Section 1 of 7Two meanings of 'pure'
Everyday meaning
- Everyday meaning In daily life 'pure' often means natural or clean,
- such as pure orange juice, even though that juice contains many different substances.
Chemistry meaning
- Chemistry meaning A pure substance is a single element or a single compound with nothing else mixed in.
- Pure water contains only H2O molecules.
Mixtures
- Mixtures A mixture contains two or more substances that are not chemically joined, so each keeps its own properties.
- Tap water is a mixture, because it contains H2O along with dissolved ions and chlorine.
Section 2 of 7Comparing the two
| Property | Pure substance | Mixture |
|---|---|---|
| Composition | One element or one compound only | Two or more substances, in any proportion |
| Melting point | Sharp, one fixed temperature | Melts over a range of temperatures |
| Boiling point | Sharp, one fixed temperature | Boils over a range of temperatures |
| Examples | Distilled water, oxygen, sodium chloride | Air, sea water, brass, tap water |
Section 3 of 7Using melting and boiling points to test purity
1
Heat a small sample slowly in a melting point apparatus or in a water bath, so the temperature rises steadily.
2
Record the temperature at which the sample first starts to melt and the temperature at which it has finished melting.
3
Compare the measured values with the data-book value for the pure substance.
4
If the sample melts sharply at the expected temperature it is pure, but if it melts over a range it is a mixture.
Section 4 of 7Why impurities change the behaviour
Purity and melting or boiling behaviour
Pure sample
- Pure sample All the particles are identical, so they all need the same energy to change state and the change happens at one temperature.
- Pure water melts at 0 °C and boils at 100 °C.
Impure sample
- Impure sample The mixture of particles changes state gradually, producing a melting range or boiling range.
- Generally an impurity lowers the melting point and raises the boiling point of the substance.
Closer to the data value
- Closer to the data value The closer the measured melting or boiling point is to the data-book value, the purer the sample is.
Section 5 of 7Worked example
Question
- Question Sulfur has a melting point of 115 °C.
- A student heats a sample that begins to melt at 100 °C and finishes melting at 113 °C.
- Is it pure?
Answer
- Answer The sample is impure, because it melts over a 13 °C range instead of at a single sharp temperature.
- Its melting range also lies below the data value of 115 °C, which is typical of an impure sample.
Real-world use
- Real-world use Pharmaceutical companies check melting points to confirm drug purity, and food producers test purity to meet safety standards.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Pure substance | A single element or compound containing no other substances. |
| Mixture | Two or more substances that are not chemically combined. |
| Melting point | The temperature at which a solid changes to a liquid. |
| Boiling point | The temperature at which a liquid changes to a gas throughout the liquid. |
| Impurity | An unwanted substance mixed in with a sample. |
Section 7 of 7Exam technique
1
Calling any natural or clean product 'pure'. In chemistry, pure means one substance only, so mineral water and fresh air are mixtures.
2
Saying a mixture has no fixed melting point. It does melt, but over a range of temperatures rather than at one sharp value.
3
Mixing up the effect of impuritiesthey usually lower the melting point and raise the boiling point.
4
Forgetting to give evidence in an answer. Quote the measured range and compare it with the data value.