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Section 3: Solutions and Solubility

Solutions can be made of liquids, gases, or even solids. However, when we speak of solutions, we commonly think of the solution being in the liquid phase, such as coffee, tea, juice, household bleach, shampoo, gasoline, and ink. Some solutions are also in the gas phase; air is a gaseous solution consisting mainly of nitrogen and oxygen, plus much smaller amounts of argon, carbon dioxide, and a few other gases, all mixed evenly together at the molecular level. There can even be solutions that are in the solid phase; a good example of these are metal alloys, like dental fillings, and these will be revisited in Unit 13.

The component that represents the largest fraction of a solution is called the "solvent," and its other components are called the "solutes." The solvent is the substance in which other materials (the solutes) dissolve. For example, the solvent in tea is water, and the solutes are all of the various molecules released by the tea leaves. (Note that the tea leaves themselves do not dissolve and become part of the solution—they are steeped to release flavor, then filtered out.) (Figure 8-3)

Solubility

A solvent cannot always dissolve a given amount of a solute. When table salt (sodium chloride, NaCl) is added to water, it makes a salty solution. But if we keep adding salt, it doesn't keep getting saltier beyond a certain point. The salt that was initially added to the water dissolves, but eventually additional salt just sits in a pile on the bottom of the container. At this point when no more solute can dissolve, we say the solution is saturated. At a given temperature, a particular solvent can dissolve a set amount of a particular solute, and then no more. This amount, usually measured in grams per volume of solvent, is said to be the solubility of the solute in the solvent at that temperature.

For example, table salt has a solubility of 36 grams per 100 mL of water at room temperature. This means that we can dissolve 36 grams of salt in every 100 mL of water we have. If more salt is added to the solution, it will just sit on the bottom of the container. (Figure 8-4) Solubilities are always given for a particular temperature of solvent. For table salt, if the temperature is raised, the 100 mL of water can dissolve more salt; if it is lowered, the water will hold less salt. This is not true for all solutes. Some of them are actually more soluble at lower temperatures.

Precipitation

If we prepared a saturated solution of salt at 25°C and then cooled the solution, we would see little salt crystals begin to appear in the liquid. This process of molecules coming together to form crystals is called "precipitation." (Figure 8-5) By cooling the solution, we have made the solution supersaturated, which means there is more solute than the solution normally can dissolve. The process of precipitation creates a solid that is called the "precipitate." Precipitation is the opposite of dissolution. When a substance dissolves, its molecules spread themselves out throughout the solvent. When it precipitates, the scattered molecules come together to form solid particles or, in some cases, crystals.