Most substances around us are mixtures, not pure. Telling the difference, and separating one from the other, is a large part of practical chemistry.
In chemistry, pure means a single substance with nothing else in it, not the everyday sense of 'natural' or 'clean'. Pure orange juice is chemically a mixture.
A pure substance has a specific melting point and boiling point.
A mixture contains two or more substances that are not chemically bonded, so each keeps its chemical identity and they can be separated by physical means.
When a solid dissolves, the solute spreads through the solvent to make a solution. Nothing is destroyed, so the mass of the solution equals the mass of solute plus solvent, and the solute can be recovered by evaporating the solvent off.
Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration, because of their random motion.
It is readily seen in liquids and gases, where particles can move freely; in a solid it is extremely slow. It is faster in gases than in liquids, and faster when hot, because the particles move more quickly.
The technique depends on what is being separated:
A pure substance melts and boils at a specific temperature. Impurities change that: a mixture often melts and boils over a range of temperatures instead.
Chromatography gives another test: under suitable conditions a pure substance normally gives one spot, while a mixture may give more than one.
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