General Characteristics of Gases
Measuring the Pressure of a Gas
Pressure is defined as force per unit area. With respect to the atmosphere, pressure is the result of the weight of a mixture of gases. This pressure, which is called atmospheric pressure, air pressure, or barometric pressure, is approximately equal to the weight of a kilogram mass on every square centimeter of surface exposed to it. This weight is about 10 newtons.
The pressure of the atmosphere varies with altitude. At higher alititudes, the weight of the overlying atmosphere is less, so the pressure is less, Air pressure also varies somewhat with weather conditions as low- and high-pressure areas move with weather fronts. On the average, however, the air pressure at sea level can support a column of mercury 760 mm in height. This average sea-level air pressure is known as normal atmospheric pressure, also called standard pressure.
The instrument most commonly used for measuring air pressure is the mercury barometer. In a mercury barometer, the atmospheric pressure exerted on the mercury is forced into a tube in which the column of mercury is measured in mm.
In gas-law problems pressure may be expressed in various units. One standard atmosphere (1 atm) is equal to 760 milimeter of mercury 9760 mm Hg) or 760 torr, a unit named for Evangelista Torricelli. In the SI system, the unit of pressure is the pascal (Pa), named in honor of the scientist of the same name. Standard pressure in pascals is 101,325 Pa or 101.325 kPa
Kinetic-Molecular Theory
By indirect observations, the kinetic-molecular theory has been arrived at to explain the forces between molecules and teh energy the molecules possess. There are three basic assumptions t the kinetic-molecular theory:
1. Matter in all its forms (solid, liquid, and gas) is composed of extremely small particles. In may cases these are called molecules. The space occupied by the gas particle themselves is ignored in comparison withe volume of the space they occupy.
2. The particles of matter are in constant motion. In solids, this motion is restricted to a small space. In liquids, the particles have a more random pattern but still are restricted to a kind of rolling over one another. In a gas, the particles are in continuous, random, straight-line motion.
3. When these particles collide with each other or withe the walls of the container, there is no loss of energy.
Some Particular Properties of Gases
As the temperature of a gas is increased, its kinetic energy is increased, thereby increasing the random motion. At a particular temperature not all the particles have the same kinetic energy, but the temperature is a measure of the average kinetic energy of teh particles. A graph of the various kinetic energies would resemble a normal bell-shaped curve with the average found at the peak of the curve.
When the temperature is lowered, the gas reaches a point at which the kinetic energy can no longer overcome the attractive forces between the particles (or molecules) and teh gas condenses to a liquid. The temperature at which this condensation occurs is related to the type of substance the gas is composed of and the type of bonding in the molecules themselves. This relationship of bond type to condensation point (or boiling point) is pointed out in the Bonding section.