Question

what is the difference beetween a pure subtances and non-pure substances ?

Answer 1

Short answer
- A pure substance contains only one kind of matter with a fixed, definite composition (an element or a compound).
- A non-pure substance is a mixture of two or more different substances whose composition can vary.

Key differences
- Composition: Pure substances have a fixed composition (e.g., H2O always 2 H : 1 O by atoms). Mixtures can have variable proportions (e.g., saltwater can be 1% salt or 10% salt).
- Identity of components: In a pure substance the component(s) are chemically bonded (elements or compounds). In a mixture the components retain their individual chemical identities.
- Separation: Pure substances cannot be separated into other substances by physical methods; mixtures can be separated by physical methods (filtration, distillation, chromatography, etc.).
- Physical properties: Pure substances show sharp melting and boiling points. Mixtures usually melt/boil over a range and their properties depend on composition.
- Chemical formula: Pure substances can be represented by a chemical formula (NaCl, O2); mixtures cannot have a single chemical formula.

Examples
- Pure substances: distilled water (H2O), pure oxygen (O2), sodium chloride (NaCl).
- Mixtures (non-pure): air (N2, O2, CO2, etc.), seawater, brass (Cu + Zn alloy), milk, soil.

Note
- “Pure” is often relative—commercial samples can contain small impurities. In practice chemists use analytical tests to determine purity.