pure substances and mixtures

A pure substance is a form of matter that has a definite composition and distinct properties. It may consist of only one type of atoms (examples: Fe(s), N2(g), S8(s)), or of only one compound where the atoms are connected in a defined way, (examples: FeS(s), CH4(g), NaNO3(s), H2O(l)). Many substances called “pure” in everyday life like “Pure Honey”, “Pure Silk” etc. are not pure in a scientific sense but they are mixtures. A homogeneous substance has the same composition and properties throughout the sample (down to microscopic but not to atomic dimensions).      Examples: aluminium, sea water, pure gas, mixture of gases. A phase is a homogeneous section of matter. It can be:       - solid: Cu(s),       - liquid: H2O(l),       - gas: O2(g),      - a homogeneous mixture, i.e. a solution, e.g. an aqueous solution of a salt, a mixture of gases (air !). A heterogeneous substance consists of two or more phases.       Example: concrete, milk, foam. Solutions are homogeneous mixtures (one phase only) of two or more components. We distinguish:  - Solid solutions, called “alloys”.       Examples: brass (mixture of copper and zinc),bronze (mixture of copper and tin),amalgam (metal dissolved in mercury). - Liquid solutions,            Examples: sea water, sweetened tea, wine. - Gaseous solutions      Examples: air (21% O2 + 78% N2 + 1% other gases). Mixtures of gases are always homogeneous. In liquid solution one may distinguish the solvent, the main part of the solution, and the solute, the substance dissolved in the solvent. The solute may be gaseous, liquid or solid. Examples: CO2(g) (solute) dissolved in H2O(l) (solvent),   alcohol(l) (solute) dissolved in toluene(l) (solvent),   LiCl(s) (solute) dissolved in alcohol(l) (solvent). There is no easily detectable, obvious difference between a solution and a pure substance. Only the identification of the components (e.g. by separation techniques) proves a sample to be a homogeneous mixture and not a pure substance.