Section 1 of 8Two meanings of pure
| Everyday meaning | Chemistry meaning |
|---|---|
| Nothing artificial has been added, for example 'pure orange juice' | A single element or compound with nothing else mixed in |
| May still be a mixture of many substances | Cannot be separated by physical methods |
| Describes the origin or quality of a product | Has a fixed, sharp melting point and boiling point |
Section 2 of 8Pure substances versus mixtures
Pure substance
- Pure substance Contains only one type of element or compound.
- Examples are distilled water, oxygen and sodium chloride crystals.
Mixture
- Mixture Two or more substances that are not chemically joined, so each keeps its own properties.
- Air, seawater and alloys are mixtures.
Separating a mixture
- Separating a mixture The parts of a mixture can be separated by physical methods such as filtration, evaporation, crystallisation, distillation and chromatography.
- A compound can only be split into its elements by a chemical reaction.
Section 3 of 8Melting and boiling points as a test
Testing purity
Pure solid
- Pure solid Melts at one exact temperature; pure ice melts at 0 °C.
Pure liquid
- Pure liquid Boils at one exact temperature; pure water boils at 100 °C at normal pressure.
Impure sample
- Impure sample Melts or boils over a range of temperatures, and the melting point is lower than expected while the boiling point is higher.
Data tables
- Data tables Compare the measured values with data-book values to identify a substance.
Section 4 of 8Interpreting a heating curve
1
Heat the sample steadily and record its temperature at regular intervals.
2
A pure substance gives a flat line at the melting point while it changes state.
3
A mixture gives a slope through the change of state instead of a flat section.
4
Compare the temperature of the flat section with the known value for the pure compound.
Section 5 of 8Worked example: is the sample pure?
| Sample | Measured melting point | Data-book value | Conclusion |
|---|---|---|---|
| A | 80 °C exactly | 80 °C | Likely to be pure, because it melts sharply at the expected value |
| B | 74 to 79 °C | 80 °C | Impure, because it melts over a range and below the expected value |
| C | 0 °C exactly | 0 °C | Consistent with pure water ice |
| D | Boils between 101 and 106 °C | 100 °C | Impure, probably water with a dissolved solid |
Section 6 of 8Why purity matters
Medicines and food
- Medicines and food Impurities may be harmful, so drugs and additives must be checked.
- Chemists test purity with melting points and chromatography before a product is sold.
Manufacturing
- Manufacturing Chemicals of known purity give predictable reactions and products.
- Impurities can waste reactants or spoil the quality of the final product.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Pure substance | A single element or compound not mixed with any other substance. |
| Mixture | Two or more substances physically combined but not chemically bonded. |
| Melting point | The temperature at which a solid turns to a liquid. |
| Boiling point | The temperature at which a liquid boils into a gas. |
| Impurity | An unwanted substance mixed with a pure substance. |
Section 8 of 8Exam technique
1
Treating 'pure' on a food label as meaning chemically pure.
2
Saying that impurities raise the melting point; they usually lower it and widen the range.
3
Believing a pure substance has no boiling point range at all, when it is a sharp, fixed value.
4
Writing that air is a compound; it is a mixture of gases.