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hscscience Chem · Y11
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Module 1 · L2 of 21 30 min ⚡ +50 XP in Learn · +25 to complete

Properties of Elements, Compounds and Mixtures

Today's hook, Mix iron filings and sulfur and you can still pull the iron out with a magnet. Heat that same mixture and the magnet stops working. Something has changed.
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Warm up and recall

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

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
01
Recall, your gut answer first
+5 XP warm-up

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?

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What you'll master, and the words for it

03
What you'll master
Know

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
Understand

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
Can do

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
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Physical Properties Used to Characterise Substances

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Physical Properties Used to Characterise Substances
core concept

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.

What it measures
Temperature of solid → liquid transition
Temperature of liquid → gas transition
Ability to conduct electric current
Resistance to scratching
Amount dissolved per volume of solvent
Mass per unit volume (g cm⁻³)
Classification significance
Sharp and fixed for pure substances; variable range for mixtures
Same principle, pure substances boil at a single precise temperature
Metals and ionic solutions conduct; covalent molecular substances generally do not
Covalent network solids very hard; ionic solids moderate; metals malleable
Ionic compounds often soluble in water; most covalent compounds are not
Characteristic value; can help identify unknown samples by comparison
Key principle: Pure substances have fixed, characteristic physical properties. If two samples share identical melting point, boiling point, density, and solubility, they are almost certainly the same substance.

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.

physical property fixed and characteristic density chemical identity

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.

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Sharp vs Gradual: How Properties Differ

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Sharp vs Gradual: How Properties Differ
core concept
A pure substance melts at a flat plateau (fixed melting point); a mixture melts over a sloping range.
Pure Substances: Sharp, Fixed Transitions
Elements and compounds both have sharp, fixed melting and boiling points that do not vary from sample to sample, regardless of sample size. Pure water always boils at exactly 100°C at sea level. Pure iron always melts at 1538°C. This consistency reflects uniform particle composition.
Mixtures: Variable Transitions Over a Range
Mixtures do not have sharp melting or boiling points. Instead, they change state over a range of temperatures. Salt water boils above 100°C, and the exact temperature depends on concentration. Impure solids begin to soften over a range rather than at a precise point. This happens because different components in the mixture interact with each other, and different amounts of energy are needed to separate them depending on the local composition.
Read this as evidence, not an absolute definition. Sharp-versus-range behaviour is reliable diagnostic evidence measured under controlled conditions (above all, constant pressure). Real exceptions exist: some substances decompose before they melt (so they show no clean melting point), different polymorphs of one compound can melt at different temperatures, boiling points shift with pressure (water boils below 100°C up a mountain), and azeotropes are mixtures that boil at a single constant temperature, like a pure substance. Always combine the thermal behaviour with at least one other property before concluding.
A pure substance has an ordered lattice and a fixed melting point; a mixture is disordered and melts over a range.
Exam application: Sharp, fixed MP or BP → pure substance. Melting or boiling over a range → mixture. This is the single most reliable physical indicator of purity, and it appears in HSC data questions regularly.
A pure substance melts and boils at flat plateaus; a mixture changes state over a sloping range.
Compounds vs. Their Component Elements
A compound's properties are completely different from the elements it is made from. New chemical bonds form during the reaction, creating a new substance with a new internal structure and therefore new properties.
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
NaCl conductivity note: Solid NaCl doesn't conduct because ions are locked in a rigid lattice. When melted or dissolved in water, ions become mobile and can carry charge. You will revisit this in L07 (Ionic Compounds). Don't confuse "doesn't conduct as a solid" with "never conducts".

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?

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Using Properties to Identify Unknown Substances

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Using Properties to Identify Unknown Substances
core concept

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:

  1. Measure the MP, BP, density, conductivity, and solubility of the unknown sample.
  2. Check sharpness: Is the MP/BP sharp (pure substance) or gradual (mixture)?
  3. Compare measured values against reference tables of known substances.
  4. Confirm with at least two independent properties, no single property is conclusive alone.
Building on L01: Classification tells you what type of substance you have. Property measurement is the practical tool chemists use to apply that classification in the laboratory. These ideas work together in every experimental context.

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.

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Short Answer Questions

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Short Answer Questions
core concept

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

✏️ Answer in your book

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 is not attracted to a magnet and reacts with dilute acid to release hydrogen sulfide (H₂S), a toxic gas, unlike iron, which is magnetic and reacts with dilute acid to release hydrogen. Using these observations, explain why a compound's properties cannot be predicted from its elements. 4 MARKS

✏️ Answer in your book

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

✏️ Answer in your book

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.

chemical bonds new substance different from cannot be predicted

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.

