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Pressure

One of the properties of a gas is its pressure. The pressure, P, is the force exerted by the gas divided by the area on which the force is exerted. Pressure = force/area Because of gravity our atmosphere exerts a downward force and therefore a pressure on Earth's Surface. The SI unit of force is kg m/s2 and is called the newton (N). The SI unit of pressure is N/m2. It is given the name pascal (Pa) after Blaise Pascal, a French scientist. The pascal is a very small unit of pressure and so pressures are usually given in kilopascals, (kPa). Chemists have traditionally used two other units of pressure, based on the mercury barometer. A barometer is an instrument for measuring the pressure of the atmosphere. Such a barometer can be made from a glass tube about one meter in length that is closed at one end. The tube is completely filled with mercury and inverted into a container that has additional mercury. At sea level the mercury in the tube falls to a height of about 760 mm above the level of mercury in the container. The height of the mercury column is a measure of the pressure of the atmosphere. This height is used to define a non-SI unit of pressure, which is mmHg, or millimeters of mercury, also called the torr (after Evangelista Torricelli, the inventor in 1643 of the mercury barometer.) The atmosphere (atm) is a related non-SI unit of pressure equal to exactly 760 mmHg. A summary of the relationship among the units of pressure is:

1 atm = 760 mmHg = 760 torr = 1.01325 x 105 Pa = 101.325 kPa = 14.7 lb/in2

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