This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
A solute dissolved in a solvent affects the properties of the solvent or in other words alters certain of its constant characteristics. No discussion of osmotic pressure is given; the effect on the freezing point is discussed in the chapter on freezing. These characteristics may be listed as follows: a. The vapor tension is lowered.
b. The boiling point is elevated.
c. The freezing point is lowered.
d. The osmotic pressure is increased.
Vapor tension. Vapor tension may be denned as the vapor or gaseous pressure of a liquid. The vapor tension of a liquid depends upon the amount of vapor formed. Some liquids, like alcohol, evaporate rapidly and have a high vapor tension. Others, like water, evaporate more slowly. This gaseous pressure soon reaches a maximum at any given temperature. The maximum pressure at a definite temperature represents the vapor tension of the liquid.
Table 9 Solubility or Levulose (Jackson, Silsbee and Proffitt)
Temperature, degrees C.
Grams of levulose dissolved by 100 grams of water
Effect of the solute on vapor tension. When a substance is dissolved in a liquid the vapor tension of the solvent is lowered, i.e., there is less tendency to pass into the vapor state, hence the gaseous pressure is decreased.
If water is the solvent, there is a tendency with a high concentration of some substances for vapor from the air to enter the solution, thus increasing the quantity of the solvent. This is particularly true of sugar solutions, when the humidity of the air is high and the solutions are concentrated. On damp, rainy days, fondant and similar candies have to be beaten for a longer time to crystallize, unless they are cooked to a little higher temperature so as to obtain a greater concentration.
The fact that it is harder for the vapor to leave the surface of the liquid when there is a soluble substance in it is made use of in the following or similar ways to keep food moist. A covered vessel containing food loses moisture from the food until the air space is saturated with vapor. If the vessel is tight enough, the food does not dry out to an appreciable extent. If two dishes are placed in an enclosed vessel, one of them containing water and the other a heavy sugar solution, the sugar solution, since it loses vapor with difficulty and absorbs liquid, will gradually absorb water from the other dish. If two foods are placed in the same container, the one with more sugar will gradually absorb moisture from the other. This is why an apple is often put in a box with a fruit cake, the apple drying up and keeping the cake moist. Cake and bread should not be stored together, as the bread will become dry quickly.