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.
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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?
Before reading on, write your best answer. What does the difference between "sharp" and "gradual" melting tell you about what is happening at the particle level?
Core Content
Wrong: All mixtures can be separated by filtration because they contain insoluble solids.
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.
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.
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.
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
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?"
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Multiple Choice
5 random questions from a replayable lesson bank — feedback shown immediately
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).
Return to your Think First response. You should now be able to explain precisely why a sharp, fixed melting point indicates a pure substance, while a gradual melting range indicates a mixture:
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