Pressure is defined as the force per unit area. A gas exerts pressure on any surface it touches. Thus, air, which is a mixture of gases, exerts pressure on the surface of the earth. This pressure is called atmospheric pressure. It depends on the mass of air above the surface and thus will be affected by wind conditions, humidity and by the relative elevation, compared to sea level, at the surface of the earth where the measurement is made.
Pascal (Pa) is the SI unit of pressure, which is defined as the force of one Newton exerted on an area of one square meter (kg×m/sec2 / m2 = kg/m×sec2).
Atmospheric pressure can support a column of liquid, as shown in Fig. 1 below. Assume that all three tubes in the figure were originally filled with liquid, then inverted into the dishes of liquid, as shown. The pressure of the atmosphere on the surface of liquid in each dish is just balanced by the pressure due to its mass of the column of liquid above the surface of the dish. If the liquid in the tube exerted a greater pressure than the atmosphere, then the level of the liquid would fall until the remaining mass of liquid in the tube produced the same pressure as the atmosphere. If the atmosphere exerted a greater pressure, then the opposite would occur, the liquid would rise in the column until the forces were equal. The same atmospheric pressure is able to support a higher column of liquid 2 than of liquid 1. Since the pressure exerted by the liquid depends on its mass, liquid 2 must be less dense than liquid 1.
Measurement of the height of a column of liquid, supported by a gas, in a tube of uniform diameter, is a convenient way of measuring pressure. The very dense liquid metal mercury is usually used to measure pressure. Atmospheric pressure at sea level typically supports a column of mercury 760.0 mm high, which is defined as a pressure of 1 atm. The SI unit of pressure is the Pascal (Pa) but laboratory measurements are often expressed as mmHg, or mm of Hg, which is also defined as a torr.) Thus, 1 atmosphere of pressure would exert 101.3 kPa. The density of Hg is 13.6 g/ cm3 and that of water is about 1 g/ cm3. A column of mercury is therefore 13.6 times heavier than a column of water of the same height and diameter, so that the column of mercury will exert a pressure 13.6 times as great as the same column filled with water. Thus, a given atmospheric pressure can support a column of water which is 13.6 times as high as the column of mercury. This is one reason mercury is used in barometers and manometers for measuring pressure, a water barometer would have to be 34 feet high.
Most of the gas law studies were conducted in Europe where there was no central heating. Consequently, the laboratories were generally quite cold. Since the properties of gases were so dependent upon temperature and pressure, it became necessary to define ‘standard’ conditions for examining the gas laws. Over time, this became known as ‘standard temperature and pressure’, or STP and was initially defined as 0oC and 1 atmosphere of pressure. Because of the work done by Lord Kelvin, it became necessary to use the absolute temperature, or the Kelvin scale, and so STP became 273.15 K and 1 atmosphere pressure.