1.2 Pure Substances and Mixtures

A pure chemical substance is any matter that has a fixed chemical composition and characteristic properties.  Oxygen, for example, is a pure chemical substance that is a colorless, odorless gas at 25°C.  Very few samples of matter consist of pure substances; instead, most are mixtures, which are combinations of two or more pure substances in variable proportions in which the individual substances retain their identity.  Air, tap water, milk, blue cheese, bread, and dirt are all mixtures.  If all portions of a material are in the same state, have no visible boundaries, and are uniform throughout, then the material is homogeneous.  Examples of homogeneous mixtures are the air we breathe and the tap water we drink.  Homogeneous mixtures are also called solutions.  Thus, air is a solution of nitrogen, oxygen, water vapor, carbon dioxide, and several other gases; tap water is a solution of small amounts of several substances in water.  Although most solutions we encounter are liquid, solutions can also be solid.  The gray substance still used by some dentists to fill tooth cavities is a complex solid solution that contains 50% mercury and 50% of a powder that contains mostly silver, tin, and copper, with small amounts of zinc and mercury.  Solid solutions of two or more metals are commonly called alloys.

If the composition of a material is not completely uniform, then it is heterogeneous (e.g., chocolate chip cookie dough, blue cheese, and dirt). Mixtures that appear to be homogeneous are often found to be heterogeneous after microscopic examination. Milk, for example, appears to be homogeneous, but when examined under a microscope, it clearly consists of tiny globules of fat and protein dispersed in water (Figure 1.5).

Figure 1.5 Milk, a heterogenous mixture.  Under a microscope, whole milk is actually a heterogeneous mixture composed of globules of fat and protein dispersed in water.

The components of heterogeneous mixtures can usually be separated by simple means.  Solid-liquid mixtures such as sand in water or tea leaves in tea are readily separated by filtration, which consists of passing the mixture through a barrier, such as a strainer, with holes or pores that are smaller than the solid particles.  In principle, mixtures of two or more solids, such as sugar and salt, can be separated by microscopic inspection and sorting.  More complex operations are usually necessary, though, such as when separating gold nuggets from river gravel by panning. First solid material is filtered from river water; then the solids are separated by inspection. If gold is embedded in rock, it may have to be isolated using chemical methods.  Homogeneous mixtures (solutions) can be separated into their component substances by physical processes that rely on differences in some physical property, such as differences in their boiling points. Two of these separation methods are distillation and crystallization.  Distillation makes use of differences in volatility, a measure of how easily a substance is converted to a gas at a given temperature.  Figure 1.6 shows a simple distillation apparatus for separating a mixture of substances, at least one of which is a liquid.  The most volatile component boils first and is condensed back to a liquid in the water-cooled condenser, from which it flows into the receiving flask. If a solution of salt and water is distilled, for example, the more volatile component, pure water, collects in the receiving flask, while the salt remains in the distillation flask.  The solution of salt in water is heated in the distilling flask until it boils. 

Figure 1.6. The distillation of a solution of table salt in water.

The resulting vapor is enriched in the more volatile component (water), which condenses to a liquid in the cold condenser and is then collected in the receiving flask.  Mixtures of two or more liquids with different boiling points can be separated with a more complex distillation apparatus. One example is the refining of crude petroleum into a range of useful products: aviation fuel, gasoline, kerosene, diesel fuel, and lubricating oil (in the approximate order of decreasing volatility).  Another example is the distillation of alcoholic spirits such as brandy or whiskey. This relatively simple procedure caused more than a few headaches for federal authorities in the 1920s during the era of Prohibition, when illegal stills proliferated in remote regions of the United States.

Crystallization separates mixtures based on differences in solubility, a measure of how much solid substance remains dissolved in a given amount of a specified liquid.  Most substances are more soluble at higher temperatures, so a mixture of two or more substances can be dissolved at an elevated temperature and then allowed to cool slowly (Figure 1.7). 

Figure 1.7. The crystallization of sodium acetate from a concentrated solution of sodium acetate in water.

Most mixtures can be separated into pure substances, which may be either elements or compounds.  An element, such as gray, metallic sodium, is a substance that cannot be broken down into simpler ones by chemical changes; a compound, such as white, crystalline sodium chloride, contains two or more elements and has chemical and physical properties that are usually different from those of the elements of which it is composed.  With only a few exceptions, a particular compound has the same elemental composition (the same elements in the same proportions) regardless of its source or history.  The chemical composition of a substance is altered in a process called a chemical change.  The conversion of two or more elements, such as sodium and chlorine, to a chemical compound, sodium chloride, is an example of a chemical change, often called a chemical reaction.

Figure 1.8 Classification of matter.