highlighted rule to your notes before the check below.
Fill the gap: When balancing a chemical equation you may only change the [___] in front of a formula — changing the numbers inside the formula would produce a completely different substance.
We just saw that atoms are rearranged in chemical reactions and mass is conserved. That raises a question: what does this look like in a dramatic, real-world context with multiple simultaneous indicators? This card answers it → fireworks combust metal salts, producing multiple indicators and oxide products that confirm chemical change.
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 |
Firework combustion is an exothermic chemical change: different metal salts produce characteristic flame colours (Na → yellow, Sr → red, Cu → green/blue) and new metal oxide products that cannot be recovered — confirming irreversible chemical change.
Pause — write the highlighted point into your book.
Explain it: A red firework contains strontium nitrate. When ignited, observers see a brilliant red flash, feel heat, and notice white smoke. Identify three indicators of chemical change present and name the new substance responsible for each. (2–3 sentences)
Worked examples · reveal as you go
Classify each of the following as a physical or chemical change. Justify each answer by identifying whether a new substance is formed.
(a) Melting chocolate (b) Burning natural gas (c) Dissolving sugar in water (d) Rust forming on iron
A student adds a piece of zinc metal to a test tube containing dilute sulfuric acid. They observe: the zinc gradually disappears, bubbles form vigorously, and the test tube becomes warm. Identify all indicators of chemical change present and name the products formed.
Key Principles — This Lesson
Common errors · the 3 traps that cost marks
Common misconception
A temperature change always means a chemical change has occurred.
Fix: 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.
Bubbling always means a chemical change
Students see fizzing when a carbonated drink is opened and classify it as a chemical change.
Fix: Bubbling from a dissolved gas escaping (e.g. opening a soft drink) is a physical change — no new substance is formed. Bubbling that indicates a chemical change occurs when gas is a new reaction product (e.g. Zn + HCl → ZnCl₂ + H₂). Always ask: "Is the gas a new substance formed by the reaction, or was it already present in dissolved form?"
Mass appears to decrease when gas escapes — conservation is violated
When a reaction in an open beaker produces a gas, the balance reading drops, leading students to conclude that mass is not conserved.
Fix: Conservation of mass applies to the total system, including escaped gases. In an open container, gaseous products leave the measured system, making the apparent mass drop. In a sealed container, total mass is constant. HSC answers should state: "Mass is conserved in a closed system; in an open system the measured mass decreases because gaseous products escape to the surroundings."
Quick-fire practice · 5 reps +2 XP per reveal
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.
Correction: Dissolving salt in water is a physical change. NaCl dissociates into Na⁺ and Cl⁻ ions in solution but remains chemically sodium chloride — no new substance is formed. The salt can be recovered by evaporating the water.
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.
Correction: Dissolving ammonium nitrate in water is a physical change (endothermic). No new substance is formed — the NH₄⁺ and NO₃⁻ ions remain chemically ammonium nitrate in solution. The temperature change here results from the energy absorbed during dissolution, not from a chemical reaction.
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.
Correction: Both rusting and burning are chemical changes — both produce new substances. Rusting produces iron(III) oxide (Fe₂O₃), a new substance with different properties from iron. The colour change at the surface is evidence of a new substance, not evidence of a physical change. Student C confused observable appearance with chemical identity.
Q1 (4 marks): 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.
Q2 (4 marks): 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)
The 1871 Chicago Fire — like all fires — involved both types of change and both types of combustion. At the active fire fronts, plenty of air meant complete combustion: carbon → CO₂, hydrogen → H₂O (both chemical changes, producing new substances). In enclosed spaces and smouldering zones, restricted oxygen produced CO through incomplete combustion — also a chemical change, but with a toxic product. The physical changes were there too: wax melting, wood charring, glass cracking — but it was the chemical changes that determined whether the products were harmless or deadly.
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.
Pick your answer, then rate your confidence — that tells the system what to drill next.
Q1. 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.
Q2. 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)
Q3. 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)
📖 Comprehensive answers (click to reveal)
Activity 1 — Spot + Fix
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.
Activity 2 — Fireworks Table
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.
❓ Multiple Choice
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).
Short Answer Model Answers
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].
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