The Foundations of Chemistry

Because no two different substances have identical sets of chemical and physical prop-
erties under the same conditions, we are able to identify and distinguish among different
substances. For instance, water is the only clear, colorless liquid that freezes at 0°C, boils
at 100°C at one atmosphere of pressure, dissolves a wide variety of substances (e.g.,
copper(II) sulfate), and reacts violently with sodium (Figure 1-4). Table 1-1 compares sev-
eral physical properties of a few substances. A sample of any of these substances can be
distinguished from the others by observing their properties.

CHEMICAL AND PHYSICAL CHANGES

We described the reaction of magnesium as it burns in the oxygen of the air (see Figure
1-1). This reaction is a chemical change.In any chemical change,(1) one or more sub-
stances are used up (at least partially), (2) one or more new substances are formed, and
(3) energy is absorbed or released. As substances undergo chemical changes, they demon-
strate their chemical properties. A physical change,on the other hand, occurs with no
change in chemical composition.Physical properties are usually altered significantly as
matter undergoes physical changes (Figure 1-3). In addition, a physical change may
suggest that a chemical change has also taken place. For instance, a color change, a
warming, or the formation of a solid when two solutions are mixed could indicate a
chemical change.
Energy is always released or absorbed when chemical or physical changes occur. Energy
is required to melt ice, and energy is required to boil water. Conversely, the condensa-
tion of steam to form liquid water always liberates energy, as does the freezing of liquid

1-

One atmosphereof pressure is the

average atmospheric pressure at

sea level.

1-4 Chemical and Physical Changes 9

TABLE 1-1 Physical Properties of a Few Common Substances

(at 1 atm pressure)

Solubility at 25°C

(g/100 g)

Melting Boiling In In ethyl Density

Substance Point (°C) Point (°C) water alcohol (g/cm^3 )

acetic acid 16.6 118.1 infinite infinite 1.

benzene 5.5 80.1 0.07 infinite 0.

bromine 7.1 58.8 3.51 infinite 3.

iron 1530 3000 insoluble insoluble 7.

methane 182.5 161.5 0.0022 0.033 6.67 10 ^4

oxygen 218.8 183.0 0.0040 0.037 1.33 10 ^3

sodium chloride 801 1473 36.5 0.065 2.

water 0 100 — infinite 1.

See the Saunders Interactive

General Chemistry CD-ROM,

Screens 1.11, Chemical Change,

and 1.12, Chemical Change on the

Molecular Scale.