Chemistry • Year 11 • Module 1 • Lesson 1
Properties and Classification of Matter
Apply your classification skills to real measurement data, case studies and structured comparisons at Band 4–5 level.
1. Interpret measurement data, classifying five mystery substances
A student measured properties of five laboratory samples using standard equipment. The table below records the results. 10 marks
| Sample | Appearance | Melting point (°C) | Boiling point changes with concentration? | Can be decomposed chemically? |
|---|---|---|---|---|
| W | Clear colourless liquid, uniform | 0, sharp, fixed | No | Yes, by electrolysis to H2 and O2 |
| X | Shiny orange solid, uniform | 1085, sharp, fixed | No | No |
| Y | Clear liquid, uniform | Varies with salt concentration | Yes | No (only by physical means) |
| Z | White solid granules mixed with black powder; two distinct regions visible | No single fixed point | n/a | Not as a unit (components retain individual properties) |
| V | Yellow-green gas, uniform | −101, sharp, fixed | No | No |
1.1 Complete the table below by classifying each sample and providing a one-sentence justification using only the data above. 7 marks
| Sample | Classification | Justification using the data |
|---|---|---|
| W | ||
| X | ||
| Y | ||
| Z | ||
| V |
1.2 For samples W and Y: both are clear colourless (or near-colourless) liquids that look uniform. Identify the one piece of data in the table that most clearly distinguishes them, and explain what it tells you about each. 2 marks
1.3 Why can appearance alone never definitively determine whether a substance is a pure substance or a homogeneous mixture? Use samples from the table to support your answer. 1 mark
2. Interpret graph, melting point as evidence of purity
A student heated three different 5 g samples (P, Q, R) in a capillary tube apparatus and recorded temperature vs time. The graph below shows the heating curves. 8 marks
Figure 2. Heating curves for 5 g samples P, Q, R heated at a constant rate. Illustrative data.
2.1 For each sample, state whether it is most likely a pure substance or a mixture, and justify your answer using the shape of the heating curve. 3 marks (1 per sample)
2.2 Read the melting point of Sample P from the graph. Using the decision tree from the lesson, state whether Sample P is an element or compound, and explain what additional information you would need to determine this. 2 marks
2.3 A student concludes: “Sample Q is a heterogeneous mixture because its temperature keeps rising.” Identify one flaw in this reasoning and write a more accurate conclusion. 2 marks
2.4 Samples P and R both show flat plateaus. Suggest why their melting point temperatures differ. 1 mark
3. Compare element, compound and mixture across five features
Complete the three-column table below. For each feature, write a concise description that contrasts all three types. 10 marks)
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Number of atom types | |||
| Is composition fixed? | |||
| Can be separated by physical means? | |||
| Example formula or name | |||
| Australian real-world example |
4. Predict and justify, the Broken Hill ore body
The Broken Hill silver–lead–zinc ore body in far-western NSW contains several minerals mixed together, including galena (PbS), sphalerite (ZnS) and gangue (waste rock). Miners extract the ore and process it in a concentrator to separate the different minerals.
5 marks
4.1 Classify the raw ore body (PbS, ZnS, gangue mixed together) as one of: element, compound, homogeneous mixture, or heterogeneous mixture. Justify your classification using two pieces of evidence from the description. 3 marks
4.2 After smelting, a silver–lead alloy is produced. Predict the classification of this alloy and explain why its classification differs from that of galena (PbS). 2 marks
Q1.1, Classification table
W: Compound, sharp fixed melting point (pure substance); can be decomposed by electrolysis into two different elements (H2 and O2), so it is chemically breakable → compound. (This is water, H2O.) X: Element, sharp fixed melting point (pure substance); cannot be decomposed chemically; contains only one type of atom (copper, Cu). Y: Homogeneous mixture, boiling point changes with concentration (variable composition); looks uniform (one visible phase) → homogeneous mixture. (This is a salt solution.) Z: Heterogeneous mixture, two distinct visible regions (white solid + black powder), no single fixed melting point → heterogeneous mixture. V: Element, sharp fixed melting point (pure substance); cannot be decomposed chemically → element. (This is chlorine gas, Cl2.)
