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hscscience Chem · Y11
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Module 3 · L1 of 12 35 min ⚡ +50 XP in Learn · +25 to complete

Physical & Chemical Change

In 1871, the Great Chicago Fire destroyed much of the city. In any large fire, the gases formed depend on the chemistry: where air is plentiful, carbon burns completely to CO₂; in enclosed, oxygen-poor spaces, incomplete combustion produces carbon monoxide (CO), which is far more acutely toxic. CO₂ is not acutely poisonous in the same way, but it is not harmless either, at high concentration it displaces oxygen and it is a greenhouse gas. The same fuel, two very different chemical reactions.

Today's hook, In 1871, the Great Chicago Fire destroyed much of the city. In any large fire, the gases formed depend on the chemistry: where air is plentiful, carbon burns completely to CO₂; in enclosed, oxygen-poor spaces, incomplete combustion produces carbon monoxide (CO), which is far more acutely toxic. CO₂ is not acutely poisonous in the same way, but it is not harmless either, at high concentration it displaces oxygen and it is a greenhouse gas. The same fuel, two very different chemical reactions.
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Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions
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Recall, your gut answer first
+5 XP warm-up

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?

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What you'll master, and the words for it

03
What you'll master
Know

Key facts

  • Common observable indicators of chemical change
  • Examples of physical and chemical changes
  • The Law of Conservation of Mass
Understand

Concepts

  • Why new substance formation defines chemical change
  • How atoms are rearranged (not created/destroyed)
  • Why chemical equations must be balanced
Can do

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
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Key terms
Physical change
A change that alters the form or state of a substance without producing any new substance.
Chemical change
A change that produces one or more new substances with different chemical properties.
Law of Conservation of Mass
The principle that total mass of reactants equals total mass of products in a chemical reaction.
Reversible reaction
A reaction where the forward and reverse reactions happen at the same time in a closed system. As products build up, the reverse reaction speeds up; once the forward and reverse rates become equal, the amounts of reactants and products stop changing, a dynamic balance called equilibrium. Neither reactants nor products are ever fully used up. (Note: the mathematical equilibrium model, Kc and Kp, is studied in Year 12 Module 5.)
Irreversible reaction
A reaction that proceeds essentially to completion in one direction only, the reverse reaction does not happen to any measurable extent, so no dynamic balance is ever reached. (Note: the mathematical equilibrium model for reversible reactions is studied in Year 12 Module 5.)
Synthesis reaction
A reaction where two or more reactants combine to form a single product.
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Common Indicators of Chemical Change

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Common Indicators of Chemical Change
core concept

You can't always see bonds breaking and forming, but chemical reactions leave observable clues. Several common indicators are shown below. No single observation is proof on its own, each must be interpreted, and the defining test is whether a new substance has formed.

Colour change, gas, precipitate, temperature change or a consumed reactant signal a new substance.
HSC extended response tip: Name the specific indicator and explain what new substance caused it. "A yellow precipitate formed, indicating that lead iodide (PbI2) was produced" scores more marks than just "a precipitate formed."
Common error, temperature alone: A temperature change alone does not confirm a chemical change. Dissolving ammonium nitrate in water causes a temperature drop but is a physical change. Always look for multiple indicators or evidence of a new substance.

Common observable indicators of chemical change include: colour change, gas evolved (bubbling/fizzing), a precipitate forming, a temperature change, and a reactant being consumed alongside evidence of new products. No single indicator is proof on its own, in HSC answers name the specific new substance responsible for each.

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?

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Modelling Atom Rearrangement & Conservation of Mass

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Modelling Atom Rearrangement & Conservation of Mass
core concept

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:

H2 + O2 → H2O   ✗ UNBALANCED 2 O on left, 1 O on right
2H2 + O2 → 2H2O   ✓ BALANCED 4 H and 2 O on each side

Modelling this with coloured balls (atoms) and sticks (bonds) makes the rearrangement visible, the same atoms appear on both sides, just connected differently.

