1Pure means one substance
To a scientist, a pure substance contains only one substance: one element or one compound. Tap water, sea water and milk are all mixtures.
pure substancemixture
Lesson 8.1 notes
Pure substances and mixtures
What 'pure' means to a scientist, and how melting and boiling points show whether something is pure.
2The melting point test
A pure substance melts at one fixed temperature: a flat line on the graph. A mixture melts over a range of temperatures.
melting point
3Freezing point
As a pure liquid cools, the temperature stays the same while it freezes. Its freezing point is the same as its melting point.
freezing point
4Impurities lower the melting point
Salt on icy roads works because salty water freezes below 0 °C. Impurities lower the melting point.
impurity
5Dissolved salt raises the boiling point
Pure water boils at a steady 100 °C. Salt water boils above 100 °C, and its temperature keeps rising as it boils.
boiling point
6Identifying a pure liquid
Every pure substance has its own boiling point. Measure it and compare it with a table to help work out what it is.
identify
7Alloys are mixtures
24-carat gold is pure gold. 9-carat gold is an alloy: gold mixed with silver and copper, which makes it harder.
alloy
Explore: Purity tester
Add some impurity to the solid, then heat it and watch where it melts.
Common mistake: 'Pure' on a label is not the scientific meaning
Labels use 'pure' to mean nothing has been added. To a scientist, pure means only one substance. Pure orange juice and pure mineral water are both mixtures.
Remember
- A pure substance is one element or one compound, with nothing mixed in.
- Pure substances melt and boil at fixed, sharp temperatures.
- Impurities lower the melting point and spread it over a range. Dissolved solids raise the boiling point.
Going further at GCSE
GCSE: Pure substances and mixtures → GCSE: Mixtures and separation →