Chemistry Reference
In-Depth Information
Some decades ago it was assumed that students do not have any preconceptions or
knowledge in chemistry: good preparation for a chemistry lesson only had to
decide, which new terms were to be introduced in which order and with which
experiments or models. Empirical studies, however, showed that learners have
preconceptions for many topics and that these preconceptions do not match today's
scientific concepts. For that reason a first basic question of chemistry education is:
which preconceptions exist for which topics and how can we effect conceptual
change? Often the preconceptions are simply called “false” - without considering
that students make correct observations and create individual mental models on the
base of their observations. Therefore, these conceptions should better be called:
- Conceptions of everyday life
- Primary or prescientific conceptions
- Student precomprehension or preconception
- Misconcepts or misconceptions
Iron wool example
. One example might back up this opinion. A ball of light gray
shiny iron wool is to be weighed and heated in a roaring Bunsen flame (see
“Introduction”). It can be observed that the iron wool glows and turns black. If
students are asked whether this black substance is of equal weight or lighter or
heavier than before, then the majority of students reply that it should be lighter. The
reason for this assumption is that wood or charcoal disappear while burning and
only a small amount of ash remains; alcohol even burns completely and without any
residue left behind.
This experience has been gained for 12-15 years and is now transferred to every
combustion process: therefore it should not be called false, but primary or presci-
entific. It is advantageous to talk about preconceptions with the students before
introducing the scientific concept, or to introduce the scientific idea first and to
come afterwards to the images of the involved students. In every case apart from
talking it is necessary to show experiments - they may convince students more than
talking.
For the iron-wool experiment the mass of a portion of metal is to be weighed
before and after combustion, the higher mass is to be discussed on the background
of the mass of oxygen which reacts with iron and forms solid black iron oxide.
Increasing mass can also be observed in the experiment “candle on the balance,”
when the invisible gaseous combustion products water vapor and carbon dioxide
are chemically bound. A discussion on the basis of this experimental experience
may reduce primary conceptions for the benefit of lasting scientific concepts. But
these learning processes cannot be made in one lesson - they have to be developed
in continuously problem-oriented lessons on the combustion process during a
number of weeks.
Student conceptions on combustion astonishingly show elements of the historic
Phlogiston theory: apparently there exist parallels between student conceptions and
the development of historic perception processes in science. Therefore, it makes
sense to study the development of historical theories which made radical changes
