When a chef adds salt to boiling water, the water boils at a slightly higher temperature than before. That single observation reveals something fundamental about how mixtures and pure substances behave differently — and why chemists measure properties so carefully.
Core Content
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.
| Property | What it measures | Classification significance |
|---|---|---|
| Melting point | Temperature of solid → liquid transition | Sharp and fixed for pure substances; variable range for mixtures |
| Boiling point | Temperature of liquid → gas transition | Same principle — pure substances boil at a single precise temperature |
| Electrical conductivity | Ability to conduct electric current | Metals and ionic solutions conduct; covalent molecular substances generally do not |
| Hardness | Resistance to scratching | Covalent network solids very hard; ionic solids moderate; metals malleable |
| Solubility | Amount dissolved per volume of solvent | Ionic compounds often soluble in water; most covalent compounds are not |
| Density | Mass per unit volume (g cm⁻³) | Characteristic value; can help identify unknown samples by comparison |
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 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.
Insert two heating curves side by side: (left) pure substance — flat plateau at exactly one temperature during melting, then another at boiling; (right) mixture — gradual sloping transition over a temperature range. Label axes: Temperature (°C) vs Time. Annotate plateaus and slopes.
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 |
In analytical chemistry, physical properties are measured and compared against reference data to identify unknown substances. The method is logical and systematic:
Worked Examples
| Sample A — sharp MP, insolubleMelts at a single precise temperature (327°C). Insoluble in water. A sharp, fixed MP confirms pure substance — could be element or compound. | Sharp MP = pure substance. 327°C matches lead (Pb) in reference data but identifying the substance isn't required here — only classifying it. Insolubility is a second consistent data point, not a classification criterion on its own. |
| Sample B — gradual transition over 20°CTransitions from soft to liquid over 60–80°C (a 20°C range). No single melting point. Broad range → mixture. | A temperature range is the key signal. Different components in the mixture each contribute to the gradual transition. Pure substances cannot show this behaviour — there is always one sharp point. |
| Sample C — sharp transition, sublimesSharp transition at −78°C confirms pure substance. Sublimation (solid → gas directly, no liquid) is an unusual but valid physical property unique to certain pure substances. | −78°C is the sublimation point of dry ice (CO₂). Sublimation is not a classification category — it's just an unusual property of a specific pure substance. The sharpness of the transition is what confirms purity. |
| Step 1 — What changed?Na and Cl₂ are elements. NaCl is a compound — it contains Na⁺ and Cl⁻ ions in a 1:1 ratio, held together by ionic bonds in a rigid 3D lattice. The chemical reaction created new bonds and a new structure. | A chemical reaction doesn't mix element properties — it creates an entirely new substance. This is the single most important point in this lesson. |
| Step 2 — Why do properties change?The ionic lattice in NaCl has strong electrostatic forces between oppositely charged ions. This is a fundamentally different structure from metallic sodium or diatomic Cl₂. The new structure determines the new properties. | New bonds → new structure → new properties. You cannot average or predict NaCl's properties from Na and Cl₂. This must be measured experimentally. |
| Step 3 — Specific property linksHigh MP (801°C): ionic lattice requires much more energy to break than metallic Na bonds (MP 98°C) or the weak forces between Cl₂ molecules (MP −101°C). Safe in water: no free Na metal (reactive) and no free Cl₂ (toxic) — only stable Na⁺ and Cl⁻ ions in solution. | This is the HSC exam pattern: structure → bond type → energy needed → observed property. Practise making this chain explicit. |
Activities
1 Substance X: melts sharply at 660°C, insoluble in water, excellent electrical conductor as a solid. Classify and justify.
2 Substance Y: begins to melt at 45°C, fully liquid at 62°C, does not conduct electricity in any state. Classify and justify.
3 Substance Z: melts sharply at 1455°C, silvery solid, excellent conductor, insoluble in water. Is it more likely an element or a compound? Give two pieces of evidence.
Question: "A substance melts over the range 40–65°C. Is it a pure substance or a mixture?"
Question: "Why is sodium chloride (NaCl) not explosive in water like sodium metal?"
Question: "A solid does not conduct electricity, but when dissolved in water it conducts well. Is it a metal or an ionic compound?"
Multiple Choice
Click an option to check. One attempt only.
1. Which property best distinguishes a pure substance from a mixture?
2. A substance begins to melt at 55°C and is completely liquid by 78°C. This is most consistent with:
3. Which best explains why NaCl has a much higher melting point than either Na (98°C) or Cl₂ (−101°C)?
4. A student wants to confirm whether an unknown white powder is a pure compound or a mixture. Which combination provides the strongest evidence?
5. Data for four substances:
| Substance | Melting behaviour | Conductivity (solid) |
|---|---|---|
| W | Melts sharply at 1064°C | Excellent |
| X | Melts over 120–160°C | None |
| Y | Melts sharply at 183°C | None |
| Z | Melts sharply at 327°C | Excellent |
Which substance is most likely a mixture?
Short Answer
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
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.
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.
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.
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).
Tick when you've finished all activities and checked your answers.