Pure substances contain only atoms or molecules of that substance. For example, oxygen and sulfur.
It is important to be able to test the purity of a substance. This is done by measuring the physical properties of the substance: z z
zz zz
Melting point Boiling point Density.
For example, pure water will freeze at 0 °C and boil at 100 °C. If there are any impurities in the water, these values will change.
Impurities can be added to water in order to influence its physical properties. For example, adding salt to water lowers its freezing point, causing ice to melt.
In winter, grit is spread on roads. Grit is made up of crushed rock and salt. The salt is used to stop ice forming. It does this by reducing the freezing point of any water on the road surface. This means that temperatures would have to drop lower than 0 °C before the water on the road would form ice.
How do impurities affect the physical properties of substances?
1. Plan, design, carry out and write up an investigation to find out the effect that different salts have on the rate at which an ice cube melts.
2. Grit is also called ‘rock salt’.
What is the purpose of the crushed rock in grit? Discuss in groups and use diagrams to explain your answer.
z
Fig. 11.1.5 A gritter truck
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Measuring the effect of impurities
It takes 58 g of salt to raise the boiling point of a litre of water by just one half of a degree Celsius. How much salt would have to be added to raise the boiling point of water to 103 °C?
