Raising of the Boiling Point

The raising of the boiling jmnt. The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid becom equal to the atmospheric pressure. At the boiling point of the pure solvent the vapour pressure of the solution is less than one atmosphere, and hence the solution must have a higher boiling point than the pure solvent. This is shown in Fig. 25, in which AB is the vapour pressure curve of 

A more general equation, apphcable to more concentrated solutions, may be obtained as follows. Integrating I between the boiling point of the solvent and the boiling point T of the solution, we have 

The raising of the boiling point can therefore be calculated from the vapour pressure of the solution at the boiling point of the pure solvent, if the heat of evaporation remains constant in the interval T — Tq. Otherwise a mean value of L must be substituted in equation (5a). It is easy to show that (5) approximates to (4) when L=Lq, and is so nearly equal to unity that we may put 

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The freezing point of a solution of a nonvolatile solute is always lower than the pure solvent and the boiling point is always higher. It is the number of solute particles that determines the amount of the lowering of the freezing point and raising of the boiling point. 

The lowering of the vapour pressure of water by ammonium iodide measured by G. Tammann 9 shows that the fall is 12"5 mm. for JN-soln. 25"1 mm. for N-soln. and 243 5 for lON-soln. According to L. C. de Coppet, the soln. of a mol. of the salt in water lowers the temp, of maximum density 1T1°. The degree of ionization calculated by S. M. Johnston from the raising of the boiling point of water by normal soln. of ammonium iodide agrees with the value of N. Zelinsky and S. Krapiwin and S. Arrhenius from the electrical conductivities of soln. of a mol. of the salt in v litres of water ... 

One type of extrinsic deviation is found in the lowering of the freezing point or the raising of the boiling point for small liquid droplets from that for the bulk state. Such effects are usually attributed to the absence of phase transition nuclei. The absence of such nuclei stems from the fact that the bulk material from which the aerosol particles are formed probably contains only minute traces of foreign material (nuclei) per unit volume, so that there is only a very small probability that any small aerosol particle will contain even one nucleus. This circumstance results in the situation that nearly all aerosol particles formed by vapor condensation and subsequent cooling well below the melting point of the parent material are likely to be in a... 

The formula for the lowering of the freezing point is thus quite analogous to the formula for the raising of the boiling point, and differs from it in that the heat of fusion takes the place of the heat of evaporation. 

In dilute solutions LqCq = L is the heat of evaporation for unit volume (1 litre) of the solvent. The raising of the boiling point is therefore proportional to the molal concentration of... 

I. Proportionality between raising of the boiling point and concentration. 

The equations of 1 (p. 228 el seq.), relating the vapour pressure of the solution to the lowering of the freezing point and the raising of the boiling point, now enable us to calculate the osmotic pressure in terms of these quantities. 

The raising of the boiling point and the lowering of the freezing point are therefore proportional to the molal concentration. The constant of proportionality can be calculated from the latent heat of evaporation or fusion, and from the boiling or freezing point of the solvent. 

The osmotic pressure of mixtures of solutiom. According to van t Hoif s theory, the osmotic pressure of a solution depends only on the number of the dissolved molecules and not on their nature. Hence the osmotic pressure of a solution containing several substances is equal to the sum of the osmotic pressures due to the substances individually. Thus Dalton s law of partial pressures applies also to solutions. The relative lowering of the vapour pressure, raising of the boiling point, and depression of the freezing point are likewise proportional to the total molal concentration of the solution. Quantities of this kind, which do not depend on the chemical nature, but only on the number of the molecules present, were termed colligative by Ostwald. 

Any physical effect of the solute on the solvent is a colligative property. The lowering of the freezing point and the raising of the boiling point are examples of colligative properties. Only nonvolatile solutes have predictable effects on boiling point, but besides that requirement, the identity of the solute is relatively unimportant. 

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