Gases - Fundamentals of Chemistry - page 317

finger is removed, the mercury level falls. There are two forces on the mercury
in the tube: the force of gravity pulling down and the force due to air pressure
pushing up. When these two forces balance each other, the mercury in the tube
stops falling. The greater the air pressure, the higher the mercury stands in the
tube above the level of mercury in the dish. A simple unit of gas pressure is
millimeters of mercury (abbreviated mm Hg). The vertical height of the mer-
cury column, corresponding to h in Figure 12.1, is a measure of pressure. A unit
equal to the pressure necessary to support 1 mm Hg is the torr, named for Evan-
gelista Torricelli (1608-1647), an Italian physicist who discovered the principle
of the barometer. At sea level at on a “normal” day, the atmosphere can
hold the mercury at a height of 760 mm; its pressure is 760 torr. Another unit
of gas pressure, the atmosphere (atm), or standard atmosphere, is therefore
defined as 760 torr:

Closed end

Nearly
perfect
vacuum

0°C

1 atm 760 torr 760 mm Hg

Note the difference between 1 atmosphere pressure and atmospheric pressure.
The former is a constant. The latter varies widely from place to place and also
varies over time at the same place. Atmospheric pressure is often referred to as
barometric pressure.
The SI unit of pressure is the pascal (Pa), which is such a small unit that
the kilopascal (kPa) is used for atmospheric pressures under ordinary conditions.
Chemists do not often use either the pascal or kilopascal. Since some modern
reference topics present pressure data in these units, however, this conversion
is presented here:

Mercury

h

10 5 Pa 1.000 atm 1.013 101.3 kPa

Snapshot Review

❒ Pressure is defined as force per unit area.

❒ Fluids (gases and liquids) exert a pressure at any point within them in

all directions.

ChemSkill Builder 7.1

❒ We can measure a gas pressure by the vertical height of mercury it

can hold up.

❒ The most important units of pressure in this course are the

atmosphere (atm) and the torr (1 atm 760 torr)

Air pressure Air pressure

A. If we double the force exerted on a wall and also double the area of the
wall, what happens to the pressure?
B. Convert 782 torr to atmospheres.

Mercury Petri dish

Figure 12.1 Simple Barometer (not drawn to scale)
A simple barometer is made with a long tube closed at one end. Fill the tube completely with mercury. Hold a finger over the
open end, invert the tube and put the open end under the surface of mercury in a Petri dish, and hold the tube vertically. The
mercury in the tube falls to a height, h, determined by the pressure of the air on the surface of the mercury in the Petri dish.
There is essentially no gas pressure in the tube above the mercury. Caution: Mercury is toxic; use rubber gloves when
handling it.