Solvent Properties of Water

Why do some chemicals dissolve in water while others do not  

Hydrocarbons (compounds that contain only carbon and hydrogen) are nonpolar. The favorable ion-dipole and dipole-dipole interactions responsible for the solubility of ionic and polar compounds do not occur for nonpolar compounds, so these compounds tend not to dissolve in water. The interactions between nonpolar molecules and water molecules are weaker than dipolar interactions. The permanent dipole of the water molecule can induce a temporary dipole in the nonpolar molecule by distorting the spatial arrangements of the electrons in its bonds. Electrostatic attraction is possible between the induced dipole of the nonpolar molecule and the permanent dipole of the water molecule (a dipole-induced dipole interaction), but it is not as strong as that between permanent dipoles. Hence, its consequent lowering of energy is less than that produced by the attraction of the water molecules for one another. The association of nonpolar molecules with water is far less likely to occur than the association of water molecules with themselves. 

A full discussion of why nonpolar substances are insoluble in water requires the thermodynamic arguments that we shall develop in Chapters 4 and 15. However, the points made here about intermolecular interactions will be useful background information for that discussion. For the moment, it is enough to know that it is less favorable thermodynamically for water molecules to be associated with nonpolar molecules than with other water molecules. As a result, nonpolar molecules do not dissolve in water and are referred to as hydrophobic ( water-hating, from the Greek). Hydrocarbons in particular tend to sequester themselves from an aqueous environment. A nonpolar solid leaves undissolved material in water. A nonpolar hquid forms a two-layer system with water an example is an oil shck. The interactions between nonpolar molecules are called hydrophobic interactions or, in some cases, hydrophobic bonds. 

Why do oil and water mixed together separate into layers  

Interactions between nonpolar molecules themselves are very weak and depend on the attraction between short-lived temporary dipoles and the dipoles they induce. A large sample of nonpolar molecules will always include some molecules with these temporary dipoles, which are caused by a momentary clumping of bonding electrons at one end of the molecule. A temporary dipole can induce another dipole in a neighboring molecule in the same way that a permanent dipole does. The interaction energy is low because the asso- 

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Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

In the case of nonionic but polar compounds such as sugars, the excellent solvent properties of water stem from its ability to readily form hydrogen bonds with the polar functional groups on these compounds, such as hydroxyls, amines, and carbonyls. These polar interactions between solvent and solute are stronger than the intermolecular attractions between solute molecules caused by van der Waals forces and weaker hydrogen bonding. Thus, the solute molecules readily dissolve in water. 

Franks, F. The Solvent Properties of Water, in Water — a Comprehensive Treatise (ed. Franks, F.), Vol. 2, chapter l, New York, Plenum Press 1973... 

Efflorescence. The solvent properties of water also causes efflorescence, a phenomenon whereby soluble or slightly soluble substances migrate from the interior of porous solids to the surface, where they precipitate. Efflorescence is an important factor in the decay and disintegration of many rocks, and of human-made porous materials such as ceramics, and even of some types of glass. On archaeological objects, efflorescence generally occurs mostly as a white, powdery, but sometimes consolidated accretion on the surface of the objects. Calcite, a form of calcium carbonate, is one of the most common substances to effloresce on archaeological ceramics. 

The solvent properties of water change considerably between 25 C and the critical point with the result that qualitative conclusions based on room temperature experience can be totally... 

These agents also alter the solvent properties of water so that hydrophobic interactions in the protein are weakened. They probably act by a combination of these effects. Urea- or guanidine-induced de-naturation is frequently partially or completely reversed when the concentration of the competitive hydrogen bonding agent in the protein solution is lowered by dialysis or dilution. 

Because water is the most significant solvent in our world, it is especially important that we understand the solvent properties of water. As we noted in Chapter 16, water is not a typical liquid, with most of its unusual properties arising from the extensive hydrogen bonding present among the molecules. Because of its unique nature, we must be very cautious in using simple arguments to account for the solvent properties of water. 

Water is often described as Nature s solvent therefore Chapter 3 describes the solvent properties of water. It is already used quite widely on an industrial scale, particularly in emulsion polymerization processes and hydrodistillations. However, some of the most exciting results have come in the field of synthetic... 

Mugnier, J. and Jung, G. Survival of bacteria and fungi in relation to the solvent properties of water in biopolymer gels, Appl. Env. Microb., 108, 1985. 

Ionic Interactions Hydrogen Bonds van der Waals Forces THERMAL PROPERTIES OF WATER SOLVENT PROPERTIES OF WATER Hydrophilic Molecules Hydrophobic Molecules Amphipathic Molecules Osmotic Pressure IONIZATION OF WATER Acids, Bases, and pH... 

When mixed with water, small amounts of nonpolar substances are excluded from the solvation network of the water that is, they coalesce into droplets. This process is called the hydrophobic effect. Hydrophobic ( water-hating ) molecules, such as the hydrocarbons, are virtually insoluble in water. Their association into droplets (or, in larger amounts, into a separate layer) results from the solvent properties of water, not from the relatively weak attraction between the associating nonpolar molecules. When nonpolar molecules enter an aqueous environment, hydrogen-bonded water molecules attempt to form a cagelike structure around... 

Water, owing to the fact that it possesses both acid and base properties, is termed amphiprotic. The solvent properties of water are a consequence of this ability to either accept or donate protons. Water is the most commonly used solvent for acids and bases. These interactions promote solubility and dissociation of acids and bases. 

The presence of an -OH group means that they have some of the solvent properties of water and can dissolve ionic compounds. The R— group allows them to behave as solvents to organic, covcdent substances. 

In addition, the chemical properties of water will change significantly as the temperature increases. These changes make the solvent properties of water at high temperature similar to those of polar organic solvents at room temperature (Siskin and Katritzky 1995). Table 7 shows several chemical/physical properties of water as a function of temperature. At 300°C, water exhibit a density and polarity similar to that of acetone at room temperature. The dielectric constant of water drops rapidly with... 

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