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| Solution Reactions |
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Solution reactions can happen as a result of many different solutions being used. They produce new and different products that are very useful in everyday life. The Iodine clock reaction is a classical chemical clock demonstration to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two clear solutions are mixed and at first there is no visible reaction, but after a short time delay, the liquid suddenly turns to a shade of dark blue. The iodine clock reaction exists in several variations. This is a basic solution reaction that takes place.
Precipitation is known as the formation or creation of a solid in a solution during a chemical reaction. When the solution reaction occurs, the solid created is called the precipitate, and the liquid remaining above the solid is called the supernate. Powders derived from precipitation have also historically been known as flowers.
Natural methods of precipitation include settling or sedimentation, where a solid forms over a period of time due to ambient forces like gravity or centrifugation. During chemical reactions, precipitation may also occur especially if an insoluble substance is introduced into a solution and the density happens to be greater (otherwise the precipitate would float). With soluble substances, precipitation is dramatically sped up once the solution becomes supersaturated. One of the more important stages of this solution reaction in the precipitation process is the onset of nucleation. The creation of a hypothetical solid particle includes the creation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If no energy is readily available, and no suitable nucleation surface is available, supersaturation occurs.
Precipitation or solution reactions can be used for making pigments, removing salts from water in water treatment, and in classical qualitative inorganic analysis. Precipitation is also purposeful to isolate the products of a reaction during workup. Ideally, the product of the reaction should be insoluble in the reaction solvent. Thus, it precipitates as it is created, preferably forming pure crystals. An example of this would be the synthesis of porphyrins in refluxing propionic acid. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration.
Precipitation can also occur when an antisolvent which is solvent in which the product is insoluble, is added, dramatically reducing the solubility of the desired product. Thereafter, the precipitate may easily be separated by filtration, decanting, or centrifugation. An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, and the product precipitates. Precipitation is also useful in purifying products: crude bmim-Cl is taken up in acetonitrile, and dropped into ethyl acetate, where it precipitates. In metallurgy, precipitation from a solid solution is also a useful way to strengthen alloys; this process is known as solid solution strengthening. Precipitate formation is useful in the detection of the type of cation in salt. This done because an alkali first reacts with the unknown salt to produce a precipitate which is the hydroxide of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used to separate chemically similar elements, such as the Alkali earth metals.
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