Chemistry Reference
In-Depth Information
burning wood these measurements are also possible: some matches are ignited in
the same way as the iron wool, and the combustion products weigh the same as the
educts (E1.8). The most convincing experiment is burning a few small pieces of
charcoal in a closed flask containing oxygen and observing that the pieces disappear
totally: although the coal “is gone” and nothing is to see - the flask weighs the same
before and after combustion (E1.9). Students will explain this with the formation of
the colorless gas carbon dioxide and the mass of this gas portion.
Air and other gases
. Even scientists from previous centuries did neither perceive
air as a substance nor did they differentiate the colorless gases mixed in the air. It is
similarly difficult for children today. Since air is always weightlessly around us and
warm air rises up, air is not perceived to have a mass and is not being seen as a
substance in children's eyes.
M
unch [
10
] could show with an empirical inquiry that half of the students
between 10 and 16 years of age believe that a soccer ball, which they pump up
hard with a regular air pump, is lighter than the one that is only pumped up to a
smaller extent. The mass of a certain amount of air and thus air density can be
demonstrated to the students quickly and convincingly: a glass ball is being
evacuated with a water jet pump and weighed with an analytical balance. After
100 mL of air are transferred to the glass ball from a gas syringe, it is weighed
again: a mass of 0.13 g is measured (E1.10). So air is identified as a substance with
the density of 1.3 g/L.
If densities of other gases are determined in the same way, air and other gases
can be differentiated by their densities. Oxygen, nitrogen, hydrogen, carbon diox-
ide, or butane can also be introduced experimentally with the help of a burning or
glowing wooden splint (E1.11). Additionally, the oxygen content of air of about
20% by volume can be shown by demonstrating the reaction of iron wool or
phosphorus to form solid oxides in a closed apparatus (E1.12). One can even
show that from gaseous, colorless carbon dioxide the black solid carbon can be
obtained: magnesium reacts with carbon dioxide by forming white magnesium
oxide and black solid carbon spots (E1.13).
Many false conceptions regarding gases are derived from everyday language.
Weerda [
11
] listed the following student comments:
- Fresh air is “good” air; air without oxygen is “bad”
- Chimneys need “fresh air” and give “used air”; cars give off “exhaust gas”
- Colorless gases are “air” or “like air”; water evaporates “to form air”
- Gases are combustible, are there to cook and to heat
- Gases are dangerous, explosive, and toxic
- Gases “can be liquid”; lighters contain “liquefied gas”
€
A simple experiment can be run to clarify the term “liquefied gas.” By air
displacement butane is to be filled into a gas liquefaction pump (E1.14), strong
pressure has to be placed on butane gas with the help of the piston: a drop of liquid
butane forms in the presence of gaseous butane. It needs to be clarified that lighters
and camping gas cartridges contain liquid
and
gaseous butane. It is also helpful to
