Boiling point, pure aqueous solutions

Phase diagrams for pure water (red lines) and for an aqueous solution containing a nonvolatile solute (blue lines). Note that the boiling point of the solution is higher than that of pure water. Conversely, the freezing point of the solution is lower than that of pure water. The effect of a nonvolatile solute is to extend the liquid range of a solvent. 

In simple or differential distillation, the vapor evolved from the boiling mixture is immediately removed and condensed. Unless the boiling points of the two pure components differ widely, a reasonable degree of separation is not possible. This method may be used to remove low boiling solvents from aqueous solutions. 

Some physical properties of solutions differ in important ways from those of the pure solvent. For example, pure water freezes at 0 °C, but aqueous solutions freeze at lower temperatures. We utilize this behavior when we add ethylene glycol antifreeze to a car s radiator to lower the freezing point of the solution. The added solute also raises the boiling point of the solution above that of pure water, making it possible to operate the engine at a higher temperature. 

F ure 12.10 Phase diagreim illustrating the boiling-point elevation and teezing-point depression of aqueous solutions. The dashed cunres pertain to the solution, and the solid curves to the pure solvent. 4s you can see, the boiling point of the solution is higher than that of water, and the freezing point of the solution is lower them that of water. 

The chemicals listed are In the pure state or in a saturated solution unless othenwise indicated. Compatibility Is shown to the maximum allowable temperature for which data are available. Incompatibility is shown by an x. bp = boiling point aq = aqueous... 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

After twice washing with 100 ml of diethyl ether, the aqueous phase is made alkaline with 50% caustic soda solution. The liberated base Is twice extracted with 150 ml of diethyl ether. After the ether has been evaporated, the residue Is distilled under reduced pressure and has a boiling point of 184°C/0.1 mm, np70 = i. 5539. 77 g of the pure base in the form of a viscous liquid is thus obtained. The hydrochloride, which is prepared in conventional manner, has a melting point of 128°C. 

P-Benzilmonoxime. Boil 10 g. of the pure a-monoxime for 15 minutes with 1 g. of dried animal charcoal in a quantity of pure benzene just sufficient to dissolve the a-monoxime at the boiling point. Filter off the charcoal and allow the filtrate to stand. The p-monoxime -f 0-5 CgH, crystallises slowly on cooling a further crop can be obtained by evapora tion of the mother liquid. An excellent yield of the p-monoxime, m.p. 112°, is obtained. The pure S-oxime causes no colour change with aqueous - alcoholic copper acetate solution if it is contaminated with the a-oxinie, a greenisli colour is produced. 

Preparation of Benzoyl Peroxide.1—Hydrogen peroxide (50 c.c. of about 10 per cent aqueous solution) kept well cooled in ice and continually shaken (preferably in a glass-stoppered bottle) is treated alternately with 4 A-sodium hydroxide solution and benzoyl chloride, added each a few drops at a time the solution is maintained faintly alkaline throughout. After about 30 c.c. of alkali and 15 g. of benzoyl chloride have been used up, the hydrogen peroxide has been decomposed and the benzoyl peroxide has separated in crystalline flocks, while the odour of the chloride has almost completely disappeared. The peroxide is filtered with suction, washed with water, and dried. Yield 10-12 g. Crystallised from a little alcohol, with which it should be boiled for a short time only, the substance forms beautiful colourless prisms. Melting point 106°-108° decomp. Heat a small quantity rapidly in a dry test tube over a naked flame. An especially pure product is obtained when a 1 von Pechmann and Vanino, Ber., 1894, 27, 1510. 

Plot the change in the saturated vapour pressure of water over pure water and an aqueous solution of sugar (sodium chloride) against the temperature. How does the freezing (boiling) point of water change when it contains solutes (sugar, sodium chloride) Define the cryoscopic and ebullioscopic constants of water. What is their numerical value State Raoult s laws and write their mathematical expression. How can the molecular masses of solids be determined ... 

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