This kit enables to understand what phase means, and the difference between homogeneous and heterogeneous systems.
From a physical point of view, a system consisting of a unique phase is defined as homogeneous system; if it consists of two or more phases, it is defined as heterogeneous system. Phases may be equal, from a chemical point of view, and different, from the physical point of view, in the meantime, like in the case of water-ice mixture; in the contrary, a system can be homogeneous, from a physical point of view, and heterogeneous, from a chemical point of view (e.g.: solutions). The portions of matter having a constant chemical composition are called pure substance; chemical elements and compounds are also defined as pure substances. Mixtures are the result of mixing different pure substances; the constituents of mixtures can be characterized by the same state of aggregation or by different states of aggregation. So mixtures can be heterogeneous and homogeneous. Heterogeneous mixtures consist of constituents that can be distinguished into two or more phases, in highly variable mass ratios; these constituents retain their original properties. The heterogeneous mixtures consisting of solids and liquids are called suspensions (e.g.: milk, water-mud); those consisting of immiscible liquids are called emulsions (e.g.: water-oil, watergasoline). The constituents of homogeneous mixtures, commonly known as solutions, cannot be distinguished any more, because they are mixed even on atomic scale, although they retain a lot of their original properties. Separating the constituents is simpler in the case of heterogeneous mixtures: simple methods such as settling, filtration or centrifugation, are sufficient; these methods are based on the different size, physical state and density of constituents. But homogeneous mixtures can be separated by more demanding methods such as solvent evaporation, distillation, (both methods based on the different volatility of the constituents), solvent extraction, based on the higher affinity of solvent with a constituent of the mixture, or chromatography, based on the different rate at which a solvent carries the different constituents of the mixture through a layer of inert material, by capillary action.
THEORETICAL – EXPERIMENTAL HANDBOOK