Chemistry Reference
In-Depth Information
Boiling temperature of water.
Students usually know 100
C as the boiling
temperature of water, but they are not aware of the fact that this value is only
valid for normal air pressure. A number of impressive mismatches can be generated
to introduce this relationship. You can tell the story of high altitude climbers,
observing in the altitude of 5,000 m the boiling temperature of water at 92
C
instead at 100
C. In addition you might demonstrate a corresponding experiment,
which shows the dependence of the boiling temperature on pressure: you decide the
presentation mode of the anomaly, and type or strength of the incongruence. The
relationship between pressure and boiling point can be demonstrated by the direct
connection of the water jet pump to an apparatus for boiling water. You can also
take a flask half filled with water, fill the flask totally with water vapor by boiling
the water and expelling all air, closing the flask and cooling it with a wet cloth: the
water begins to boil at temperatures below 100
C. This “boiling by cooling” is an
interesting and very extensive cognitive conflict for students (see E2.2).
Solubility of carbon dioxide.
Students know the sparkling appearance of the gas
carbon dioxide when dissolving a mineral tablet in water and they have the idea that
each tablet will generate the same specific volume of gas. Fill a measuring cylinder
with water and open it under water in a half-filled water bowl. Dissolve a mineral
tablet under the cylinder and observe a gas volume of 70 mL (see E2.3). At this
point, the students may predict the volume of gas, if a second tablet is put into the
cylinder: they will say: “the same volume.” The second tablet develops almost
200 mL of carbon dioxide - the students will be astonished by the different volume
and begin to think, to remove the occurring anomaly.
Combustion.
The experiment “burning steel wool on the balance” (E1.6)
described in Chap. 1 exemplifies a classic cognitive conflict for young people
who throughout their life have always observed the decrease of mass during
combustion: a big bunch of wood becomes a little amount of ash, alcohol disappears
completely during combustion. You have to weigh a portion of steel wool, ignite it
with the burner, and then you ask the students to predict whether the portion of
burnt iron wool is lighter, heavier, or shows no change in mass. The likely
expectation “lighter” turns out to be wrong after the sample is weighed again: the
portion of burnt iron wool is heavier than before. The students will be highly
motivated to eliminate this incongruity by thinking with the help of the teacher of
solid iron oxide, a compound formed by oxygen from the air.
Extinguishing fires.
Students know very well that all fires in everyday life are
extinguished by water. When asked, how burning fat of a deep fryer or burning
metal shavings should be treated, the response of most students will be given on the
basis of their experience: “with water, of course.” If you do this, explosions of
violent flames occur in both cases (see E2.4), and the pupils are very surprised that
the fires are not extinguished. They are very motivated to consider appropriate
explanations for these observations.
Striking experimental effects
. “No motivation without emotion” is a shortened
statement of the previously described fact that interests and attitudes develop
optimal with positive emotions. Especially in the sciences, experimental effects
may trigger positive emotions
for persons of all ages. Therefore, many
