Many useful products at home are made from mixtures. In some products, these mixtures must be consistent - the mixture must always contain the same substances in proportions that do not change. A mixture that is made according to a set formula like this is called a formulation.
Scientists need to know that substances made in the laboratory are pure. They also need to be able to test whether a substance is pure or impure. One way to test this is to find the melting point of a substance:
The melting point range of a substance is the temperature range from which the first crystal starts to melt, to the temperature at which the last crystal finishes melting.
Pure substances can be identified by comparing the melting point found in the experiment with published reference data of what the melting point should be.
Which of the following substances (A, B, C, D) are impure?
|Substance||Melting point (°C)|
|B||97 - 99|
|D||102 - 104|
Substances B and D are impure. This is because they show a broad melting point range. Substances A and C show sharp melting points and so are pure.