Chemistry • Year 11 • Module 3 • Lesson 1
Physical & Chemical Change
Apply your understanding of indicators, conservation of mass and change classification to real data, real scenarios and a diagram critique.
1. Interpret experimental data — classifying changes at Port Kembla
A student visited the Port Kembla steelworks and collected data on six industrial processes. The table below records observations. 8 marks
| Process | Observation(s) | New substance formed? | Change type |
|---|---|---|---|
| Crushing limestone | Solid breaks into smaller pieces; no colour change | ||
| Heating limestone (calcination) | White solid decomposes; CO2 gas released; solid becomes quicklime (CaO) | ||
| Melting iron ore concentrate | Red solid becomes liquid at ~1538 °C; same chemical formula | ||
| Iron ore + coke in blast furnace | Orange-red solid consumed; molten iron produced; CO2 and CO gases evolved; intense heat | ||
| Dissolving iron scrap in acid | Metal slowly disappears; bubbles form vigorously; solution turns pale green; flask warms | ||
| Rolling hot steel into sheets | Steel is deformed into thin sheets; shiny metallic appearance maintained |
1.1 Complete the “New substance formed?” and “Change type” columns in the table above. 6 marks (1 per row)
1.2 For the blast furnace process (row 4), identify three observable indicators of chemical change from the data and name the new substance responsible for each. 2 marks
2. Interpret graph — temperature change during zinc + hydrochloric acid
A student placed 0.5 g of zinc granules in 50 mL of dilute hydrochloric acid (HCl) and measured the temperature of the solution every 30 seconds. The graph below shows the data. 7 marks
Figure 2. Temperature of 50 mL HCl(aq) + 0.5 g Zn(s) over time. Room temperature = 22 °C. Reaction: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g). Illustrative data.
2.1 Describe the trend in temperature from 0 s to 480 s. 2 marks
2.2 Using the graph, estimate the maximum temperature change (ΔT) during the reaction and explain what this indicates about the energy change. 2 marks
2.3 A student concludes: “This is a chemical change because the temperature increased.” Identify the flaw in this reasoning and write a stronger conclusion with two pieces of evidence. 3 marks
3. Compare physical and chemical change across five features
Complete the two-column table below. For each feature, write a concise description that contrasts the two types of change. 10 marks (1 per cell)
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed? | ||
| Atoms rearranged? | ||
| Reversibility | ||
| Energy change involved? | ||
| Australian example |
4. Predict and justify — a WA salt lake scenario
Lake Lefroy in Western Australia is a dry salt lake. During winter rain, water dissolves sodium chloride (NaCl) from the lakebed and the lake refills. During summer, the water evaporates and white NaCl crystals reappear on the lakebed surface.
5 marks
4.1 Classify the seasonal cycle at Lake Lefroy (dissolution in winter; crystallisation in summer) as physical or chemical changes. Justify each classification. 3 marks
4.2 A geologist claims that the total mass of NaCl in the lake system (dissolved + crystalline) is the same at the start and end of any complete annual cycle. Predict whether this claim is correct and link it to Lavoisier’s Law of Conservation of Mass. 2 marks
5. Spot the error — three students, three mistakes
Each student response below contains one error in reasoning about physical and chemical change. For each, identify the specific error and write a corrected response using precise lesson vocabulary. 6 marks (1 identify + 1 correct per item)
5.1 “When zinc is added to sulfuric acid, the test tube warms up slightly. This confirms that a chemical change has occurred, because physical changes never involve temperature changes.”
Error:
Correction:
5.2 “When I burned a candle, the wax melted and then burned. Both the melting and the burning are chemical changes because energy was released and the wax changed shape.”
Error:
Correction:
5.3 “To balance the equation H2 + O2 → H2O I changed it to H2 + O2 → H2O2 so both sides have 2 oxygen atoms.”
Error:
Correction:
Q1.1 — Classification table
Crushing limestone: No new substance / Physical change. Heating limestone: Yes (CaO and CO2) / Chemical change. Melting iron ore concentrate: No / Physical change. Blast furnace: Yes (Fe, CO2, CO) / Chemical change. Dissolving iron in acid: Yes (FeCl2 or FeSO4, H2) / Chemical change. Rolling hot steel: No / Physical change.
