Factors Affecting Solubility and Colligative Properties

 Solubility is a measure of the maximum amount of solute that can be dissolved in a given amount of solvent to form a stable solution. Whereas changes in pressure have little effect on the solubility of solid or liquid solutes in a liquid solvent, pressure has a much greater influence on the solubility of a gaseous solute. A commonly observed phenomenon that supports this is the effervescence that occurs when the cap of a bottle of ordinary soda water is removed. Soda water contains carbon dioxide gas dissolved in water under pressure; when the cap is removed, the pressure of the gas on the liquid is decreased to atmospheric pressure. Since carbon dioxide gas leaves the solution at this lower pressure, it follows that the solubility of carbon dioxide in water is dependent upon the pressure of the carbon dioxide above the liquid. The results of this simple observation are summarized in Henry's Law, which states that at any specified temperature, the extent to which a gas dissolves in a liquid is directly dependent upon the pressure of the gas.

 In general, a change in temperature affects the solubility of gaseous solutes differently than it does the solubility of solid solutes, because the solubility of a gas in a liquid solvent decreases with increasing temperature. With relatively few exceptions, the solubility of solids in liquids increases with an increase in temperature. In some instances, the increase in solubility is very large; for example, the solubility of potassium nitrate in water at 25°C is about 31 grams of KNO3 per 100 grams of water and about 83 grams of KNO3 per 100 grams of water at 50°C (122°F). On the other hand, the solubility of some solutes, such as ordinary table salt, shows very little dependence on temperature. Often this difference in solubility can be used as an advantage in the preparation, isolation, or purification of substances by the process of crystallization. In general, it is not possible to arrive at any reliable generalization concerning the influence of temperature upon the solubility of liquids in liquids. In some cases the solubility increases with an increase in temperature, in some cases it decreases, and in others there is very little effect.

 Crystalline substances consist of a regular arrangement of atoms, molecules, or ions; in the latter case, the forces that hold the crystal together are electrostatic in nature. For an ionic crystal to dissolve in water, the water molecules must be able to shield the charges of the positive and negative ions from each other. The attractive forces between the ions in solution are less than those in the solid state because of the solvent molecules; hence, the ions behave more or less independently in solution. In general, the relative solubilities of ionic substances are a measure of the magnitude of the electrostatic forces that hold the crystals together.

Pure liquids have a set of characteristic physical properties (melting point, vapor pressure at a given temperature, etc.). Solutions in a solvent exhibit these same properties, but the values differ from those of the pure solvent because of the presence of the solute. Moreover, the change observed in these properties in going from the pure solvent to a solution is dependent only upon the number of solute molecules; these properties are called colligative properties. The properties of a solvent that show a predictable change upon the addition of a solute are melting point, boiling point, vapor pressure, and osmotic pressure.

Solutions exhibit higher boiling points and lower melting points than the parent solvent. The increase in boiling point and decrease in melting point is dependent upon the number of solute particles in the solution. The greater is the number of solute particles (i.e., the concentration), the greater will be the boiling point elevation and melting point depression. A common application of this effect in some parts of the world is in the use of antifreeze solutions in the cooling systems of automobiles in cold climates. "Antifreeze" compounds are usually organic liquids that are miscible with water so that large freezing point effects can be attained.

All liquids exhibit a vapor pressure, the magnitude of which depends on the temperature of the liquid. For example, water boils at 100°C, which means that at 100°C the vapor pressure of water is equal to the atmospheric pressure allowing bubbles of gaseous water (steam) to escape from the liquid state. However, the vapor pressure of a solution (at any temperature) is less than that of the solvent. Thus, boiling water ceases to boil upon the addition of salt because the salt solution has a lower vapor pressure than pure water. The salt solution will eventually boil when the temperature of the solution increases bringing about an increase in vapor pressure sufficient to again form bubbles. Note in this example that the boiling point of water increases with the addition of salt; thus, the boiling point elevation and the vapor pressure depression are related.

This property of solutions related to osmosis is perhaps the least familiar of the colligative properties, but in a sense it is more important than other well known properties. In 1748 French clergyman and physicist Jean-Antoine Nollet observed that certain animal membranes are selectively permeable to different molecules. Since then, many examples of semi permeable membranes have been discovered, including animal bladder or gut tissues, eggshell lining, and certain vegetable tissues. A semi permeable membrane may be defined as a material that allows molecules of one kind to pass through it but prevents the passage of other kinds of molecules or allows the passage of different kinds of molecules at different rates. Membranes often permit the passage of solvent molecules and prevent the passage of solute molecules. The phenomenon of osmosis is of far-reaching importance in biology, medicine, and related areas. Many animal and vegetable membranes are semi-permeable, and the process of osmosis plays an important role in the transfer of molecules through cell walls in biological processes. Osmosis is responsible in part for the germination of seeds and for the rising of sap into the branches and leaves of trees. The preservative action of sugar solutions (e.g., preserves, jellies) is believed to depend upon osmotic processes; bacteria are literally dehydrated.