Section 1 of 9Two meanings
| Context | Meaning of pure | Example |
|---|---|---|
| Everyday | Not mixed with anything harmful or added | Pure orange juice, which still contains many substances |
| Scientific | Only one element or compound, with nothing else mixed in | Distilled water, H2O only |
Section 2 of 9Properties of a pure substance
Fixed melting point
- Fixed melting point A pure solid melts at one sharp, known temperature.
Fixed boiling point
- Fixed boiling point A pure liquid boils at one temperature at a given pressure.
Identity
- Identity Comparing a measured melting point with a data book value helps identify a substance.
Section 3 of 9Effect of impurities
1Impurity added
- Impurity added A different substance is mixed with the pure one.
2Melting range
- Melting range The mixture melts over a range of temperatures.
3Changed point
- Changed point The melting point is usually lowered and the boiling point raised.
Section 4 of 9Mixtures
Definition
- Definition A mixture has two or more substances that are not chemically joined.
Separation
- Separation Physical methods such as filtration, distillation and chromatography can separate mixtures.
Formulations
- Formulations Fuels, alloys and medicines are mixtures made in measured proportions for a use.
Section 5 of 9Judging purity from data
| Observation | Conclusion |
|---|---|
| Melts sharply at the data book value | Likely pure |
| Melts over a range below the data book value | Impure; contains another substance |
| Chromatogram with one spot | Probably a single substance, but not proof |
| Chromatogram with several spots | A mixture of at least that many substances |
Section 6 of 9Worked example: is the sample pure?
1
A student heats a solid and records that it begins to melt at 76 °C and is completely liquid at 80 °C.
2
The data book says the pure compound melts at 80 °C exactly.
3
The sample melted over a range rather than at one temperature, so it is not pure.
4
The melting began below the data book value, which is what impurities usually cause.
5
Conclusion: the sample is a mixture containing at least one other substance.
Section 7 of 9Why impurities matter
Purity in practice
Medicines
- Medicines Drugs must be pure or made to an exact formulation, because impurities could be harmful or change how the medicine works.
Food and drink
- Food and drink Manufacturers test purity so products are safe and match the label.
Chemical industry
- Chemical industry Pure starting materials give reliable reactions and predictable products.
Everyday use
- Everyday use Salt on icy roads lowers the freezing point of water, a simple example of impurities changing a physical property.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Pure substance | A single element or compound with no other substances present. |
| Mixture | Two or more substances physically combined, not chemically bonded. |
| Impurity | An unwanted substance mixed in with a pure one. |
| Formulation | A mixture designed with exact amounts of components for a particular purpose. |
| Melting range | The span of temperatures over which an impure solid melts instead of a single sharp melting point. |
| Data book value | A reliable reference value for a property of a pure substance, used for comparison. |
Section 9 of 9Exam technique
1
Calling milk or air pure because they are natural or clean; scientifically they are mixtures.
2
Saying impurities raise the melting point; they usually lower it and widen the range.
3
Believing a mixture has a sharp boiling point.
4
Assuming a single chromatogram spot proves purity; a different solvent might separate it into more spots.