Tuesday, 22 December 2009

Implications Of This Model Of Exocharmic Demonstrations

The notion that some of the fascination of lecture
demonstrations results from the fact that they may be
discrepant events has an important implication:
Demonstrations don't have to be spectacular to be
effective. They should, however, contain an element
of the unexpected.

Let me offer an example, from my own experience.
When I took chemistry for the first time, I was told
that equal volumes of different gases contained the
same number of particles. Until I took physics, this
was the most absurd thing I had heard a teacher
claim to be true. I knew that gases contained empty
space, but I seriously underestimated the fraction of
the space that is empty. It therefore seemed
reasonable to expect that equal numbers of gas
particles of different size would occupy different
amounts of space. I now know I was in good
company; Dalton rejected Gay-Lussac's data on
combining volumes for the same reason. To me, and
to many of my contemporaries, Avogadro's
hypothesis was just as counterintuitive as it was to
John Dalton.

I am reasonably confident that I could have stated
Avogadro’s hypothesis, if asked to do so on an exam.
I am equally confident that I couldn’t have used
Avogadro’s hypothesis to solve a problem because I
didn’t really believe it to be true.

About 15 years ago, I learned a lecture
demonstration that provides a discrepant event that
confronts the intuitive model of gases I brought to
my first chemistry course.11 Start with a plastic 50-
mL Leur-lok syringe, a syringe cap, and a 10-penny
nail. Pull the plunger out of the barrel until the
volume reads 50 mL. Now drill a small hole
through one of the veins of the plunger into which
the nail can be inserted, as shown in Figure 1.

Push in the plunger until no gas remains in the
syringe, seal the syringe with a syringe cap, pull the
plunger back out of the barrel of the syringe, insert
the nail into the hole, and weigh the ‘empty’ syringe
to the nearest 0.001 grams with an analytical
balance. Fill the syringe with different gases* and
determine the weight of 50 mL of each gas. Now use
the molar mass of each gas to calculate the number
of gas particles in each sample.

Typical data obtained with this apparatus are given
in Table 1. Within experimental error, the number
of gas particles in each sample is the same. It might
still seem strange that equal volumes of different
gases contain the same number of particles, but it is
no longer possible to avoid this conclusion.

Although this demonstration isn’t as spectacular as
the thermite reaction, or one of the oscillating
clocks, it can still be exocharmic because it contains
an element of surprise for many students.

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