All matter is either a pure substance or a mixture, and the difference decides how you separate it.

• A pure substance (element or compound) has fixed composition and fixed properties.
• A mixture contains substances physically combined in any ratio — no chemical bond forms.
• Homogeneous mixtures look uniform throughout (a solution); heterogeneous mixtures show separate parts.
• Physical separation methods: filtration, distillation, chromatography, magnetic separation.

• Element — a substance made of one kind of atom.
• Compound — atoms of different elements chemically bonded in a fixed ratio.