Chemistry • Year 11 • Module 3 • Lesson 1
Physical & Chemical Change
Build HSC Band 5–6 extended-response technique on classifying change, applying conservation of mass, and evaluating evidence in industrial and experimental contexts.
1. Data + scenario: Lavoisier’s sealed-vessel experiment (Band 5–6)
8 marks Band 5–6
Scenario. In 1770, Antoine Lavoisier burned a weighed sample of tin (Sn) inside a sealed glass vessel containing air. He observed: (1) a grey-white solid powder (tin oxide, SnO2) formed on the surface of the tin; (2) the solid became noticeably heavier; (3) the air volume inside the vessel decreased slightly; (4) when the vessel was opened, air rushed in. Before Lavoisier, the phlogiston theory predicted that burning substances would lose mass, because they released “phlogiston” into the air. The table below shows Lavoisier’s summarised data.
| Measurement | Before burning | After burning |
|---|---|---|
| Mass of tin (g) | 12.0 | 10.4 |
| Mass of tin oxide SnO2 (g) | 0 | 16.1 |
| Mass of vessel + contents (g) | 388.0 | 388.0 |
| Colour of solid at tin surface | silver-grey (Sn) | grey-white powder (SnO2) |
Illustrative data based on Lavoisier, Traité Élémentaire de Chimie (1789).
Q1. Analyse and evaluate the experimental data above to determine whether the burning of tin is a physical or chemical change, and assess whether Lavoisier’s results support or refute the phlogiston theory. In your response you must:
- Classify the burning of tin as physical or chemical and justify with reference to at least three observable indicators from the data.
- Use the mass data to demonstrate whether the Law of Conservation of Mass holds, showing your reasoning quantitatively.
- Explain why the mass of the solid increased even though tin metal was consumed (reference oxygen from the air).
- Evaluate whether the data support or refute the phlogiston theory, using at least one specific piece of evidence.
- State one limitation of Lavoisier’s experimental design and suggest an improvement.
2. Experimental design — testing whether rusting is chemical or physical (Band 5–6)
7 marks Band 5–6
Research question. A Year 11 student argues that rusting of iron is a physical change because “only the surface colour appears to change.” Design a scientific investigation to determine whether rusting of iron produces a new substance, thereby settling whether it is a physical or chemical change.
Constraints: You have access to standard Year 11 laboratory equipment (balances, test tubes, iron nails, dilute acids, a drying oven at 80 °C, and common indicator solutions). Your investigation must take no longer than one week.
Q2. Design the investigation and present it in the format below.
- State your hypothesis (a testable prediction including the independent and dependent variables).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will test whether the product is the same as or different from the original iron.
- Explain what result would falsify your hypothesis.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Classification and indicators: The burning of tin is a chemical change because multiple indicators of new substance formation are present [1 — classification with justification]. Indicator 1: a colour change from silver-grey tin metal to grey-white SnO2 powder indicates a new substance has formed, since the new colour reflects different light-absorbing properties of SnO2 [indicator 1]. Indicator 2: the tin solid gradually disappears (is consumed) during the reaction, consistent with atoms being incorporated into a new product rather than simply dissolving physically [indicator 2]. Indicator 3: the composition of the gas inside the vessel changes (air volume decreases, air rushes in when opened), indicating oxygen was consumed to produce a new substance, confirming a chemical reaction occurred [indicator 3]. Award 1 mark for each clearly identified indicator linked to new substance formation. Accept also: temperature change (reaction may release heat).
Conservation of mass (quantitative): Total mass of vessel + contents before = 388.0 g; after = 388.0 g — no mass was lost, consistent with the Law of Conservation of Mass [1]. Within the vessel: mass of SnO2 produced = 16.1 g; mass of Sn consumed = 12.0 − 10.4 = 1.6 g still present + 10.4 g consumed — Wait: Sn remaining = 10.4 g. Sn consumed = 12.0 − 10.4 = 1.6 g. But SnO2 = 16.1 g. Mass of oxygen incorporated = 16.1 − (12.0 − 10.4) = 16.1 − 1.6 = 14.5 g from air. Total mass of products (10.4 g Sn + 16.1 g SnO2) = 26.5 g. This plus the air in the sealed vessel = 388.0 g (constant) — conservation holds [1].