Q1.2 — Blast furnace indicators (2 marks)
Any three of: (1) Gas evolved (CO2 and CO) — new gaseous substances produced. (2) Temperature change (intense heat) — exothermic combustion releasing energy. (3) Colour change (orange-red ore → grey molten iron) — new substance Fe has different properties. (4) Solid disappearing (iron ore consumed) — solid reactant converted into new substance. Award 1 mark for each correctly named indicator linked to the specific new substance.
Q2.1 — Temperature trend (2 marks)
The temperature rises steeply from 22 °C at time zero to a maximum of approximately 48 °C at around 120 s [1]. After the peak, the temperature decreases gradually as the system loses heat to the surroundings, approaching room temperature again by about 480 s [1].
Q2.2 — ΔT and energy change (2 marks)
Maximum temperature rise ΔT ≈ 48 − 22 = 26 °C [1]. This indicates the reaction is exothermic: energy is released from the chemical bonds of the reactants to the surroundings, heating the solution [1].
Q2.3 — Flaw and stronger conclusion (3 marks)
Flaw: Temperature change alone is not sufficient evidence for a chemical change; dissolving ammonium nitrate in water also causes a temperature change but is a physical change [1]. Stronger conclusion using two pieces of evidence: (1) hydrogen gas (H2) is evolved as visible bubbles, indicating a new gaseous substance is formed [1]; (2) the zinc solid gradually disappears (consumed in reaction) and the solution changes character — new substances ZnCl2(aq) and H2(g) are produced with different properties from the reactants, confirming a chemical change [1].
Q3 — Compare and contrast table
New substance formed? Physical: No, same substance remains. Chemical: Yes, one or more new substances produced. Atoms rearranged? Physical: No rearrangement of atoms; intermolecular spacing may change. Chemical: Yes, atoms are rearranged into new bonding arrangements. Reversibility: Physical: Often easily reversible (e.g. melt → freeze). Chemical: Often irreversible (or requires different conditions to reverse). Energy change: Physical: Energy involved (e.g. melting absorbs heat) but no bond making/breaking between atoms. Chemical: Energy released (exothermic) or absorbed (endothermic) as bonds are broken and formed. Australian example: Physical: Salt dissolving in water at a WA salt lake / iron ore melting. Chemical: Iron smelting at Port Kembla steelworks / bushfire combustion.
Q4.1 — Salt lake cycle classification (3 marks)
Both dissolution and crystallisation are physical changes [1]. Dissolution: NaCl dissociates into Na+ and Cl− ions in water but no new chemical species are formed — NaCl remains chemically sodium chloride [1]. Crystallisation: the same Na+ and Cl− ions re-associate into the crystal lattice; the original substance NaCl is recovered, confirming no chemical change occurred [1].
Q4.2 — Conservation of mass at Lake Lefroy (2 marks)
The claim is correct [1]. The Law of Conservation of Mass (Lavoisier) states that atoms are rearranged but never created or destroyed. In a closed system, dissolving NaCl does not destroy the atoms; they remain in solution. When the water evaporates, the same Na and Cl atoms crystallise out. Total mass of NaCl (dissolved + solid) is conserved across the annual cycle [1].
Q5 — Spot the error (6 marks)
5.1 Error: The student claims temperature change alone confirms a chemical change, and that physical changes never involve temperature changes. Both claims are incorrect [1]. Correction: Temperature change is an indicator of chemical change, not the defining criterion. Physical changes can also involve temperature changes — for example, dissolving ammonium nitrate in water is an endothermic physical change. The defining criterion for a chemical change is whether a new substance with different chemical properties is formed. In this case, the zinc + sulfuric acid reaction IS a chemical change, but the correct evidence is the formation of new substances (ZnSO4 and H2), not the temperature rise alone [1].
5.2 Error: The student classifies both melting wax and burning wax as chemical changes. Melting is incorrectly classified [1]. Correction: Melting wax is a physical change — the wax changes state from solid to liquid but remains chemically the same substance; no new substance is formed. Burning wax is a chemical change because new substances (carbon dioxide and water vapour) are produced. Energy release and shape change are not sufficient criteria to classify a change as chemical [1].
5.3 Error: The student balanced the equation by changing a subscript inside a formula (H2O changed to H2O2), which changes the identity of the substance entirely [1]. Correction: H2O2 is hydrogen peroxide, a different compound from water. When balancing equations, only coefficients (numbers in front of formulas) may be changed, not subscripts. The correctly balanced equation is: 2H2 + O2 → 2H2O [1].