Pure substances and mixtures

Many different particles may be found in a perfect cup of coffee. All the ingredients; coffee, milk, water and sugar, add their own particles into the mixture.

Milk is a mixture of many different particles; fat, protein, water, sugar and minerals.

Milk poured into a cup of black coffee.
A mixture contains different types of particles that are not joined together chemically.

Pure bottled water should contain only H2O molecules (the particles that make up the water). In reality, it is unlikely that any sample of water stays pure for very long as things will very quickly dissolve in the water. For example, gas particles in the air or particles from the walls of the water pipes are often found in 'pure' water.

A pure substance only contains one type of particle.

It is often easy to distinguish whether a material is pure by finding the temperature at which it changes state. The presence of an impurity usually changes the melting and boiling point.

Pure water will boil at 100°C and freeze at 0°C. If the water has something dissolved in it, eg salt, it will boil at a temperature higher than 100°C and freeze at a temperature lower than 0°C. Putting salt on a wet road means that the temperature has to get colder before the water freezes into ice.

Scientists can measure the melting or boiling point of a substance to determine if it is a pure substance or not.