Properties of Elements, Compounds and Mixtures | HSC Chemistry Year 11 Module 1 | HSCScience
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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A student has two white solids. Solid A melts sharply at 801°C. Solid B begins to soften at 50°C and is fully liquid by 70°C. Which one is more likely to be a pure substance and which is more likely to be a mixture?
Key facts
- The key physical properties used to characterise substances (MP, BP, density, solubility, conductivity, hardness)
- How properties differ between elements, compounds and mixtures
- The difference between sharp and gradual melting/boiling behaviour
Concepts
- Why pure substances have sharp, fixed physical properties
- Why a compound's properties differ from its constituent elements
- How to use property data to identify an unknown substance
Skills
- Use property data to classify an unknown as pure substance or mixture
- Explain with particle reasoning why mixtures boil over a range
- Identify errors in classification reasoning
A physical property can be measured or observed without changing the chemical identity of the substance. Chemists use a standard set of physical properties to characterise, compare, and classify substances.
A physical property (MP, BP, density, solubility, conductivity, hardness) can be measured without changing the substance's chemical identity. Pure substances have fixed, characteristic values; mixtures show variable or ranged values. Identical MP, BP, density and solubility across two samples → almost certainly the same substance.
Pause — copy the highlighted definition into your book before moving on.
Fill the blanks: drag each token into the matching gap.
A ___ can be measured without changing the ___ of a substance. For a pure substance these values are ___. For example, ___ is measured in g cm⁻³ and is a useful identifier.
| Substance | MP | State at 25°C | Conductivity (solid) | Behaviour in water |
|---|---|---|---|---|
| Sodium (Na) | 98°C | Soft metal | Excellent | Violent reaction → NaOH + H₂ |
| Chlorine (Cl₂) | −101°C | Toxic gas | None | Dissolves/reacts slowly |
| Sodium chloride (NaCl) | 801°C | White crystal | None (solid) | Simply dissolves — safe |
| Hydrogen (H₂) | −259°C | Colourless gas | None | No reaction at room temp |
| Oxygen (O₂) | −218°C | Colourless gas | None | No reaction at room temp |
| Water (H₂O) | 0°C | Liquid | Very low (pure) | Is water |
A pure substance shows a flat plateau on a heating curve at a sharp, fixed MP and BP. A mixture changes state over a range of temperatures with no plateau. Compound properties differ completely from constituent elements — e.g. reactive Na + toxic Cl₂ → safe NaCl (MP 801°C); solid NaCl does not conduct but molten or dissolved NaCl does.
Add the highlighted point to your notes before the check below.
Quick check: A liquid is heated and its temperature stays exactly at 78°C until the entire sample has boiled away. What does this tell you?
We just saw that pure substances have sharp, fixed transitions while mixtures have gradual ranges. That raises a question: how do chemists apply this in the laboratory to identify unknowns? This card answers it → a four-step identification procedure using multiple independent properties.
In analytical chemistry, physical properties are measured and compared against reference data to identify unknown substances. The method is logical and systematic:
- Measure the MP, BP, density, conductivity, and solubility of the unknown sample.
- Check sharpness: Is the MP/BP sharp (pure substance) or gradual (mixture)?
- Compare measured values against reference tables of known substances.
- Confirm with at least two independent properties — no single property is conclusive alone.
To identify an unknown: (1) measure MP, BP, density, conductivity, solubility; (2) check if MP/BP is sharp (pure) or gradual (mixture); (3) compare against reference data; (4) confirm with ≥ 2 independent properties — no single property is conclusive alone.
Pause — write the highlighted rule into your book.
Two truths, one lie — about identifying unknowns by physical properties. Pick the lie.
We just saw the four-step identification procedure. That raises a question: how do you turn this knowledge into full-mark exam answers? This card answers it → structure each answer around the specific property evidence and explain the particle-level reason behind it.
