Pure Substances Notes
Oxford AQA IGCSE Chemistry: Revision notes
Key facts
- In chemistry a pure substance is a single element or compound with nothing else mixed in.
- This is stricter than everyday uses of "pure", such as "pure orange juice".
- Pure substances melt and boil at one sharp, fixed temperature.
- Mixtures melt over a range of temperatures, usually starting below the pure melting point.
- Purity matters for foodstuffs and medicines, where contamination affects safety.
Pure in chemistry
A pure substance is a single element or compound made of one type of particle throughout.
In everyday language "pure" means natural or unprocessed. In chemistry it is much stricter: a pure substance has nothing else mixed in. Supermarket "pure milk" is still a mixture of many compounds.
Pure substance
Impure substance
A bottle is labelled "pure spring water". Is it a pure substance in chemistry?
Melting and boiling points
A pure substance has a sharp, fixed melting point; an impure one melts over a range and below the pure value.
Compare a sample's melting point with the value in data tables. A sharp melting point that matches suggests the sample is pure. A lower point, or a spread over a range, suggests it is impure.
| Pure substance | Impure sample | |
|---|---|---|
| Melting point | One sharp temperature | Spread over a range |
| Compared with data tables | Matches the known value | Lower than the known value |
| Boiling point | One fixed temperature | Spread over a range |
Pure substance
- Melting point:
- One sharp temperature
- Compared with data tables:
- Matches the known value
- Boiling point:
- One fixed temperature
Impure sample
- Melting point:
- Spread over a range
- Compared with data tables:
- Lower than the known value
- Boiling point:
- Spread over a range
A solid melts gradually between 78°C and 84°C. The pure substance melts sharply at 85°C. What does this show?
Collecting the data
Heat a small sample slowly and record when it starts and finishes melting.
A pure sample melts sharply at, or very close to, the known value. An impure sample starts to melt below the known value and melts gradually over a range.
- 1
Heat slowly
warm a small sample of the solid steadily
- 2
Record
the temperature when melting begins and when it finishes
- 3
Compare
with the known melting point from data tables
- 4
Conclude
sharp and matching means pure; lower or a range means impure
A sample melts sharply at the known melting point. What can you conclude?
Why purity matters
Contaminated foods and medicines can be unsafe or not work properly.
Purity testing checks that foodstuffs are not contaminated or adulterated with cheaper substances. In medicines, an impure drug may not work as intended or may contain harmful contaminants, so strict purity standards apply.
| Foodstuffs | Medicines | |
|---|---|---|
| Purity testing checks | Check for contamination | Safety |
| Also ensures | Check for cheaper substitutes | Works as intended |
Foodstuffs
- Purity testing checks:
- Check for contamination
- Also ensures:
- Check for cheaper substitutes
Medicines
- Purity testing checks:
- Safety
- Also ensures:
- Works as intended
Give one reason why purity testing is important for medicines.
Try an exam question
A scientist heats a solid and finds that it melts gradually between 150°C and 156°C. The data book gives 158°C. Explain what this shows.
[3 marks]
- [1]The sample is impure.
- [1]A pure substance melts sharply at one temperature.
- [1]The sample melts over a range and below the known value.
That's the notes covered.
Carry on to the next subtopic.