Developing the Computer Animation
To help students understand the chemical processes that
occur when a can filled with water is heated and then sealed
and cooled, we developed a computer animation depicting this
process. Before the can is heated it contains liquid water, and
the air space inside and outside the can contains air molecules
(N2, O2, and some H2O). When the can is heated, the water
boils and the water molecules enter the gas phase inside the
can, forcing most of the nitrogen and oxygen molecules out of
the can. When the can is sealed and cooled, the water vapor
condenses back to a liquid inside the can. This lowers the
pressure inside the can below atmospheric pressure, and the
can is crushed by the collisions of outer air molecules with
the surface of the can, which is depicted in Figure 1.
The animation was carefully constructed so that it would
correctly address the misconceptions identified from the students’
responses. For example, after the can was sealed and
cooled, the water molecules were shown as moving slower
than before; and eventually these molecules returned to the
liquid state, which is depicted by water molecules encased in
a circular water droplet (Fig. 1). Because the water molecules
were depicted as moving slower after the sample was capped
and cooled, this animation should dispel the belief that the
molecules inside the can are exerting more pressure on the
can than the outer molecules (i.e., that there is a pressure
buildup inside the can). The animation should also help
students realize that the condensation of the water vapor leads
to a decrease in the number of gas particles inside the can,
which translates into a decreased pressure. The pressure
differential between the decreased inner pressure and the
constant (atmospheric) outer pressure results in the gas
molecules on the outside colliding with the can’s surface more
often than the inner molecules, crushing the can.
The animation was tested on a second set of 86 students
enrolled in a first-semester introductory chemistry class
(experimental group), who received the same instruction
regarding gas laws and watched the same can-crushing
demonstration as the first set (control group), but in addition
viewed the computer animation. Both sets of students came
from the same population: students (predominantly natural
science majors) enrolled in an introductory chemistry course