Chemistry Reference
In-Depth Information
It can be shown in an experiment (E1.2) that the metal can be recovered from
black silver sulfide or black copper oxide. Additionally, it can be demonstrated that
the oxides or sulfides can be formed again in a reaction of metal with oxygen or
sulfur: chemical reactions are reversible. But this leads to the question of what
survives in the compound and what can be formed again. The answer is difficult and
can only be formulated on the level of atomic models: the nuclei of metal atoms and
nonmetal atoms do not alter in the reaction. Also the question of the difference
between red copper oxide (Cu
2
O) and black copper oxide (CuO) can only be
answered on the basis of Dalton's atomic model: with the ratio of atoms or ions
in different compounds and with different chemical structures of these substances
(see also Chap. 6).
Mixing and unmixing
. These images play an important role in students' interpre-
tation of the conversion of materials [
7
]:
- Silver sulfide is
mixed
from silver and sulfur
- Copper oxide
contains
copper and oxygen
- Water
consists
of hydrogen and oxygen
- A hydrocarbon
is built of
carbon and hydrogen
These expressions suggest a concept of mixture, similar to what has been
discussed by the Greek philosophers. A heterogeneous mixture of two substances,
like one of copper crystals and sulfur crystals, has to be shown to the learner and is
to be compared to homogeneous copper sulfide. The formulation “copper sulfide is
a compound of the elements” or “hydrogen and oxygen exist chemically bound in
water” is to be preferred to the ones stated above. Later, on the level of Dalton's
atomic model one can talk of H
2
O molecules “consisting of” H atoms and O atoms
or “containing” those atoms.
Conservation of mass
. Most students do not know of the alchemists and their
wish to turn mercury or lead into gold. But they see the “green copper roof,” and are
thinking that red copper transformed over a long period of time into “green copper,”
that silver turns into “black silver.” These arguments show that students have the
idea of the change of substances, but no idea of a chemical reaction conserving the
elements in their compounds, or gaining back the elements from compounds by
chemical reactions.
Similar preconcepts exist concerning the evaporation of water or ethanol to the
disappearance of metals in the reaction with acids, or to the removal of a fat stains
from clothes: “the water, the metal, the fat has gone away” [
7
]. Young students do
not think of water or ethanol vapor going into the air, of metals reacting with acids
forming a salt solution and hydrogen gas, or of fat that dissolves in petrol: the
substances have simply “gone away.” So the teacher has to demonstrate that, for
example, acetone is evaporating at room temperature forming a big volume of
acetone vapor in a syringe (E1.3), that magnesium is dissolving in hydrochloric acid
forming a magnesium chloride solution and the gas hydrogen (E1.4), that the
disappearance of fat is interpreted by dissolving of the fat in petrol (E1.5). Through
these experiments the “destruction” idea of young students can be turned into a
“conservation” idea.
