Figure 19. Diffusion of HydrogenHere the H 2 in the beaker that is placed over the porous cup diffuses faster
through the cup than the air can diffuse out. Consequently, there is a pressure
buildup in the cup, which pushes the gas out through the water in the lower
beaker.The chemical properties of hydrogen are:- It burns in air or in oxygen, giving off large amounts of heat. Its high heat
of combustion makes it a good fuel. - It does not support ordinary combustion.
- It is a good reducing agent in that it withdraws oxygen from many hot
metal oxides.
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 altitudes, 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 millimeters in height. This average sea-