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Worked examples

Worked examples · reveal as you go

Worked example +5 XP on full reveal

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.

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Sample A, sharp MP at 327°C, insoluble in water
A sharp, fixed melting point at one precise temperature is the defining property of a pure substance. This immediately rules out mixture. The specific value (327°C) and insolubility are consistent data that support pure substance classification.
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Sample B, begins to soften at 60°C, fully liquid by 80°C
Melting occurs over a 20°C range (60–80°C), not at a single sharp point. Variable melting behaviour is chemical evidence of a mixture, different components have different melting temperatures, so the phase change occurs gradually rather than at one moment.
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Sample C, sharp fixed transition at −78°C, sublimes
A sharp, fixed transition temperature (−78°C) confirms a pure substance. Sublimation (solid → gas directly) is not contradictory to purity, it just means this substance skips the liquid phase, but the transition still occurs at one fixed temperature.
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Classification: A = pure substance; B = mixture; C = pure substance
Remember: sharp, fixed MP or BP = pure substance; gradual melting/boiling over a range = mixture. This is the single most reliable indicator of purity in HSC exams and applies regardless of whether the substance is an element or compound.
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Worked example +5 XP on full reveal

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.

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Identify what changed: Na and Cl₂ are elements; NaCl is a compound
During the chemical reaction 2Na + Cl₂ → 2NaCl, electrons are transferred from Na atoms to Cl atoms, forming Na⁺ and Cl⁻ ions. The elements no longer exist as discrete atoms, new ionic bonds hold the ions in a rigid 3D lattice.
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Compare element properties: Na is reactive soft metal; Cl₂ is toxic gas; NaCl is white crystalline solid
The properties are completely different because a new substance has formed. The ionic lattice structure of NaCl is fundamentally different from the metallic structure of Na or the molecular structure of Cl₂. Different structure = different properties.
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Explain the behavioural change: Na reacts violently; NaCl simply dissolves, safe to eat
Na's reactivity comes from loose outer electrons ready to donate. In NaCl, those electrons are locked in the ionic lattice as part of Cl⁻ ions. The Na⁺ ion has no reactivity, it is inert. The Cl⁻ ion is not the toxic Cl₂ molecule but a chloride anion in an ionic compound. No trace of the original element properties survives in the compound.
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Final answer: NaCl has completely different properties because new ionic bonds created a new substance
This is the key principle: A compound is a new substance whose properties must be measured or looked up, they cannot be predicted or inferred from the constituent elements. The bonding and structure are entirely new, therefore the properties are entirely new.
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Misconception to fix

Common errors · the 3 traps that cost marks

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Misconception to fix

Wrong: All mixtures can be separated by filtration because they contain insoluble solids.

2

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.

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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.

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Drill, then revisit

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

1

Name three physical properties used to characterise a substance.

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A sample melts sharply at 80 °C, what does this suggest, and what would a mixture's curve look like?

3

Why does solid NaCl not conduct electricity, while molten NaCl does?

4

Compare the properties of sodium, chlorine and sodium chloride to justify why a compound is a new substance.

5

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?

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Revisit your thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

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Interactive Tool, Elements, Compounds & Mixtures Open fullscreen ↗
The Classifying Matter tool shows that a PURE SUBSTANCE differs from a mixture because…
✓

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
ApplyApply3 MARKS

Q1. 6. Explain how melting point data distinguishes a pure substance from a mixture. Refer to the shape of a heating curve for each.

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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 is not attracted to a magnet and reacts with dilute acid to release hydrogen sulfide (H₂S), a toxic gas, unlike iron, which is magnetic and reacts with dilute acid to release hydrogen. Using these observations, explain why a compound's properties cannot be predicted from its elements.

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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.

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📖 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 generally conduct as solids (delocalised 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

Sharp, fixed MP is the most reliable single indicator. Colour, conductivity, and solubility vary across both types.

A 23°C melting range confirms mixture. Pure substances always melt at a single, precise temperature.

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.

Melting point (sharp or gradual) + a second independent property is the gold standard. Single tests or qualitative observations are insufficient.

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 the atoms, producing FeS with a completely different structure (1 mark). Iron is magnetic and reacts with dilute acid to release hydrogen; sulfur is a brittle yellow non-metal that is not magnetic, yet FeS is non-magnetic and reacts with dilute acid to release hydrogen sulfide (H₂S), a toxic gas (1 mark). These properties belong to neither starting element, they arise from the new bonding and structure of the compound (1 mark). This confirms that a compound is a new substance whose properties must be determined experimentally, not inferred from its elements (1 mark). Safety: adding dilute acid to a metal sulfide releases toxic hydrogen sulfide, so this is a teacher demonstration with ventilation only, never an open student practical.

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).