Classification of Matter

Matter is made up of very tiny units called atoms. Each different type of atom is the building block of a different chemical element. Presently, the International Union of Pure and Applied Chemistry (IUPAC) recognizes 112 elements, and all matter is made up of just these types! The known elements range from common substances, such as carbon, iron, and silver, to uncommon ones, such as lutetium and thulium. About 90 of the elements can be obtained from natural sources. The remainder do not occur naturally and have been created only in laboratories. On the inside front cover you will find a complete listing of the elements and also a special tabular arrangement of the elements known as the periodic table. The periodic table is the chemist s directory of the elements. We will describe it in Chapter 2and use it throughout most of the text.

Chemical compounds are substances comprising atoms of two or more elements joined together. Scientists have identified millions of different chemical compounds. In some cases, we can isolate a molecule of a compound. A molecule is the smallest entity having the same proportions of the constituent atoms as does the compound as a whole. A molecule of water consists of three atoms: two hydrogen atoms joined to a single oxygen atom. A molecule of hydrogen peroxide has two hydrogen atoms and two oxygen atoms; the two oxygen atoms are joined together and one hydrogen atom is attached to each oxygen atom. By contrast, a molecule of the blood protein gamma globulin is made up of 19,996 atoms, but they are of just four types: carbon, hydrogen, oxygen, and nitrogen.

The composition and properties of an element or a compound are uniform throughout a given sample and from one sample to another. Elements and compounds are called substances. (In the chemical sense, the term substance should be used only for elements and compounds.) A mixture of substances can vary in composition and properties from one sample to another. One that is uniform in composition and properties throughout is said to be a homogeneous mixture or a solution. A given solution of sucrose (cane sugar) in water is uniformly sweet throughout the solution, but the sweetness of another sucrose solution may be rather different if the sugar and water are present in different proportions. Ordinary air is a homogeneous mixture of several gases, principally the elements nitrogen and oxygen. Seawater is a solution of the compounds water, sodium chloride (salt), and a host of others. Gasoline is a homogeneous mixture or solution of dozens of compounds.

Is it homogeneous or heterogeneous? When viewed through a microscope, homogenized milk is seen to consist of globules of fat dispersed in a watery medium. Homogenized milk is a heterogeneous mixture.

In heterogeneous mixtures sand and water, for example the components separate into distinct regions. Thus, the composition and physical properties vary from one part of the mixture to another. Salad dressing, a slab of concrete, and the leaf of a plant are all heterogeneous. It is usually easy to distinguish heterogeneous from homogeneous mixtures. A scheme for classifying matter into elements and compounds and homogeneous and heterogeneous mixtures is summarized in Figure below. 

Every sample of matter is either a single substance (an element or compound) or a mixture of substances. At the molecular level, an element consists of atoms of a single type and a compound consists of two or more different types of atoms, usually joined into molecules. In a homogeneous mixture, atoms or molecules are randomly mixed at the molecular level. In heterogeneous mixtures, the components are physically separated, as in a layer of octane molecules (a constituent of gasoline) floating on a layer of water molecules.

Separating Mixtures

A mixture can be separated into its components by appropriate physical means. Consider again the heterogeneous mixture of sand in water. When we pour this mixture into a funnel lined with porous filter paper, the water passes through and sand is retained on the paper. This process of separating a solid from the liquid in which it is suspended is called filtration (Figure). You will probably use this procedure in the laboratory. Conversely, we cannot separate a homogeneous mixture (solution) of copper (II) sulfate in water by filtration because all components pass through the paper. We can, however, boil the solution of copper (II) sulfate and water. In the process of distillation, a pure liquid is condensed from the vapor given off by a boiling solution. When all the water has been removed by boiling a solution of copper (II) sulfate in water, solid copper (II) sulfate remains behind.

Another method of separation available to modern chemists depends on the differing abilities of compounds to adhere to the surfaces of various solid substances, such as paper and starch. The technique of chromatography relies on this principle. The dramatic results that can be obtained with chromatography are illustrated by the separation of ink on a filter paper.

(a) Separation of a heterogeneous mixture by filtration: Solid copper(II) sulfate is retained on the filter paper, while liquid hexane passes through. (b) Separation of a homogeneous mixture by distillation: Copper(II) sulfate remains in the flask on the left as water passes to the flask on the right, by first evaporating and then condensing back to a liquid. (c) Separation of the components of ink using chromatography: A dark spot of black ink can be seen just above the water line as water moves up the paper. (d) Water has dissolved the colored components of the ink, and these components are retained in different regions on the paper according to their differing tendencies to adhere to the paper.

Decomposing Compounds

A chemical compound retains its identity during physical changes, but it can be decomposed into its constituent elements by chemical changes. The decomposition of compounds into their constituent elements is a more difficult matter than the mere physical separation of mixtures. The extraction of iron from iron oxide ores requires a blast furnace. The industrial production of pure magnesium from magnesium chloride requires electricity. It is generally easier to convert a compound into other compounds by a chemical reaction than it is to separate a compound into its constituent elements. For example, when heated, ammonium dichromate decomposes into the substances chromium (III) oxide, nitrogen, and water. This reaction, once used in movies to simulate a volcano, is illustrated in Figure below.

A chemical change: decomposition of ammonium dichromate

States of Matter

Matter is generally found in one of three states: solid, liquid, or gas. In a solid, atoms or molecules are in close contact, sometimes in a highly organized arrangement called a crystal. A solid has a definite shape. In a liquid, the atoms or molecules are usually separated by somewhat greater distances than in a solid. Movement of these atoms or molecules gives a liquid it’s most distinctive property the ability to flow, covering the bottom and assuming the shape of its container. In a gas, distances between atoms or molecules are much greater than in a liquid. A gas always expands to fill its container. Depending on conditions, a substance may exist in only one state of matter, or it may be present in two or three states. Thus, as the ice in a small pond begins to melt in the spring, water is in two states: solid and liquid (actually, three states if we also consider water vapor in the air above the pond). The three states of water are illustrated at two levels in Figure.

The picture shows a block of ice on a heated surface and the three states of water. The circular insets show how chemists conceive of these states microscopically, in terms of molecules with two hydrogen atoms joined to one of oxygen. In ice (a), the molecules are arranged in a regular pattern in a rigid framework. In liquid water (b), the molecules are rather closely packed but move freely. In gaseous water (c), the molecules are widely separated.

The macroscopic level refers to how we perceive matter with our eyes, through the outward appearance of objects. The microscopic level describes matter as chemists conceive of it in terms of atoms and molecules and their behavior. In this text, we will describe many macroscopic, observable properties of matter, but to explain these properties, we will often shift our view to the atomic or molecular level the microscopic level.