In chemistry, a pure substance contains only one type of element or compound and melts sharply at a fixed temperature. Formulations are mixtures designed to have specific properties — medicines, fuels, paints, and alloys are all formulations. Both concepts feature in GCSE calculations and practical work.
What does "pure" mean in chemistry?
In everyday English, "pure" often means clean or unprocessed — "pure orange juice" might mean juice with no added sugar. In chemistry, "pure" has a precise definition: a pure substance is one that contains only one element or one compound — nothing else.
- Pure water (H₂O) contains only water molecules. It melts at exactly 0 °C and boils at exactly 100 °C at standard atmospheric pressure.
- Pure iron contains only iron atoms, melting at exactly 1,535 °C.
- Pure ethanol melts at −114 °C and boils at 78.4 °C.
Any substance that contains two or more elements or compounds mixed together is a mixture, regardless of how clean or natural it appears. Sea water is a mixture of water, sodium chloride, and many other dissolved salts. Bronze is a mixture (an alloy) of copper and tin.
This distinction matters because pure substances have sharp, fixed melting and boiling points, while mixtures melt over a range of temperatures — a property exploited to test purity.
How do melting points reveal purity?
A pure substance melts at a sharp, specific temperature. When heated, its temperature rises steadily, then stays constant during melting (energy goes into breaking the lattice rather than raising temperature), then rises again when all is liquid.
An impure substance (a mixture) behaves differently:
- It starts melting at a temperature below the pure substance's melting point (the impurity depresses the melting point — a phenomenon called freezing point depression).
- It melts over a range of temperatures rather than at a single sharp point — say, 78–82 °C rather than exactly 80 °C.
Practical application: To test whether a synthesised sample of aspirin is pure, measure its melting point. Pure aspirin melts at 135 °C. A sample containing impurities will start melting below 135 °C and will not be fully liquid until above 135 °C — the broader the range, the more impurities are present.
| Observation | Conclusion |
|---|---|
| Sharp melting point at the expected temperature | Substance is pure |
| Melting starts below expected temperature | Impurities are present |
| Melting occurs over a range (not a sharp point) | Substance is a mixture |
| Melting point matches a different substance's value | Could be a different compound |
What is a formulation?
A formulation is a mixture that has been deliberately designed to have useful properties. Formulations are not accidental mixtures — each component is chosen for a specific purpose and the proportions are optimised.
The key phrase for GCSE: a formulation is a mixture designed for a purpose. Simply mixing two things together at random does not create a formulation.
What are examples of formulations?
Formulations are widespread in industry, medicine, and everyday life:
| Formulation | Key components | Purpose of each component |
|---|---|---|
| Medicines (tablets) | Active drug, binder, filler (lactose), coating | Drug provides the therapeutic effect; binder holds the tablet together; filler gives correct dose volume; coating controls release rate |
| Paints | Pigment, solvent (water or organic), binder (polymer resin), additives | Pigment gives colour; solvent adjusts viscosity; binder sticks pigment to surface; additives prevent fungal growth, etc. |
| Fuels (petrol/diesel) | Mixture of hydrocarbons, additives | Different hydrocarbons give the correct energy density and boiling range; additives prevent engine knocking, improve combustion |
| Fertilisers | Nitrogen compounds, phosphate salts, potassium salts | Each nutrient promotes different aspects of plant growth (N = leaves, P = roots, K = flowers and fruits) |
| Cleaning products | Surfactants, water, bleach, enzymes, perfumes | Surfactants lower surface tension; bleach breaks down stains; enzymes digest proteins/fats; perfumes mask odours |
| Alloys | Two or more metals (or a metal + non-metal) | Different metals chosen for strength, corrosion resistance, conductivity, or cost |
Why are formulations used instead of pure substances?
Pure substances often lack the combination of properties needed for a practical application. Formulations solve this by combining the advantages of multiple components.
Example: Pure aspirin is the active drug, but a pure aspirin tablet would be powder that dissolves immediately in the mouth and taste extremely bitter. The formulation adds a binder (holds the tablet shape), a filler (gives the correct physical size), and a coating (controls where in the digestive tract the drug dissolves for maximum absorption, or simply makes swallowing easier).
Example: Pure water is an excellent solvent but a poor adhesive and has no pigment. Paint formulations combine water (or an organic solvent), a polymer binder that sticks to surfaces when dry, pigment particles, and additives — producing a liquid that is easy to apply, adheres strongly, and provides colour durability.
How does chromatography relate to purity?
Chromatography (covered separately at GCSE) can separate mixtures and confirm purity. A single spot on a chromatography plate that does not move from the origin and matches the Rf value of the expected compound suggests purity. Multiple spots confirm a mixture. Combined with melting point data, chromatography is a powerful tool for confirming that a synthesised compound is pure.
Frequently asked questions
Is tap water pure in the chemistry sense?
No. Tap water contains dissolved minerals (calcium, magnesium, and sodium salts), small amounts of chlorine added during treatment, traces of other compounds, and dissolved gases. It is a mixture, not a pure substance. This is why tap water does not have a single, sharp boiling point of exactly 100 °C — dissolved salts raise the boiling point slightly (boiling point elevation). Distilled water — produced by boiling water, condensing the steam, and collecting the liquid — is much closer to chemically pure, though it still absorbs CO₂ from the air.
What is the difference between a mixture and a formulation?
All formulations are mixtures, but not all mixtures are formulations. A formulation is a mixture that has been deliberately designed with specific components in specific proportions to achieve a desired set of properties. Random mixtures — soil, sea water, the air in a room — are mixtures but not formulations. The distinction is intent and design: a pharmaceutical company designing a medicine optimises every component ratio for efficacy, stability, bioavailability, and shelf life. That product is a formulation; simply mixing drug powder with chalk at random is just a mixture.
Can a formulation be separated into its components?
Yes — the components of a formulation are mixed together rather than chemically bonded, so physical separation methods (chromatography, distillation, filtration) can separate them. Chromatography is particularly useful for separating the pigments in paints or the active ingredients in medicines. Distillation can separate fuels into their component hydrocarbons. This is in contrast to a compound (like water), where the elements are chemically bonded and can only be separated by chemical reactions (such as electrolysis of water to give hydrogen and oxygen).
Why does adding an impurity always lower the melting point rather than raise it?
When a pure substance melts, its particles must overcome the attractive forces holding the lattice together. An impurity molecule occupies a lattice site where a pure molecule would normally sit, disrupting the regularity of the lattice and weakening the overall structure. This means less energy — and therefore a lower temperature — is needed to cause melting to begin. However, complete melting still requires enough energy to fully disrupt the lattice, so the upper end of the melting range stays near the pure substance's melting point. The result is an extended range that starts below and ends near (but rarely above) the expected sharp melting point.
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