Pure SubstancesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What does 'pure' mean in chemistry?
In everyday language, 'pure' often means natural or unprocessed (e.g. 'pure orange juice'). In chemistry, the meaning is much stricter: a pure substance is a single element or compound that contains no other substance mixed in with it. A pure substance is made of only one type of particle throughout.
Do not confuse the chemistry meaning of 'pure' with the everyday/marketing meaning. 'Pure milk' in a supermarket is still a mixture of many different compounds - it is not a pure substance chemically.
Section 2
How can melting point and boiling point data show purity?
Pure elements and pure compounds each melt and boil at one fixed, sharp temperature. Mixtures, in contrast, melt and boil over a range of temperatures, and this range typically has a lower melting point and less sharp boiling point than the pure substance would have.
This means melting point and boiling point data can be used to assess how pure a sample is:
- A sharp melting point matching a known reference value suggests the sample is pure.
- A melting point that is lower than expected, or that occurs over a range rather than at one temperature, suggests the sample is impure.
Section 3
How is this data collected and interpreted?
- A small sample of the solid is heated slowly and steadily.
- The temperature is recorded at the moment melting begins and the moment it is complete.
- This is compared against the known melting point of the pure substance from data tables.
A pure sample melts sharply at (or very close to) the known value. An impure sample starts melting below the known value and melts gradually over a range of temperatures, rather than sharply at one point.
When answering exam questions on this, state both effects of an impurity: it lowers the melting point AND widens the melting range - one mark for each idea.
Section 4
Why does purity matter in everyday substances?
Measuring purity is important beyond the chemistry lab. In foodstuffs, purity testing checks that products have not been contaminated or adulterated with cheaper substitute substances. In drugs and medicines, purity is critical for safety and effectiveness - an impure drug may not work as intended, or may contain harmful contaminants, so strict purity standards are enforced during manufacture.
A pharmaceutical company testing a new batch of medicine could use melting point analysis to check the drug has not been contaminated during production.
Must Know
- A pure substance (chemistry definition) is a single element or compound with nothing else mixed in.
- This is different from the everyday/marketing use of 'pure'.
- Pure substances melt and boil at one sharp, fixed temperature.
- Mixtures melt over a range of temperatures, usually starting below the pure substance's melting point.
- Melting point data can therefore be used to test how pure a sample is.
- Purity testing matters for foodstuffs and drugs, where contamination affects safety and quality.
That's the notes covered.
Carry on to the next subtopic.