Q1.2, Distinguishing W and Y (2 marks)
The most useful distinguishing piece of data is whether the boiling point changes with concentration [1]. Sample W has a fixed melting point of 0°C and a fixed boiling point, indicating it is a pure substance, its composition does not vary. Sample Y has a boiling point that changes with salt concentration, confirming its composition is variable and therefore it is a mixture [1].
Q1.3, Appearance alone insufficient (1 mark)
Appearance alone cannot determine purity because a homogeneous mixture looks just as uniform as a pure substance [1]. Samples W (pure compound) and Y (homogeneous mixture) both appear as clear uniform liquids, yet they have fundamentally different classifications based on composition data. The appearance “uniform” refers to visual phase, not chemical identity.
Q2.1, Classification from heating curves (3 marks)
Sample P, pure substance. The curve shows a horizontal flat plateau at 80°C, indicating all energy is being used to melt the substance at a fixed temperature. A sharp fixed melting point is characteristic of a pure substance [1]. Sample Q, mixture. The curve shows a gradual, continuous rise with no flat plateau. In a mixture, the melting occurs over a temperature range because different components melt at different temperatures, and the composition shifts as melting proceeds [1]. Sample R, pure substance. Like P, it shows a sharp plateau (at 63°C), indicating a pure substance with a distinct fixed melting point [1].
Q2.2, Melting point of P; element or compound? (2 marks)
The melting point of Sample P reads as 80°C from the plateau on the graph [1]. Knowing that P is a pure substance, the heating curve cannot distinguish between an element and a compound. Additional information needed: the formula or composition of the substance (whether it contains one or two or more element types). If the formula has only one element symbol, element; if it has two or more different element symbols, compound [1]. Accept also: carrying out a chemical decomposition test (e.g. electrolysis) to determine if it can be broken down into simpler substances.
Q2.3, Flaw in student reasoning (2 marks)
Flaw: the absence of a flat plateau indicates a mixture, but the type of mixture (heterogeneous vs homogeneous) cannot be determined from a heating curve alone. The student incorrectly uses a thermal property to determine the uniformity type [1]. More accurate conclusion: Sample Q is most likely a mixture (because it has no sharp melting point), but whether it is homogeneous or heterogeneous requires observing the sample directly, heating curve data alone cannot distinguish the two [1].
Q2.4, Why plateaus differ (1 mark)
Samples P and R are different pure substances (with different chemical identities), so they have different melting points. Each pure substance has a characteristic, fixed melting point that is unique to that substance, this is one of the properties that defines and identifies it [1].
Q3, Compare and contrast table
Atom types: Element, one type only. Compound, two or more types, bonded in fixed ratio. Mixture, two or more types, physically combined, not bonded. Fixed composition: Element, yes. Compound, yes (always the same ratio). Mixture, no (can vary). Physical separation: Element, no (no components to separate). Compound, no (would require breaking chemical bonds, i.e. chemical means). Mixture, yes (e.g. filtration, distillation, magnetic separation). Example formula: Element, Fe, Cu, O2, Cl2. Compound, NaCl, H2O, CO2. Mixture, ocean water, air, granite. Australian example: Element, copper ore (Cu) from Mount Isa mine. Compound, iron oxide (Fe2O3) ore at Pilbara, WA. Mixture, ocean water off Sydney coast (homogeneous); Broken Hill ore body (heterogeneous). Accept any valid Australian examples.
Q4.1, Broken Hill ore body (3 marks)
The raw ore body is a heterogeneous mixture [1]. Evidence 1: multiple different mineral components are present (PbS, ZnS and gangue), so composition is not fixed and varies from location to location in the ore body [1]. Evidence 2: different minerals are described as “mixed together” with distinct visible regions (different coloured minerals), indicating non-uniform composition with multiple phases, the defining feature of a heterogeneous mixture [1]. Accept also: the minerals can be physically separated in a concentrator (flotation process), consistent with a mixture.
Q4.2, Silver–lead alloy vs galena (2 marks)
The silver–lead alloy is a homogeneous mixture [1]. It differs from galena (PbS) because galena is a compound two different elements (Pb and S) are chemically bonded in a fixed 1:1 ratio; it has a definite composition that cannot vary and can only be separated by chemical means. The alloy is a physical blend of silver and lead with no chemical bonds between Ag and Pb atoms; its composition can vary (different Ag:Pb ratios are possible) and the metals can in principle be separated by physical processes such as fractional distillation [1].