Pure or impure?

Everyday mixtures

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.

An impurity is a small quantity of another substance in an otherwise pure substance. Products such as medicines must not contain any impurities that could make them unsafe.

Distinguishing between pure substances and mixtures

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:

  • pure substances have sharp melting points
  • mixtures melt over a range of temperatures

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.

curriculum-key-fact
An impure substance is a type of mixture, so melting points can be used to find out if a substance is pure or impure. Impure substances tend to have a slightly lower melting point than the pure substance, and a broader melting temperature range.

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.

Question

Which of the following substances (A, B, C, D) are impure?

SubstanceMelting point (°C)
A100
B97 - 99
C82
D102 - 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.