Water molecules solubility

The pentahydrate consists of flat, shiny prismatic crystals density 1.58 g/cm3 loses three water molecules at 100°C melts at 120°C losing all water molecules soluble in water and alcohol aqueous solutions strongly alkaline insoluble in ether. 

The lower members of the series are liquids soluble in water and volatile in steam. As the number of carbon atoms in the molecule increases, the m.p. and b.p. rise and the acids become less soluble in water and less volatile. The higher fatty acids are solids, insoluble in water and soluble in organic solvents. 

The aluminium ion, charge -I- 3. ionic radius 0.045 nm, found in aluminium trifluoride, undergoes a similar reaction when a soluble aluminium salt is placed in water at room temperature. Initially the aluminium ion is surrounded by six water molecules and the complex ion has the predicted octahedral symmetry (see Table 2.5 ) ... 

The anhydrous chloride is prepared by standard methods. It is readily soluble in water to give a blue-green solution from which the blue hydrated salt CuClj. 2H2O can be crystallised here, two water molecules replace two of the planar chlorine ligands in the structure given above. Addition of dilute hydrochloric acid to copper(II) hydroxide or carbonate also gives a blue-green solution of the chloride CuClj but addition of concentrated hydrochloric acid (or any source of chloride ion) produces a yellow solution due to formation of chloro-copper(ll) complexes (see below). 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Solubility in Water A familiar physical property of alkanes is contained m the adage oil and water don t mix Alkanes—indeed all hydrocarbons—are virtually insoluble m water In order for a hydrocarbon to dissolve m water the framework of hydrogen bonds between water molecules would become more ordered m the region around each mole cule of the dissolved hydrocarbon This increase m order which corresponds to a decrease m entropy signals a process that can be favorable only if it is reasonably... 

Solubility in Water Alkyl halides and alcohols differ markedly from one another m their solubility m water All alkyl halides are insoluble m water but low molecular weight alcohols (methyl ethyl n propyl and isopropyl) are soluble m water m all pro portions Their ability to participate m mtermolecular hydrogen bonding not only affects the boiling points of alcohols but also enhances their water solubility Hydrogen bonded networks of the type shown m Figure 4 5m which alcohol and water molecules asso ciate with one another replace the alcohol-alcohol and water-water hydrogen bonded networks present m the pure substances... 

In aqueous solution intermolecular association between carboxylic acid molecules IS replaced by hydrogen bonding to water The solubility properties of carboxylic acids are similar to those of alcohols Carboxylic acids of four carbon atoms or fewer are mis cible with water m all proportions... 

Amines that have fewer than six or seven carbon atoms are soluble m water All amines even tertiary amines can act as proton acceptors m hydrogen bonding to water molecules... 

Solubility can often be decreased by using a nonaqueous solvent. A precipitate s solubility is generally greater in aqueous solutions because of the ability of water molecules to stabilize ions through solvation. The poorer solvating ability of nonaqueous solvents, even those that are polar, leads to a smaller solubility product. For example, PbS04 has a Ks of 1.6 X 10 in H2O, whereas in a 50 50 mixture of H20/ethanol the Ks at 2.6 X 10 is four orders of magnitude smaller. 

D C Red No. 36 (27) is an unsulfonated pigment. It contains no groups capable of salt formation and is thus insoluble direcdy on coupling. Its chlorine group ortho to the azo group results in a stericaHy hindered molecule with low solubiUty and excellent light stabiUty. The unsulfonated dyes Citms Red No. 2 (8) and D C Red No. 17 (20) are insoluble in water but soluble in aromatic solvents. 

Because of their zwitterionic nature, amino acids are generally soluble in water. Their solubility in organic solvents rises as the fat-soluble portion of the molecule increases. The likeliest impurities are traces of salts, heavy metal ions, proteins and other amino acids. Purification of these is usually easy, by recrystallisation from water or ethanol/water mixtures. The amino acid is dissolved in the boiling solvent, decolorised if necessary by boiling with Ig of acid-washed charcoal/lOOg amino acid, then filtered hot, chilled, and set aside for several hours to crystallise. The crystals are filtered off, washed with ethanol, then ether, and dried. 

Ethers are more volatile than alcohols of the same molar mass because their molecules do not form hydrogen bonds to one another (Fig. 19.2). They are also less soluble in water because they have a lower ability to form hydrogen bonds to water molecules. Because ethers are not very reactive and have low molecular polarity, they are useful solvents for other organic compounds. However, ethers are flammable diethyl ether is easily ignited and must be used with great care. 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

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