Chemistry Reference
In-Depth Information
5.2 Teaching Processes: Purpose, Selection Criteria,
and Forms of Experiments
The scientific purposes of experiments are also educational functions, as for
example the process of gaining understanding through setting up and proving
hypotheses. If students cannot interpret special unknown facts they can discover
the scientific interpretation by the “discovery teaching method”: a hypothesis is
required, isolated cases are to be deduced. These cases need examination through
planning, running, and analyzing experiments (see a detailed description of this
inquiry-based teaching by Schmidkunz and Lindemann [
15
]). In chemical educa-
tion experiments also have roles, which do not have an equivalent in the scientific
field. These purposes (P) will be discussed using examples of experiments (E) with
alkaline hydroxides and their solutions.
P1. Motivation.
E5.11: Sodium Hydroxide on the Balance
Students are familiar with weighing from their everyday life: they weigh a choco-
late bar to find out that it weighs exactly 100 g and its mass does not change in the
earth's gravitation field. In the suggested experiment, students can observe that
the mass of a special portion of substance increases while weighing. Regarding this
cognitive conflict, students are motivated to explain this phenomenon. Since the
second component can only be added from air and since the sodium hydroxide
pellets deliquesce aqueous, students might expect aerial water vapor. Sodium
hydroxide, indeed, absorbs water vapor from air: this characteristic of hydroxides
is called “hygroscopic.” But sodium hydroxide also reacts with aerial carbon
dioxide to form sodium carbonate. This experiment provides an interesting intro-
duction as well as the motivation to learn more about those reactions.
P2. Stimulate Curiosity.
E5.12: Reactions of Sodium Hydroxide with Air
Surprisingly, the balance shows the increase of the mass of sodium hydroxide (see
E5.11). Since the deliquescence of the sample is a hint for water vapor in the air, the
question arises, if other components of air are responsible for the mass increase, if
nitrogen, oxygen or carbon dioxide might also be responsible. The students are
asked to think of an apparatus to verify their suggestion. A simple way is to use test
tubes, which are filled with some pellets of sodium hydroxide and a particular gas -
connected to a gas syringe, which contains the same gas. The result: only carbon
dioxide reacts with solid sodium hydroxide as can be deduced from a volume
decrease in the gas syringe.
P3. Testing of Hypotheses.
E5.13: Reactions of Sodium Hydroxide Solution with Carbon Dioxide
Since pure water as well as solid sodium hydroxide react with carbon dioxide gas,
the solution in water - sodium hydroxide solution - may also react with carbon
dioxide. An hypothesis might be: sodium hydroxide solution dissolves carbon