Why the solid increased in mass: Tin reacted with oxygen from the air inside the vessel, incorporating oxygen atoms into the new compound SnO2. The mass of the solid increased because the new substance contains both tin and oxygen atoms; the oxygen came from the limited air supply, which is why the air volume decreased [1].
Phlogiston evaluation: The phlogiston theory predicts mass loss during burning (phlogiston released). Lavoisier’s data shows the solid gained mass (from 12.0 g to 16.1 g SnO2), directly contradicting the phlogiston prediction [1]. Furthermore, the total vessel mass stayed constant (388.0 g), meaning no “substance” escaped — refuting the idea of phlogiston being released. The data strongly refute the phlogiston theory and support the oxygen-based combustion model [1].
Limitation and improvement: One limitation is that the experiment used a sealed vessel: if the seal was not perfect, air could escape or enter, compromising the mass measurements [1]. Improvement: use a calibrated airtight vessel and repeat the experiment three times to improve reliability; alternatively, use a modern digital balance with 0.001 g precision to detect small mass changes more accurately [1].
Marking criteria summary (8 marks): 1 = classifies as chemical + at least three indicators linked to new substance formation (accept any valid three); 1 = correct quantitative conservation of mass reasoning using the table; 1 = explains mass increase via oxygen incorporation from air; 1 = evaluates phlogiston theory using a specific data point; 1 = clear refutation with second evidence; 1 = names one valid limitation; 1 = states one improvement; 1 = uses precise chemical terminology throughout (SnO2, exothermic/endothermic, conservation of mass, chemical change, indicator).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If rusting of iron is a chemical change, then the rust (iron oxide, Fe2O3) formed on an iron nail exposed to water and oxygen will have different chemical and physical properties from the original iron metal. Independent variable: whether the nail is exposed to water and oxygen (rusting conditions) or not. Dependent variable: the properties of the solid product (colour, reactivity with dilute HCl, magnetic behaviour). Controlled variables: nail mass (5 g each), temperature (room temperature), nail surface area (same grade of iron nails). [1 — hypothesis with IV and DV]
Procedure: (1) Mass two identical iron nails (5.00 g each). Place nail A in a test tube with 5 mL distilled water and leave exposed to air for 7 days; store nail B dry in a sealed vial. (2) After 7 days, photograph and record colour and mass of both nails. (3) Test the magnetic attraction of nail A (rusted) and nail B (original) using a magnet — iron is strongly magnetic; Fe2O3 is non-magnetic or weakly magnetic. (4) Scrape 0.5 g of the rust from nail A and place in 10 mL dilute HCl; place 0.5 g of shavings from nail B in 10 mL of the same concentration HCl. Observe the rate of reaction, colour of solution, and whether gas is evolved. Record differences. [1 — four clear steps including a chemical property test]
Falsification: If the rust product has the same reactivity with acid, the same magnetic behaviour, the same colour, and the same mass per gram as the original iron, the hypothesis would be falsified — there would be no evidence of a new substance [1].
Limitations: (1) Rust formation may be incomplete after 7 days, meaning some original iron may remain mixed with the rust, making property comparisons less clear [1]. (2) Scraping rust off the nail may introduce contaminants from the nail surface, affecting acid-reactivity results [1].
Improvement: Repeat the experiment with a minimum of three nails per condition to improve reliability; use a control without water or oxygen to confirm that oxygen and water are both required for rusting [1].
What the results would show: Fe2O3 reacts much more slowly with HCl than iron metal, produces a yellow-brown solution (FeCl3) rather than a colourless one (FeCl2), and is non-magnetic — all consistent with a new substance having been formed, confirming a chemical change [1].
Marking criteria summary (7 marks): 1 = testable hypothesis naming IV and DV; 1 = four steps with a chemical property test (acid reactivity, magnetic test, or solubility); 1 = states what would falsify the hypothesis; 1 = one valid limitation; 1 = second valid limitation; 1 = one specific improvement; 1 = precise chemical terminology (Fe2O3, independent/dependent/controlled variable, chemical change, new substance).