A gas sample obeys a number of laws that relate its volume to its pressure, temperature, and mass. How are these properties measured? Mass and volume are familiar concepts and can be measured with familiar apparatus. Temperature can be measured on any scale -- Celsius, Fahrenheit, or Kelvin; however, if the temperature is to be used in a calculation involving gases, the Kelvin scale must be used. Standard temperature for gases, the temperature at which the properties of different gases are compared, is 273 K (0°C).

Pressure is defined as force per unit area and is measured in units that have dimensions of force per unit area. For example, the air pressure in tires is measured in pounds per square inch (psi). The pressure of the atmosphere is frequently measured with a mercury barometer.

Pressure can be more easily understood if we consider how a barometer measures pressure. The basic features of a mercury barometer are shown in Figure 9.6. In preparing a barometer, a glass tube at least 760 mm long and closed at one end is filled with mercury and then carefully inverted into a pool of mercury. The level of the mercury in the column will fall slightly and then become steady.

FIGURE 9.6 A mercury barometer. The height of mercury in the column is proportional to the pressure of the atmosphere.

The height of the column of mercury measures the pressure of the atmosphere. To understand this concept, consider the pressure on the surface of the mercury pool at the base of the column. Above this surface rises the "sea" of air (the atmosphere) that surrounds the Earth. On each square centimeter of the surface, we can visualize a 20-km column of air pressing down. On the surface under the mercury column, the mercury is pressing down. The two pressures must be equal. If they were not, mercury would be flowing into or out of the column, and the height of the column would not be steady. The atmosphere must be exerting a pressure equal to that exerted by the mercury column. Remember that pressure is force per unit area. The total area under the atmosphere or under the column of mercury is not critical, because the force that is measured is the force on each unit of area under the column or the atmosphere, not the total force.

When this experiment is performed in dry air at sea level and at 0°C, the column of mercury is 760 mm high; therefore, we say that the atmosphere is exerting a pressure equal to that of 760 mm of mercury. This amount of pressure has been defined as one atmosphere (1 atm) of pressure and designated as standard pressure. Thus STP is used to mean standard temperature and pressure or standard conditions (0°C, 1 atm). The values of standard pressure measured in units other than atmospheres are shown:

Each of these relationships can be used as a conversion factor, as shown in the following problems.

In dry air at sea level, the average air pressure is 1 atm. Atmospheric pressure decreases as altitude increases, because the sea of air above becomes less dense. Our bodies become adjusted to the normal pressure of the altitude at which we live. Minor problems of adjustment can occur when we move from sea level to the mountains, and vice versa. Major problems develop at higher altitudes. Commercial jet-aircraft cabins must therefore be pressurized, because humans cannot survive the low pressure of the atmosphere at the altitudes at which jet aircraft fly. For the same reason, travelers in space must wear pressurized suits.

Barometers measure the pressure of the atmosphere. Manometers measure the pressure of isolated gas samples. Some manometers measure pressure with a column of mercury, like a mercury barometer. This type of manometer has a U-shaped tube partially filled with mercury (Figure 9.7). One end of the tube is open to a chamber holding a gas sample, and other end is open to the atmosphere. If the mercury level on the side of the tube open to the gas sample is lower than that on the side open to the atmosphere, the pressure of the gas is greater than that of the atmosphere by an amount equal to the difference in height between the two mercury columns. If the mercury level on the side of the gas sample is higher than that on the side open to the atmosphere, the pressure of the gas is less than the atmospheric pressure by the difference in the heights of the two columns.

FIGURE 9.7 A manometer. The height difference between the mercury levels in the two sides of the tube measures the pressure difference between the gas sample and the atmosphere.