6. Explain how melting point data distinguishes a pure substance from a mixture. Refer to the shape of a heating curve for each. 3 MARKS
7. Iron (Fe) is a grey reactive metal. Sulfur (S) is a yellow non-metal that burns in air. Iron sulfide (FeS) is a dark grey solid that does not react with dilute acids or oxygen under normal conditions. Using these observations, explain why a compound's properties cannot be predicted from its elements. 4 MARKS
8. A chemist has two clear liquids: Liquid A boils at exactly 100°C regardless of sample size. Liquid B boils between 100°C and 108°C depending on the sample. The chemist claims both are pure water. Evaluate this claim. 5 MARKS
For exam answers on heating curves: pure → flat plateau at one fixed T; mixture → sloped curve, no plateau. For "compound properties" questions: explain that new chemical bonds produce a new substance with entirely new properties. Always quote ≥ 2 independent properties before concluding pure or mixture.
Pause — copy the highlighted strategy into your book before moving on.
Fill the blanks: complete this short-answer scaffold about iron + sulfur vs iron sulfide.
When iron and sulfur react, ___ form between the atoms. The product, FeS, is a ___ with properties ___ those of the elements. This is why the properties of a compound ___ by simply looking at the elements it contains.
Worked examples · reveal as you go
A student records these properties: Sample A melts sharply at 327°C and is insoluble in water. Sample B begins to soften at 60°C and is fully liquid by 80°C. Sample C melts sharply at −78°C and sublimes (goes directly from solid to gas). Classify each sample.
Sodium (Na) explodes in water. Chlorine (Cl₂) is a toxic gas. Sodium chloride (NaCl) is safe to eat and simply dissolves in water. Explain, using bonding and structure, why NaCl has such different properties.
Common errors · the 3 traps that cost marks
Misconception to fix
Wrong: All mixtures can be separated by filtration because they contain insoluble solids.
Misconception to fix
Right: Filtration only separates insoluble solids from liquids. Homogeneous mixtures like salt water require crystallisation or distillation. The separation method depends on the physical properties of the components, not just whether it is a mixture.
Predicting a compound's properties from its elements
Students sometimes argue that NaCl must be "reactive like sodium" or "toxic like chlorine". A compound forms new chemical bonds, so its properties are completely new — they cannot be inferred from the constituent elements.
Fix: Always quote the compound's own measured properties (MP, state, behaviour) rather than reasoning from the elements.
Quick-fire practice · 5 reps +2 XP per reveal
Name three physical properties used to characterise a substance.
A sample melts sharply at 80 °C — what does this suggest, and what would a mixture's curve look like?
Why does solid NaCl not conduct electricity, while molten NaCl does?
Compare the properties of sodium, chlorine and sodium chloride to justify why a compound is a new substance.
An unknown white solid has MP 801 °C, density 2.16 g cm⁻³, dissolves in water and conducts when molten. What is it likely to be, and what additional check would confirm the identification?
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Pick your answer, then rate your confidence — that tells the system what to drill next.
Q1. 6. Explain how melting point data distinguishes a pure substance from a mixture. Refer to the shape of a heating curve for each.
Q2. 7. Iron (Fe) is a grey reactive metal. Sulfur (S) is a yellow non-metal that burns in air. Iron sulfide (FeS) is a dark grey solid that does not react with dilute acids or oxygen under normal conditions. Using these observations, explain why a compound's properties cannot be predicted from its elements.
Q3. 8. A chemist has two clear liquids: Liquid A boils at exactly 100°C regardless of sample size. Liquid B boils between 100°C and 108°C depending on the sample. The chemist claims both are pure water. Evaluate this claim.
📖 Comprehensive answers (click to reveal)
Activity 1 — Classification Drill
1. X → Pure substance (element — specifically aluminium, Al). Sharp, fixed MP at 660°C confirms pure substance. Excellent conductivity as a solid indicates metallic bonding → must be a metal element (most metal compounds don't conduct as solids).
2. Y → Mixture. Melting over a 17°C range (45–62°C) is the key signal — pure substances never melt over a range. Non-conductivity is consistent with many substance types, so it doesn't help narrow down further.
