What Is the Difference Between a Mixture and a Compound?
Learn the difference between a mixture and a compound for AQA GCSE Chemistry, with a salt-water example and the key separation mistake to avoid in exams.
This article covers 4.1.1.1 and 4.1.1.2 of the AQA GCSE Chemistry 8462 specification: a mixture contains substances that are not chemically combined, while a compound contains two or more elements chemically combined in fixed proportions. This explanation applies to both Foundation and Higher Tier. It assumes you are studying separate GCSE Chemistry.
How are they different?
In a mixture, each substance keeps its chemical properties. The amounts of the substances can vary, and they can be separated by physical processes such as filtration, distillation or chromatography. Separating a mixture does not make a new substance. For example, a mixture might contain a small amount of one substance or a large amount of it without changing what those substances are.
In a compound, elements have reacted to form a new substance. The elements are present in a fixed proportion, shown by the compound’s chemical formula. To separate a compound into its elements, you need a chemical reaction, not just a physical separation. Changing the proportion of its elements would mean you no longer had the same compound.
A quick example
Salt water is a mixture of salt and water. You can add different amounts of salt, and you can separate the water from the salt by distillation. Salt itself is a compound: sodium and chlorine are chemically combined in a fixed proportion. Dissolving salt in water does not turn the salt and water into a new compound.
Distillation works because the water evaporates and is then condensed and collected, leaving the salt behind. This separates the substances in the mixture; it does not split the salt into sodium and chlorine. Notice that the answer depends on which substances you are comparing: salt water is a mixture, but the salt within it is a compound.
Common exam mistake: saying a compound is simply two substances mixed together. Look for the key distinction: chemically combined means a compound; not chemically combined means a mixture.
