This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Solubility means the amount of a specific substance that will dissolve in a given volume of a specified solvent. If the solvent is not mentioned it is understood to be water. Solubility is generally expressed as the number of grams of solute that will dissolve in 100 cc. of solvent, and as the temperature affects solubility it is usually mentioned. If the temperature is not stated it is understood to be room temperature or 20° to 22°C.
Factors determining the solubility. Temperature, the fineness of division and the nature of the solute, and the nature of the solvent determine the solubility to a great extent. Some substances are highly, others only slightly, soluble in water, and still others are highly insoluble in this solvent. Some substances will dissolve best in alcohol, or chloroform, or ether, or benzine. Some liquids, like alcohol and water, will dissolve in all proportions, but there is a limit to the solubility of all substances of a crystalline form.
Fineness of division of the solute. A larger amount of the crystalline solute is soluble if it is very fine divided or powdered than if left in large crystals. This does not refer to the ease or quickness of solution but to the total number of grams dissolved in 100 cc. of the solvent. This increase in solubility is due to the increased surface energy of the smaller particles in the solvent. The diameter of the particles must be less than lu to increase the solubility to any great extent.
Temperature. The solubility of most crystalline substances in water is increased by heating, though some are little affected by temperature change, and a few are less soluble in hot water than in cold. Sodium chloride, common salt, is an example of a substance that is little affected by temperature change. At 0°C. about 35.6 grams will dissolve in 100 cc. of water, and about 39 grams at 100°. Calcium hydroxide is only half as soluble at 100° as at 20°. The solubility of the sugars is increased by elevation of temperature. At 0°C, 179 grams of sucrose will dissolve; at 100°C. 487 grams will dissolve.
Addition of other substances. Two forms of the same compound may give different solubilities. Thus anhydrous lactose (without water of crystallization) has one solubility and the hydrated form has another. Sometimes the addition of another substance, organic or inorganic, to the solution will increase or decrease the solubility of the solute. If, after all the sucrose possible is dissolved, some potassium acetate, sodium chloride, or many other salts, is added to the solution, more sucrose can be dissolved. This may affect the solubility of sugar in candy making, for it is known that different waters, with different proportions and kinds of minerals, do not always give identical results, with the same sugar, for the same kind of candy.
Saturated solutions. Once all the solute is dissolved that can possibly be held by a definite amount of solvent at a constant temperature, any excess of the solute remains unchanged. The saturated solution is one that contains all the dissolved solute that it can take up when in contact with undissolved solute. In other words, it is a solution which when placed with excess of the solute at a definite temperature is in equilibrium, i.e., there is neither increase nor decrease in concentration of the solute.
Supersaturated solutions. One should not define a saturated solution as one containing all the solute it can hold, for a supersaturated solution holds more than a saturated one. If water is heated to 70°C. and all the sucrose added that can be dissolved by the water at this temperature, one has a saturated solution. There should be no excess of the solid left with the liquid. If this solution is carefully cooled to 40°C. without stirring more sucrose will .be in solution than could have been if the water had not been heated above 40°. The cooling of a saturated solution leads to the formation of crystals, and any excess beyond saturation is gradually precipitated as the temperature drops, though crystallization may not begin immediately. But some substances, like the sugars, require longer than others for crystallization to commence, unless the solution is stirred or agitated.