Perhaps the most straightforward way to begin our study of chemistry is to examine some fundamental ways in which matter is classified and described. Two of the principal ways of classifying matter are according to its physical state, as a gas, liquid, or solid, and according to its composition, as an element, compound, or mixture.
A sample of matter can be a gas, a liquid, or a solid. These three forms of matter are called the states of matter. The states of matter differ in some of their simple observable properties. A gas (also known as vapor) has no fixed volume or shape; rather, it conforms to the volume and shape of its container. A gas can be compressed to occupy a smaller volume, or it can expand to occupy a larger one. A liquid has a distinct volume independent of its container but has no specific shape. It assumes the shape of the portion of the container that it occupies. A solid has both a definite shape and a definite volume; it is rigid. Neither liquids nor solids can be compressed to any appreciable extent.
The properties of the states can be understood on the molecular level (Figure 1.4). In a gas the molecules are far apart and are moving at high speeds, colliding repeatedly with each other and with the walls of the container. In a liquid the molecules are packed more closely together, but still move rapidly, allowing them to slide over each other; thus, liquids pour easily. In a solid the molecules are held tightly together, usually in definite arrangements, in which the molecules can wiggle only slightly in their otherwise fixed positions. Thus, solids have rigid shapes.
Most forms of matter that we encounter--for example, the air we breathe (a gas), gasoline for cars (a liquid), and the sidewalk on which we walk (a solid)--are not chemically pure. We can, however, resolve, or separate, these kinds of matter into different pure substances. A pure substance (usually referred to simply as a substance) is matter that has a fixed composition and distinct properties. For example, water and ordinary table salt (sodium chloride), the primary components of seawater, are pure substances.
We can classify substances as either elements or compounds. Elements are substances that cannot be decomposed into simpler substances. Each element is composed of only one kind of atom [Figure 1.5(a and b)]. Compounds, in contrast, are composed of two or more elements, and thus contain two or more kinds of atoms [Figure 1.5(c)].
FIGURE 1.5 Each element contains a unique kind of atom. Elements might consist of individual atoms, as in (a), or molecules, as in (b). Compounds contain two or more different atoms connected or bonded together as in (c). A mixture contains the individual units of its components, shown in (d) as both atoms and molecules.
Most of the matter we encounter consists of mixtures of different substances. Mixtures are combinations of two or more substances in which each substance retains its own chemical identity and hence its own properties [Figure 1.5(d)]. Whereas pure substances have fixed compositions, the compositions of mixtures can vary. For example, a cup of sweetened coffee can contain either little sugar or a lot. The substances making up a mixture (such as sugar and water) are called components of the mixture.
Some mixtures, such as sand, rocks, and wood, do not have the same composition, properties, and appearance throughout the mixture. Such mixtures are heterogeneous [Figure 1.6(a)] Mixtures that are uniform throughout are homogeneous. Air is a homogeneous mixture of the gaseous substances nitrogen, oxygen, and smaller amounts of other substances. The nitrogen in air has all the properties that pure nitrogen does because both the pure substance and the mixture contain the same nitrogen molecules. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures [Figure 1.6(b)]. Homogeneous mixtures are also called solutions. Air is a gaseous solution; gasoline is a liquid solution; brass is a solid solution.
Figure 1.7 summarizes the classification of matter into mixtures, compounds, and elements. The distinction between physical and chemical changes is examined in Section 1.3.
FIGURE 1.7 Classification of matter resulting in the categories of compounds and elements.
Because each component of a mixture retains its own properties, we can separate a mixture into its components by taking advantage of the differences in their properties. For example, a heterogeneous mixture of iron filings and gold filings could be sorted individually by color into iron and gold. A more clever approach would be to use a magnet to attract the iron filings, leaving the gold ones behind. We can also take advantage of an important chemical difference between these two metals: Many acids dissolve iron but do not dissolve gold. Thus, if we put our mixture into an appropriate acid, the iron will dissolve and the gold will be left behind. The two could then be separated by filtration, a procedure illustrated in Figure 1.8. We would have to use other chemical reactions, which we will learn about later, to transform the dissolved iron back into metal.
We can separate homogeneous mixtures into their components in similar ways. For example, water has a much lower boiling point than table salt. If we boil a solution of salt and water, the water will evaporate and the salt will be left behind. We could use a tube with cold walls (a condenser) to change the water vapor back into liquid (Figure 1.9). This process is called distillation.
FIGURE 1.9 A simple apparatus for the separation of a sodium chloride solution (salt water) into its components. Boiling the solution evaporates the water, which is condensed and collected in the receiving flask. After all the water has boiled away, pure sodium chloride remains in the boiling flask.
The differing abilities of substances to adhere to the surfaces of various solids such as paper and starch can also be used to separate mixtures. This is the basis of chromatography (literally "the writing of colors"), a technique that can give beautiful and dramatic results. An example of the chromatographic separation of ink is shown in Figure 1.10.
At the present time 112 elements are known. These elements vary widely in their abundance, as shown in Figure 1.11. For example, over 90 percent of Earth's crust consists of only five elements: oxygen, silicon, aluminum, iron, and calcium. In contrast, just three elements (oxygen, carbon, and hydrogen) account for over 90 percent of the mass of the human body.
FIGURE 1.11 Elements, in percent by mass, in (a) Earth's crust (including oceans and atmosphere) and (b) the human body.
Some of the more familiar elements are listed in Table 1.2, along with the chemical abbreviations--or chemical symbols--that we use to denote them. All the known elements and their symbols are listed on the front inside cover of this text. The table in which the symbol for each element is enclosed in a box is called the periodic table. In the periodic table the elements are arranged so that closely related elements are grouped together. We describe this important tool in more detail in Section 2.4.
Notice that the symbol for each element consists of one or two letters, with the first letter capitalized. These symbols are often derived from the English name for the element (first and second columns in Table 1.2), but sometimes they are derived from a foreign name instead (third column). You will need to know these symbols and to learn others as we encounter them in the text.
Most elements can interact with other elements to form compounds. Hydrogen gas, for example, burns in oxygen gas to form water. Conversely, it is possible to decompose water into its component elements by passing an electrical current through the water, as shown in Figure 1.12. As seen in Table 1.3, the properties of water bear no resemblance to the properties of its component elements. Furthermore, the composition of water is not variable. Pure water, regardless of its source, consists of 11 percent hydrogen and 89 percent oxygen by mass. This macroscopic composition of water corresponds to its molecular composition, which consists of two hydrogen atoms combined with one oxygen atom.
FIGURE 1.12 Decomposition of the compound water into the elements hydrogen and oxygen by passing a direct electrical current through it.
The observation that the elemental composition of a pure compound is always the same is known as the law of constant composition (or the law of definite proportions). It was first put forth by the French chemist Joseph Louis Proust (1754-1826) in about 1800. Although this law has been known for almost 200 years, the general belief persists among some people that a fundamental difference exists between compounds prepared in the laboratory and the corresponding compounds found in nature. However, a pure compound has the same composition and properties regardless of its source. Both chemists and nature must use the same elements and operate under the same natural laws. Differences in composition and properties between substances indicate that the compounds are not the same or that they differ in purity.