The Foundations of Chemistry

1.Gases can be compressed into smaller volumes; that is, their densities can be

increased by applying increased pressure.

2.Gases exert pressure on their surroundings; in turn, pressure must be exerted to

confine gases.

3.Gases expand without limits, and so gas samples completely and uniformly occupy

the volume of any container.

4.Gases diffuse into one another, and so samples of gas placed in the same container

mix completely. Conversely, different gases in a mixture do not separate on standing.

5.The amounts and properties of gases are described in terms of temperature, pres-

sure, the volume occupied, and the number of molecules present. For example, a

sample of gas occupies a greater volume when hot than it does when cold at the

same pressure, but the number of molecules does not change.

PRESSURE

Pressureis defined as force per unit area—for example, pounds per square inch (lb/in.^2 ),

commonly known as psi.Pressure may be expressed in many different units, as we shall

see. The mercury barometeris a simple device for measuring atmospheric pressures.

Figure 12-1a illustrates the “heart” of the mercury barometer. A glass tube (about 800

mm long) is sealed at one end, filled with mercury, and then carefully inverted into a dish

of mercury without air being allowed to enter. The mercury in the tube falls to the level

at which the pressure of the air on the surface of the mercury in the dish equals the grav-

itational pull downward on the mercury in the tube. The air pressure is measured in terms

12-3

Investigating four variables at once
is difficult. In Sections 12-4 through
12-8 we shall see how to study these
variables two at a time. Section 12-9
will consolidate these descriptions into
a single relationship, the ideal gas
equation.

436 CHAPTER 12: Gases and the Kinetic–Molecular Theory

Diffusion of bromine vapor in air. Some liquid bromine (dark reddish brown) was placed in

the small inner bottle. As the liquid evaporated, the resulting reddish brown gas diffused.

Br 2 (gas)

Br 2 (liquid)

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 12.2, Properties of Gases:
Pressure.