Balancing rule: When balancing equations, change coefficients (numbers in front of formulas) only, never change subscripts inside a formula. Changing H2O to H2O2 changes the substance entirely, that would be hydrogen peroxide, a different compound.
Common error, changing subscripts: Students often try to balance equations by changing subscripts rather than coefficients. Writing H2O3 instead of placing a 2 in front of H2O is chemically wrong, it changes what substance you're describing.
Historical insight: Lavoisier's discovery required sealed containers to capture gases. Before this, chemists thought burning released a mysterious substance called "phlogiston" into the air, mass appeared to decrease because the gas escaped unweighed.
Watch atoms rearrange, same colours appear on both sides · choose any reaction · atoms are never created or destroyed Interactive

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.

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Fireworks, Chemical Change in Action

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Fireworks, Chemical Change in Action
core concept

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. A firework "star" contains a fuel, an oxidiser, and a metal colourant salt; when ignited, the fuel-oxidiser mixture combusts to produce hot gases and new solid oxides, while the metal colourant emits characteristic coloured light as its excited electrons relax. Each colour corresponds to a different metal:

Colour Metal salt Indicator of chemical change
RedStrontium compoundsColour change, gas evolved, temperature increase
GreenBarium compoundsColour change, gas evolved, temperature increase
BlueCopper compoundsColour change, gas evolved, temperature increase
YellowSodium compoundsColour change, gas evolved, temperature increase
🎆 Real-World Anchor, Fireworks: Fireworks demonstrate several indicators of chemical change at once, light (colour) emission, gas evolved, a temperature increase, and new solid oxide products forming. The original metal salt is consumed and cannot be recovered. This is the real-world anchor for this lesson and will reappear in Short Answer Q3.
Common error: The colour of a firework is not a physical property of the metal, it is produced by the chemical reaction. The metal itself may be grey or silver. Electron transitions during combustion produce the colour as electrons fall back to lower energy levels.

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 star contains a strontium colourant salt together with a fuel and oxidiser. When ignited, observers see a brilliant red flash, feel heat, and notice white smoke. Identify three indicators of chemical change present and explain the chemistry behind each (for the colour, say where the light comes from). (2–3 sentences)

Cross-lesson links: This lesson introduces the foundation concepts, physical vs chemical change and the six reaction types. In L02 you will apply this directly to synthesis and decomposition reactions with balanced equations.
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Worked examples

Worked examples · reveal as you go

Worked Example 1, Classifying Physical and Chemical Changes +5 XP on full reveal

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

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Chocolate changes from solid to liquid but is still chemically chocolate. No new substance is formed. Physical change.
(a) Melting chocolate
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Burning methane produces carbon dioxide and water vapour, new substances with different properties from methane. Chemical change.
(b) Burning natural gas
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Sugar molecules disperse through water but remain chemically sucrose. Can be recovered by evaporation. No new substance. Physical change.
(c) Dissolving sugar in water
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Produces rust, hydrated iron(III) oxides (commonly modelled as Fe2O3), a new reddish-brown material with different properties from grey iron metal. Chemical change.
(d) Rust forming on iron
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Worked Example 2, Identifying Indicators of Chemical Change +5 XP on full reveal

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.

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Zinc metal is consumed, indicating it has reacted and formed a new substance (not just dissolved physically).
Indicator 1, Reactant consumed
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Bubbles forming, gas is evolved. Zinc reacting with sulfuric acid produces hydrogen gas (H2); the teacher can confirm this by performing the "squeaky pop" test, holding a lit splint to a small test tube of the collected gas behind a safety screen.
Indicator 2, Gas evolved
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The test tube becomes warm, indicating an exothermic chemical reaction releasing energy to the surroundings.
Indicator 3, Temperature increase
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Zinc sulfate (ZnSO4) is also formed in solution.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Products and balanced equation
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Safety, teacher-supervised practical. Reactions of metals with acids release flammable hydrogen gas. Any "squeaky pop" ignition test is done by the teacher on a small sample of gas, using a lit splint behind a safety screen with no other ignition sources nearby. Wear safety glasses, use only small quantities of dilute acid, work in a well-ventilated room, and neutralise and dispose of waste as directed. These are school-laboratory procedures, do not attempt them at home.
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What you'll master, and the words for it

