1.2 Classifications of Matter


Let's begin our study of chemistry by examining some fundamental ways in which matter is classified and described. Two 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).

States of Matter

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.


Figure 1.4 The three physical states of water are water vapor, liquid water, and ice. In this photo we see both the liquid and solid states of water. We cannot see water vapor. What we see when we look at steam or clouds is tiny droplets of liquid water dispersed in the atmosphere. The molecular views show that the molecules in the solid are arranged in a more orderly way than in the liquid. The molecules in the gas are much farther apart than those in the liquid or the solid.


Pure Substances

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 distinct properties and a composition that doesn't vary from sample to sample. Water and ordinary table salt (sodium chloride), the primary components of seawater, are examples of pure substances.

All substances are either elements or compounds. Elements cannot be decomposed into simpler substances. On the molecular level, each element is composed of only one kind of atom [Figure 1.5 (a and b)]. Compounds are substances composed of two or more elements, so they contain two or more kinds of atoms [Figure 1.5(c)]. Water, for example, is a compound composed of two elements, hydrogen and oxygen. Figure 1.5(d) shows a mixture of substances. Mixtures are combinations of two or more substances in which each substance retains its own chemical identity.


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 chemically joined together, as in (c). A mixture contains the individual units of its components, shown in (d) as both atoms and molecules.


Elements

At the present time 114 elements are known. These elements vary widely in their abundance, as shown in Figure 1.6. For example, only five elements account for over 90% of the Earth's crust: oxygen, silicon, aluminum, iron, and calcium. In contrast, just three elements (oxygen, carbon, and hydrogen) account for over 90% of the mass of the human body.


Figure 1.6 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—used 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 in vertical columns so that closely related elements are grouped together. We describe this important tool in more detail in Section 2.5.



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, but sometimes they are derived from a foreign name instead (last column in Table 1.2). You will need to know these symbols and to learn others as we encounter them in the text.

Compounds

Most elements can interact with other elements to form compounds. Hydrogen gas, for example, burns in oxygen gas to form water. Conversely, water can be decomposed into its component elements by passing an electrical current through it, as shown in Figure 1.7. Pure water, regardless of its source, consists of 11% hydrogen and 89% oxygen by mass. This macroscopic composition corresponds to the molecular composition, which consists of two hydrogen atoms combined with one oxygen atom. As seen in Table 1.3, the properties of water bear no resemblance to the properties of its component elements. Hydrogen, oxygen, and water are each unique substances.



Figure 1.7 Water decomposes into its component elements, hydrogen and oxygen, when a direct electrical current is passed through it. The volume of hydrogen (on the right) is twice the volume of oxygen (on the left).


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 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. When two materials differ in composition and properties, we know that they are composed of different compounds or that they differ in purity.

Mixtures

Most of the matter we encounter consists of mixtures of different substances. Each substance in a mixture retains its own chemical identity and hence its own properties. Whereas pure substances have fixed compositions, the compositions of mixtures can vary. A cup of sweetened coffee, for example, can contain either a 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 1.8(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 1.8(b)]. Homogeneous mixtures are also called solutions. Figure 1.9 summarizes the classification of matter into elements, compounds, and mixtures.


Figure 1.8 (a) Many common materials, including rocks, are heterogeneous. This close-up photo is of malachite, a copper mineral. (b) Homogeneous mixtures are called solutions. Many substances, including the blue solid shown in this photo (copper sulfate), dissolve in water to form solutions.


SAMPLE EXERCISE 1.1

"White gold," used in jewelry, contains two elements, gold and palladium. Two different samples of white gold differ in the relative amounts of gold and palladium that they contain. Both are uniform in composition throughout. Without knowing any more about the materials, how would you classify white gold?

Solution Let's use the scheme shown in Figure 1.9. Because the material is uniform throughout, it is homogeneous. Because its composition differs for the two samples, it cannot be a compound. Instead, it must be a homogeneous mixture. Gold and palladium can be said to form a solid solution with one another.

PRACTICE EXERCISE

Aspirin is composed of 60.0% carbon, 4.5% hydrogen, and 35.5% oxygen by mass, regardless of its source. Is aspirin a mixture or a compound?

Answer: a compound because of its constant composition


Figure 1.9 Classification scheme for matter. At the chemical level all matter is classified ultimately as either elements or compounds.