Answer:
The composition of a pure substance is always the same, regardless of the source, but the composition of a mixture can vary.
Mixtures can be separated into their components by physical changes; some pure substances can be separated into components by chemical change.
Step-by-step explanation:
A chemically pure substance has a well defined and constant composition. The composition of a chemically pure substance remains the same irrespective of its source. Also, the components of a chemically pure substance may be separated by chemical changes.
Mixtures have a variable composition depending on their respective sources. The composition of a mixture varies with the source of the mixture and mixtures are separated by physical processes.