Question

1. How do you know if a substance is a pure substance and not a mixture?
2. How is a mixture different from a pure substance?

Answer 1

Short answers first:
- A pure substance has a fixed, definite composition and uniform properties — either a single element or a single chemical compound.
- A mixture contains two or more different substances combined physically (not chemically) and therefore has a variable composition and properties that are combinations of its components.

How you can tell a sample is a pure substance (practical tests and signs)
- Fixed composition / constant properties: samples of a pure substance always show the same composition and characteristic physical properties (density, refractive index, melting point, boiling point).
- Sharp melting or boiling point: a pure solid normally melts at a sharp, well-defined temperature; a mixture melts over a range. Likewise a pure liquid has a single boiling point at a given pressure.
- Chromatography: a pure substance gives a single spot/peak; a mixture gives multiple spots/peaks.
- Spectroscopy: a pure compound or element produces one consistent spectrum (IR, NMR, mass spec, atomic emission/absorption) without extra peaks from other substances.
- Chemical/elemental analysis: yields the exact expected elemental percentages/formula for a compound or a single element.
- Homogeneity: visually and microscopically a pure substance is uniform throughout (no separate phases or visible particles).

How a mixture differs from a pure substance (key differences)
- Composition: pure substance — fixed (one formula); mixture — variable proportions of components.
- Separation: pure compounds/elements require chemical reactions (or for elements, nuclear changes) to change into other substances; mixtures can generally be separated into their components by physical methods (filtration, distillation, chromatography, decanting, magnets, evaporation).
- Properties: pure substance has specific, reproducible physical and chemical properties; a mixture’s properties are a weighted combination of its components and can change with composition.
- Identity of components: in a mixture each component keeps its own chemical identity (e.g., salt in saltwater is still NaCl); in a pure compound the atoms are chemically bonded and exhibit new properties (e.g., NaCl as a compound behaves differently from Na and Cl separately).
- Examples: pure water (H2O) vs saltwater (mixture of H2O and dissolved NaCl); oxygen gas (O2, pure element) vs air (mixture of N2, O2, CO2, etc.).

If you want, tell me the substance you’re examining and I can recommend the best test (melting/boiling point, chromatography, spectroscopy, etc.).