The Foundations of Chemistry

water to form ice. The changes in energy that accompany these physical changes for wa-

ter are shown in Figure 1-5. At a pressure of one atmosphere, ice always melts at the same

temperature (0°C), and pure water always boils at the same temperature (100°C).

MIXTURES, SUBSTANCES, COMPOUNDS, AND ELEMENTS

Mixturesare combinations of two or more pure substances in which each substance re-

tains its own composition and properties. Almost every sample of matter that we ordi-

narily encounter is a mixture. The most easily recognized type of mixture is one in which

different portions of the sample have recognizably different properties. Such a mixture,

which is not uniform throughout, is called heterogeneous.Examples include mixtures of

salt and charcoal (in which two components with different colors can be distinguished

readily from each other by sight), foggy air (which includes a suspended mist of water

droplets), and vegetable soup. Another kind of mixture has uniform properties through-

out; such a mixture is described as a homogeneous mixtureand is also called a

solution.Examples include salt water; some alloys,which are homogeneous mixtures of

metals in the solid state; and air (free of particulate matter or mists). Air is a mixture of

gases. It is mainly nitrogen, oxygen, argon, carbon dioxide, and water vapor. There are

only trace amounts of other substances in the atmosphere.

An important characteristic of all mixtures is that they can have variable composition.

(For instance, we can make an infinite number of different mixtures of salt and sugar by

varying the relative amounts of the two components used.) Consequently, repeating the

same experiment on mixtures from different sources may give different results, whereas

the same treatment of a pure sample will always give the same results. When the distinc-

tion between homogeneous mixtures and pure substances was realized and methods were

developed (in the late 1700s) for separating mixtures and studying pure substances, con-

sistent results could be obtained. This resulted in reproducible chemical properties, which

formed the basis of real progress in the development of chemical theory.

1-

Figure 1-5 Changes in energy that accompany some physical changes for water. The

energy unit joules (J) is defined in Section 1-13.

10 CHAPTER 1: The Foundations of Chemistry

1.00 g ice at 0°C 1.00 g liq H 2 O

at 0°C

1.00 g liq H 2 O

at 100°C

1.00 g steam

at 100°C

334 J absorbed

334 J released

418 J absorbed

• 418 J released

+2260 J absorbed

–2260 J released

A heterogeneous mixture of two minerals: galena (black) and quartz (white).

By “composition of a mixture,” we
mean both the identities of the
substances present and their relative
amounts in the mixture.

The blue copper(II) sulfate solution in
Figure 1-4c is a homogeneous mixture.