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Mixtures can be separated by physical means, while compounds can only change composition via a chemical reaction. Since the salt was separated by evaporation of the water without evidence of a chemical reaction we know that the salt water was a mixture. Evaporation, the process that removed the water from the salt, is a physical change.
Some other physical processes for separating mixtures are:
Distillation - separating liquids with different boiling points by boiling off each substance step-wise.
Magnetism - iron-containing particles can be separated out of a mixture using a magnet.
Screening - a screen can be used to separate substances that have particles of different sizes.
An example of a compound (which is a pure substance) being separated by chemical means is the application of electric current to water to produce hydrogen and oxygen gases.
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