Pure substances and formulations
In everyday English "pure" often means "uncontaminated" — pure orange juice means no additives. In chemistry, "pure" has a stricter meaning: a single element or a single compound, with no other substances mixed in.
Pure substance — chemical definition
A pure substance:
- Has a fixed composition.
- Has a specific, sharp melting point and boiling point at standard pressure.
Example: pure water melts at exactly 0 °C and boils at exactly 100 °C.
Impure substances
Impure substances (mixtures) have a range of melting and boiling temperatures — and the m.p. is lower and the b.p. is higher than the pure version.
This is called melting/boiling point depression/elevation — adding salt to ice lowers its m.p., which is why we salt roads in winter.
Test of purity
Measure the melting (or boiling) point and compare with a data book value.
- If it matches exactly → likely pure.
- If it melts/boils over a range or away from the expected value → impure.
This is widely used in chemistry labs to confirm a synthesised compound.
Formulations
A formulation is a mixture designed for a specific purpose, where the composition is carefully controlled to deliver desired properties. Each component has a specific job.
Common examples
| Formulation | Components & job |
|---|---|
| Paint | Pigment (colour), solvent (carrier), binder (sticks to surface), additives |
| Medicine (tablet) | Active drug, binder, filler (bulk), coating |
| Cleaning product | Surfactant, water, fragrance, colourant |
| Fuel | Hydrocarbon mix, additives (stabilisers, anti-knock) |
| Alloy | Mix of metals for desired hardness/conductivity (C2.4) |
| Fertiliser (NPK) | Nitrogen, phosphorus, potassium compounds |
A formulation is not a single chemical compound — it's a deliberate mixture.
✦Worked example
A student finds a substance melts over the range 75–82 °C, while pure naphthalene melts sharply at 80 °C. What can be concluded?
The substance is likely impure naphthalene, since it has a melting range and the m.p. is lower than expected.
⚠Common mistakes
- "Pure milk" — chemistry definition: milk is a mixture, so it's not pure.
- Saying formulations are compounds — they're mixtures with controlled composition.
- Forgetting that impurities lower m.p. AND broaden the range.
- Confusing the everyday meaning of "pure" with chemical purity.
Links
Builds on C1.1 (mixtures) and C2.5 (states of matter). Sets up C8.2 (chromatography) and C10.8 (alloys as formulations).
AI-generated · claude-opus-4-7 · v3-deep-chemistry