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:''This article talks about the melting and freezing point depression due to mixtures of compounds. For depression due to small particle size, see melting point depression.'' Freezing-point depression describes the process in which adding a solute to a solvent decreases the freezing point of the solvent. Examples include salt in water, alcohol in water, or the mixing of two solids such as impurities in a finely powdered drug. In the last case, the added compound is the solute, and the original solid is thought of as the solvent. The resulting solution or solid-solid mixture has a lower freezing point than the pure solvent or solid. This phenomenon is what causes sea water, (a mixture of salt (and other things) in water) to remain liquid at temperatures below , the freezing point of pure water. ==Uses== The phenomenon of freezing point depression has many practical uses. The radiator fluid in an automobile is a mixture of water and (glycol ) (antifreeze). As a result of freezing point depression, radiators do not freeze in winter (unless it is extremely cold, e.g. . Road salting takes advantage of this effect to lower the freezing point of the ice it is placed on. Lowering the freezing point allows the street ice to melt at lower temperatures, preventing the accumulation of dangerous, slippery ice. The maximum depression of the freezing point of the commonly used sodium chloride in water is about , so if the ambient temperature is lower, it becomes ineffective and other salts are used, such as calcium chloride, magnesium chloride or a mixture of many. These salts are a little aggressive to metals, especially iron, so in airports they need a safer media such as sodium formate, potassium formate, sodium acetate, potassium acetate and others. Freezing-point depression is used by some organisms that live in extreme cold. Such creatures have evolved means through which they can produce high concentration of various compounds such as sorbitol and glycerol. This elevated concentration of solute decreases the freezing point of the water inside them, preventing the organism from freezing solid even as the water around them freezes, or as the air around them becomes very cold. Examples of organisms that produce antifreeze compounds include some species of arctic-living fish such as the rainbow smelt, which produces glycerol and other molecules to survive in frozen-over estuaries during the winter months. In other animals, such as the spring peeper frog (''Pseudacris crucifer''), the molality is increased temporarily as a reaction to cold temperatures. In the case of the peeper frog, freezing temperatures trigger a large scale breakdown of glycogen in the frog's liver and subsequent release of massive amounts of glucose into the blood.〔L. Sherwood et al., ''Animal Physiology: From Genes to Organisms'', 2005, Thomson Brooks/Cole, Belmont, CA, ISBN 0-534-55404-0, p. 691–692〕 With the formula below, freezing-point depression can be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called cryoscopy (Greek ''cryo'' = cold, ''scopos'' = observe "observe the cold"〔BIOETYMOLOGY- Biomedical Terms of Greek Origin (bioetymology.blogspot.com )〕) and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the van 't Hoff factor ''i'' by first determining ''m''B and then comparing it to ''m''solute. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing ''m''B with the amount of solute dissolved. In this case, ''i'' must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was, but it was included in textbooks at the turn of the 20th century. As an example, it was still taught as a useful analytic procedure in Cohen's ''Practical Organic Chemistry '' of 1910,〔Julius B. Cohen ''Practical Organic Chemistry'' 1910 (Link to online text )〕 in which the molar mass of naphthalene is determined using a ''Beckmann freezing apparatus''. Freezing-point depression can also be used as a purity analysis tool when analysed by differential scanning calorimetry. The results obtained are in mol%, but the method has its place, where other methods of analysis fail. This is also the same principle acting in the melting-point depression observed when the melting point of an impure solid mixture is measured with a melting point apparatus, since melting and freezing points both refer to the liquid-solid phase transition (albeit in different directions). In principle, the boiling point elevation and the freezing point depression could be used interchangeably for this purpose. However, the cryoscopic constant is larger than the ebullioscopic constant and the freezing point is often easier to measure with precision, which means measurements using the freezing point depression are more precise. FPD measurements are used in the dairy industry to ensure that milk has not had extra water added. Milk with FPD of over 0.509 m *C is considering to be unadulterated. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Freezing-point depression」の詳細全文を読む スポンサード リンク
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