Purity of Substances
Science: KS3, topic 2.1.13
Melting and Boiling in Pure Substances
Pure substances melt or boil at sharp temperatures. Mixtures melt or boil over a range of temperatures. Graphs can be used to see this.
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Pure substances
- Pure substances melt or boil at sharp temperatures.
- E.g. Water boils at 100°C.
- Graphs can be used to see that melting and boiling happens at sharp points.
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Mixtures
- Mixtures melt or boil over a range of temperatures.
- For example, salt in water (salt solution) boils between 90 and 110°C.
- Graphs can be used to see that melting and boiling happen over a range of temperatures.
Chemically pure
- We describe things as being ‘pure’ in our everyday lives.
- For example, you might buy ‘pure orange juice’. Here, pure means that nothing else was added to the orange juice when it was made.
- But when we talk about pure substances in chemistry, we mean substances that contain only one element or compound.
- If a substance contains 'impurities', it does not contain only one element or compound.
Test yourself on Purity of Substances
More impurities in a sample will...
What does it mean if a solid melts over a range of temperatures, instead of at one exact point?
What can a graph of temperature during melting or boiling show about a sample?
A bottle of water is labelled 'pure spring water', but the water still contains dissolved minerals. Is this water chemically pure?