A sample of pure baking soda, regardless of its source or size, will be a
white solid containing 57.1% sodium, 1.2% hydrogen, 14.3% carbon, and 27.4%
oxygen. The sample will dissolve in water. When heated to 270°C the sample
will decompose, giving off carbon dioxide and water vapor and leaving a residue
of sodium carbonate. Thus, by definition, baking soda is a pure substance because
it has a constant composition and a unique set of properties, some of which
we have listed. The properties we have described hold true for any sample of
baking soda. These properties are the kinds in which we are interested.
A note about the term pure; in this text, the word pure means a single substance, not a mixture of substances. As used by the U.S. Food and Drug Administration (USFDA), the term pure means "fit for human consumption." Milk, whether whole, 2% fat, or skim, may be pure (fit for human consumption) by public health standards, but it is not pure in the chemical sense. Milk is a mixture of a great many substances, including water, butterfat, proteins, and sugars. Each of these substances is present in different amounts in each of the different kinds of milk (Figure 1.1).
|FIGURE 1.1 Pure substances versus mixtures. The labels on a carton of milk and a box of baking soda show that milk is a mixture and baking soda is a pure substance.|
A mixture consists of two or more pure substances. Most of the matter we see around us is composed of mixtures. Seawater contains dissolved salts; river water contains suspended mud; hard water contains salts of calcium, magnesium, and iron. Both seawater and river water also contain dissolved oxygen, without which fish and other aquatic life could not survive.
Unlike the constant composition of a simple substance, the composition of a mixture can be changed. The properties of the mixture depend on the percentage of each pure substance in it. Steel is an example of a mixture. All steel starts with the pure substance iron. Refiners then add varying percentages of carbon, nickel, chromium, vanadium, or other substances to obtain steels of a desired hardness, tensile strength, corrosion resistance, and so on. The properties of a particular type of steel depend not only on which substances are mixed with the iron but also on the relative percentage of each. One type of chromium-nickel steel contains 0.6% chromium and 1.25% nickel. Its surface is easily hardened, a property that makes it valuable in the manufacture of automobile gears, pistons, and transmissions. The stainless steel used in the manufacture of surgical instruments, food-processing equipment, and kitchenware is also a mixture of iron, chromium, and nickel; it contains 18% chromium and 8% nickel. Steel with this composition can be polished to a very smooth surface and is very resistant to rusting.
You can often tell from the appearance of a sample whether it is a mixture. For example, if river water is clouded with mud or silt particles, you know it is a mixture. If a layer of brown haze lies over a city, you know the atmosphere is mixed with pollutants. However, the appearance of a sample is not always sufficient evidence by which to judge its composition. A sample of matter may look pure without being so. For instance, air looks like a pure substance but it is actually a mixture of oxygen, nitrogen, and other gases.
Rubbing alcohol is a clear, colorless liquid that looks pure but is actually
a mixture of isopropyl alcohol and water, both of which are clear, colorless
liquids. As another example, you cannot look at a piece of metal and know whether
it is pure iron or a mixture of iron with some other substance such as chromium
or nickel. Figure 1.2 shows the relationships between different kinds of matter.
|FIGURE 1.2 Classification of matter.|