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Formula reference · this lesson
core formula
📐

Key Principles, This Lesson

No calculation formulas, classification is conceptual
Key principle: Conservation of atoms, atoms are rearranged in chemical reactions, never created or destroyed Balancing rule: Total atoms on left = total atoms on right Test for chemical change: Is a new substance with different properties formed?
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Common misconception

Common errors · the 3 traps that cost marks

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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.

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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?"

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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."

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Drill, then revisit

Quick-fire practice · 5 reps +2 XP per reveal

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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.

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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.

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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.

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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.

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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)

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Revisit your thinking

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 an acutely 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 relatively low-hazard (CO₂, H₂O) or acutely toxic (CO).

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.

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Interactive Tool, Chemical vs Physical Change Open fullscreen ↗
True or false? The Chemical vs Physical Change tool demonstrates that dissolving salt in water is a physical change because the salt can be recovered by evaporation and no new substance is formed.
✓

Multiple choice

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Multiple choice
+5 XP per correct · +25 XP all-correct

Pick your answer, then rate your confidence. That tells the system what to drill next.

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Short answer
UnderstandBand 34 MARKS

Q1. 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.

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ApplyBand 44 MARKS

Q2. 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)

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EvaluateBand 55 MARKS

Q3. Fireworks produce brilliant colours through the combustion of a fuel-oxidiser mixture together with a metal colourant salt. When a red firework star (containing a strontium salt) is ignited, observers see a red flash, feel heat, and notice white strontium oxide (SrO) smoke. (a) Identify three observable indicators of chemical change present and, for each, explain the chemistry responsible (for the colour, state where the light comes from). (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)

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📖 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 rust (hydrated iron(III) oxides, modelled as Fe₂O₃), a new material, 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: Light emission (red flash), Yes, indicator of chemical change. The red light is emitted by excited strontium species as their electrons relax to lower energy levels, this is energy emission, not a new substance; the new solid substance formed in the reaction is strontium oxide (SrO)

Row 2: Gas evolved, Yes, indicator. The fuel-oxidiser mixture combusts to produce hot gases (such as CO₂ and N₂); any loud bang is the rapid expansion of these gases, not itself a separate product

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 the original strontium salt

Question A: The ignition is a chemical change because new substances are formed (e.g. strontium oxide SrO, plus combustion gases such as CO₂ and N₂ from the fuel-oxidiser reaction) with different properties from the starting materials. Multiple indicators confirm this, light emission, gas evolved, temperature increase, and new solid product. The original substances are 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

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.

The copper sulfate was recovered unchanged; no new substance was produced. Applying heat to evaporate is a physical process here.

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.

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.

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 forms a patina (basic copper carbonate) [1]. Accept any two valid indicators with 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: light emission (red flash), the red light is emitted by excited strontium species as their electrons relax to lower energy levels, this is energy emission, not a new substance, but it accompanies the high-temperature combustion [1]. Indicator 2: gas evolved, the fuel-oxidiser mixture combusts to produce hot gases such as CO₂ and N₂ (any bang is rapid gas expansion, not a separate product) [1]. Indicator 3: temperature change (heat felt), exothermic combustion releases energy as heat [1]. (b) The student is incorrect because: first, the coloured light accompanies a combustion reaction, it is evidence of a chemical change rather than a simple change of physical state [1]; second, the formation of white SrO smoke confirms a new substance with different chemical properties from the original strontium salt has been produced, a defining criterion for chemical change [1].