What is the lowering of vapor pressure

5.3 Solutions and mixed states

5.3.1 Boiling point increase (Ebullioscopy)

By adding common salt (dissolved substance) to water as a solvent, the work function of the solvent molecules (water) increases, as additional attractive forces occur between the molecules of the dissolved substance (common salt) and the molecules of the solvent (water). The higher work function means that fewer molecules of the solvent can pass from the liquid to the gaseous phase at the same temperature than with the pure substance.

If the vapor pressure of the dissolved substance is negligibly small compared to the vapor pressure of the solvent, the vapor pressure decrease is proportional to the concentration of the solute. This lowering of the vapor pressure causes an increase in the boiling point Ts and it applies to the increase in the boiling point

Raoult's law.

Here designated R. the general gas constant, TS. the boiling temperature, the heat of vaporization and c the concentration of the solute.

The increase in boiling point is therefore proportional to the concentration of the dissolved substance. The dependence on the solvent is in the so-called "ebullioscopic constant" who value for water Has.34


First of all, in the experiment, the water boils at 99.4 ° C without the addition of a solvent due to the lower air pressure compared to normal pressure. After adding 30 g of common salt, the boiling temperature rises to 101.3 ° C. A further 30 g of NaCl cause the boiling point to rise to 103.6 ° C.

If 30 g of NaCl (M (NaCl) = 58 g / mol) are dissolved in 250 ml of water, with complete dissociation in Na+ and Cl- the boiling point around increase.

Experiment: increasing the boiling point with table salt

34 see Demtröder (1998, chap. 10.4.3.b, p. 329f) or Gerthsen (1993, chap. 5.7.3, p. 252 f)

© University of Würzburg, Tilo Hemmert 2000