3. Z → Most likely an element (nickel, Ni). Evidence: (1) Sharp, fixed MP at 1455°C → pure substance, not a mixture. (2) Excellent conductivity as a solid → metallic bonding → strongly suggests an elemental metal. Most ionic or covalent compounds with metals do not conduct as solids.
Activity 2 — Error Spotting
Response 1 — Error: Student A attributed the broad melting range to measurement error. A 25°C span (40–65°C) is far too large to be explained by thermometer inaccuracy. Correct response: The substance is a mixture. A broad melting range is chemical evidence of variable composition — different components begin to melt at different temperatures, producing a gradual transition rather than a sharp plateau. Measurement error would produce a deviation of ±1–2°C at most, not 25°C.
Response 2 — Error: Student B claimed compounds retain reduced versions of element properties ("sodium still retains some reactivity"). Correct response: Compounds do not retain any properties of their constituent elements. When Na and Cl₂ react, entirely new ionic bonds form between Na⁺ and Cl⁻, creating a new lattice structure. NaCl is not reactive with water — it simply dissolves. No trace of sodium's explosivity or chlorine's toxicity survives in the compound.
Response 3 — Error: Student C concluded "metal" from the conductivity in solution data. Correct response: The substance is an ionic compound. Metals always conduct as solids (free electrons). A substance that doesn't conduct as a solid but does conduct in solution indicates an ionic compound — solid ions are immobilised in the lattice, but dissolving releases mobile Na⁺/Cl⁻ (or equivalent) ions that can carry charge.
❓ Multiple Choice
1. B — Sharp, fixed MP is the most reliable single indicator. Colour, conductivity, and solubility vary across both types.
2. C — A 23°C melting range confirms mixture. Pure substances always melt at a single, precise temperature.
3. A — NaCl's ionic lattice with strong electrostatic forces accounts for its high MP. B (averaging rule) doesn't apply to compounds. C is false (many compounds melt below their elements). D is wrong — NaCl is a compound, not a mixture.
4. D — Melting point (sharp or gradual) + a second independent property is the gold standard. Single tests or qualitative observations are insufficient.
5. B — Substance X melts over a 40°C range → mixture. W, Y, Z all have sharp, fixed MPs → pure substances.
Short Answer Model Answers
Q6 (3 marks): A pure substance has a sharp, fixed MP — on a heating curve, temperature holds constant at a flat plateau during the solid → liquid transition (1 mark). A mixture melts over a temperature range — the heating curve shows a gradual upward slope rather than a flat plateau during transition (1 mark). The width of the melting range reflects the degree of impurity — a wider range indicates more mixed composition; a narrower range indicates closer to pure (1 mark).
Q7 (4 marks): When Fe and S react chemically, new bonds form between Fe and S atoms, producing FeS with a completely different atomic arrangement (1 mark). Iron is grey, metallic, and reactive with oxygen; sulfur is yellow and burns readily — yet FeS is dark grey and resistant to reaction with oxygen (1 mark). This is because FeS has new bonds (ionic/covalent character) that are entirely absent from either element — the properties arise from the new structure (1 mark). This confirms that a compound is a new substance whose properties must be determined experimentally — they cannot be calculated or inferred from the properties of its elements (1 mark).
Q8 (5 marks): The claim is correct for Liquid A but incorrect for Liquid B (1 mark). Liquid A boils at exactly 100°C regardless of sample size — this is consistent with pure water, which has a fixed, characteristic BP of 100°C at standard pressure; the consistency across sample sizes confirms uniform composition (1 mark). Liquid B boils between 100–108°C and the BP varies by sample — this is a mixture, not pure water (1 mark). A fixed BP is a defining property of a pure substance; a BP that changes with sample or concentration indicates dissolved solutes (1 mark). Liquid B is most likely an aqueous salt solution — dissolved ions elevate the boiling point (boiling point elevation), and the exact BP depends on concentration, explaining the observed variation (1 mark).
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