| Solubility is a measure of the maximum amount of | | | | is dependent only upon the number of solute |
| solute that can be dissolved in a given amount of | | | | molecules; these properties are called colligative |
| solvent to form a stable solution. Whereas changes in | | | | properties. The properties of a solvent that show a |
| pressure have little effect on the solubility of solid or | | | | predictable change upon the addition of a solute are |
| liquid solutes in a liquid solvent, pressure has a much | | | | melting point, boiling point, vapor pressure, and osmotic |
| greater influence on the solubility of a gaseous solute. | | | | pressure. |
| A commonly observed phenomenon that supports this | | | | Solutions exhibit higher boiling points and lower melting |
| is the effervescence that occurs when the cap of a | | | | points than the parent solvent. The increase in boiling |
| bottle of ordinary soda water is removed. Soda water | | | | point and decrease in melting point is dependent upon |
| contains carbon dioxide gas dissolved in water under | | | | the number of solute particles in the solution. The |
| pressure; when the cap is removed, the pressure of | | | | greater is the number of solute particles (i.e., the |
| the gas on the liquid is decreased to atmospheric | | | | concentration), the greater will be the boiling point |
| pressure. Since carbon dioxide gas leaves the solution | | | | elevation and melting point depression. A common |
| at this lower pressure, it follows that the solubility of | | | | application of this effect in some parts of the world is |
| carbon dioxide in water is dependent upon the | | | | in the use of antifreeze solutions in the cooling |
| pressure of the carbon dioxide above the liquid. The | | | | systems of automobiles in cold climates. "Antifreeze" |
| results of this simple observation are summarized in | | | | compounds are usually organic liquids that are miscible |
| Henry's Law, which states that at any specified | | | | with water so that large freezing point effects can be |
| temperature, the extent to which a gas dissolves in a | | | | attained. |
| liquid is directly dependent upon the pressure of the | | | | |
| gas. | | | | All liquids exhibit a vapor pressure, the magnitude of |
| | | | | which depends on the temperature of the liquid. For |
| In general, a change in temperature affects the | | | | example, water boils at 100°C, which means that at |
| solubility of gaseous solutes differently than it does the | | | | 100°C the vapor pressure of water is equal to the |
| solubility of solid solutes, because the solubility of a gas | | | | atmospheric pressure allowing bubbles of gaseous |
| in a liquid solvent decreases with increasing | | | | water (steam) to escape from the liquid state. |
| temperature. With relatively few exceptions, the | | | | However, the vapor pressure of a solution (at any |
| solubility of solids in liquids increases with an increase in | | | | temperature) is less than that of the solvent. Thus, |
| temperature. In some instances, the increase in solubility | | | | boiling water ceases to boil upon the addition of salt |
| is very large; for example, the solubility of potassium | | | | because the salt solution has a lower vapor pressure |
| nitrate in water at 25°C is about 31 grams of KNO3 | | | | than pure water. The salt solution will eventually boil |
| per 100 grams of water and about 83 grams of KNO3 | | | | when the temperature of the solution increases |
| per 100 grams of water at 50°C (122°F). On the | | | | bringing about an increase in vapor pressure sufficient |
| other hand, the solubility of some solutes, such as | | | | to again form bubbles. Note in this example that the |
| ordinary table salt, shows very little dependence on | | | | boiling point of water increases with the addition of salt; |
| temperature. Often this difference in solubility can be | | | | thus, the boiling point elevation and the vapor pressure |
| used as an advantage in the preparation, isolation, or | | | | depression are related. |
| purification of substances by the process of | | | | |
| crystallization. In general, it is not possible to arrive at | | | | This property of solutions related to osmosis is |
| any reliable generalization concerning the influence of | | | | perhaps the least familiar of the colligative properties, |
| temperature upon the solubility of liquids in liquids. In | | | | but in a sense it is more important than other well |
| some cases the solubility increases with an increase in | | | | known properties. In 1748 French clergyman and |
| temperature, in some cases it decreases, and in others | | | | physicist Jean-Antoine Nollet observed that certain |
| there is very little effect. | | | | animal membranes are selectively permeable to |
| | | | | different molecules. Since then, many examples of |
| Crystalline substances consist of a regular | | | | semi permeable membranes have been discovered, |
| arrangement of atoms, molecules, or ions; in the latter | | | | including animal bladder or gut tissues, eggshell lining, |
| case, the forces that hold the crystal together are | | | | and certain vegetable tissues. A semi permeable |
| electrostatic in nature. For an ionic crystal to dissolve in | | | | membrane may be defined as a material that allows |
| water, the water molecules must be able to shield the | | | | molecules of one kind to pass through it but prevents |
| charges of the positive and negative ions from each | | | | the passage of other kinds of molecules or allows the |
| other. The attractive forces between the ions in | | | | passage of different kinds of molecules at different |
| solution are less than those in the solid state because | | | | rates. Membranes often permit the passage of solvent |
| of the solvent molecules; hence, the ions behave more | | | | molecules and prevent the passage of solute |
| or less independently in solution. In general, the relative | | | | molecules. The phenomenon of osmosis is of |
| solubilities of ionic substances are a measure of the | | | | far-reaching importance in biology, medicine, and |
| magnitude of the electrostatic forces that hold the | | | | related areas. Many animal and vegetable membranes |
| crystals together. | | | | are semi-permeable, and the process of osmosis |
| | | | | plays an important role in the transfer of molecules |
| Pure liquids have a set of characteristic physical | | | | through cell walls in biological processes. Osmosis is |
| properties (melting point, vapor pressure at a given | | | | responsible in part for the germination of seeds and for |
| temperature, etc.). Solutions in a solvent exhibit these | | | | the rising of sap into the branches and leaves of trees. |
| same properties, but the values differ from those of | | | | The preservative action of sugar solutions (e.g., |
| the pure solvent because of the presence of the | | | | preserves, jellies) is believed to depend upon osmotic |
| solute. Moreover, the change observed in these | | | | processes; bacteria are literally dehydrated. |
| properties in going from the pure solvent to a solution | | | | |