Chemistry Reference
In-Depth Information
condition of ideal gases. For example, if you have a sample of
hydrogen gas at high temperature and low pressure, the par-
ticles of the gas don't occupy a very significant amount of the
space occupied by the sample. The lower the density of a real
gas, the closer it will approach this characteristic of an ideal gas.
2. Ideal gases are treated as if there is no attraction between par-
ticles. Again, many real gases can approach this characteristic
of ideal gases. Because intermolecular forces decrease over
distance, less dense gases are more like ideal gases. Any char-
acteristic that results in weaker intermolecular forces will make
a real gas more like an ideal gas. For example, non-polar gases
are more like ideal gases than polar gases, if all other variables
are the same. Small molecules, with less subatomic particles
and taking up less significant space, tend to come closest to
approaching ideal gas characteristics.
Pressure is defined as a force exerted over a given area, as shown in the
following formula:
force
area
Pressure =
Notice that the formula for pressure looks similar to the formula for
density. In this case the pressure is directly related to the force, so an in-
crease in force leads to an increase in pressure. Pressure is inversely pro-
portional to the surface area, so that an increase in the area will lead to a
decrease in the pressure.
You have experienced the consequences of this formula in many real-
life situations. Think of the shape of a nail. The head of a nail is relatively
broad, meaning that the area that you strike with a hammer is relatively
large. The formula for pressure shows us that, by increasing the area of the
surface of the head of the nail, we decrease the pressure per unit of area.
The tip of a nail, which is designed to plunge into a wood surface, is rela-
tively small. The pressure formula shows us that when the area is small, we
exert a relatively large pressure per unit of area.
Another example of where this formula comes into play involves a per-
son standing on a frozen lake when the ice starts to crack. You may have
heard that it is advisable to lie down on the surface of the ice, if you find
yourself in this situation. The logic behind this can be seen in the pressure
formula. By lying down on the surface of the ice, you will increase the area
of your body that is in contact with the ice. Because the pressure that you
exert on the surface of the ice is inversely proportional to the area of contact,
you will be less likely to break through.
