Chemistry Reference
In-Depth Information
in the definition of temperature. Remember that we define temperature
as the average kinetic energy of the particles of a substance. Even in a sample
of water with a temperature of 22
o
C, there are some molecules of water
with exceptionally high kinetic energy, just as you can score a 100% on
your chemistry examination even if the class average is only 70%. When a
molecule of water that is close to the surface gains enough individual ki-
netic energy to escape the molecular attraction of its neighbors, it can en-
ter the gas phase, even when the temperature of the sample is below the
boiling point.
Can you see why having sweat or other water evaporate off the surface
of your skin can cool you off? What would happen to the average grade in
your chemistry class if your teacher removed all of the “A” grades from the
average? The class average, deprived of the highest grades, would go down.
What happens to the sample of water, when the molecules with the highest
temperature escape as gas? The average temperature of the sample, de-
prived of the “hottest” molecules, goes down, leaving you feeling cooler.
Gases have no definite shape and no definite volume. They will expand
to fill the container that they are made to occupy. Ten molecules of a gas
can occupy as much space as ten million molecules of a gas, as they move
freely around a container at relatively high speeds. The intermolecular
forces of attraction between molecules in the gas phase are so weak that
the particles are allowed to travel quite far away from each other. Gases
differ significantly from the other phases of matter in that, the particles of
a gas are often separated by very great distances. You already know that
most of an atom is made up of empty space. So, too, most of a gas sample is
made up of empty space, to the point that the size of a gas sample typically
has little to do with the size of the actual particles.
For example, if you look at a party balloon filled with helium, the ac-
tual helium atoms would occupy a very tiny fraction of that space if you
could get them to stop moving around. The balloon is really held open by
the pressure exerted by the helium atoms, as they crash into the walls of
the balloon at high speed. You can get a better understanding of this by
leaving a balloon in a freezer for a few hours. You will notice that the size
of the balloon decreases significantly, not because the atoms in the balloon
shrink or escape, but because the atoms of helium have lost some of their
kinetic energy and are not exerting as much pressure on the inside of the
balloon. As the balloon returns to room temperature, its volume will in-
crease, as the atoms of helium gain kinetic energy from the room.
