Chemistry Reference
In-Depth Information
Fig. 1.6
Mental model of a student regarding the combustion of magnesium [
8
]
Even advanced students with 3-4 years of chemistry instruction sometimes
cannot give up their destruction theory. Pfundt [
9
] reported about a student who
stated: “concerning the formula CO
2
, the black substance carbon should be pro-
duced from carbon dioxide, but it is impossible to gain a black solid from a colorless
gas” [
9
].
In every case the preconcepts concerning combustion processes have to be
discussed in lectures, especially regarding these two facts: the reaction with oxygen
as a part of air, and the formation of colorless and invisible gases through the
combustion of candles, wood, or paper. It is possible to show with experiments the
mass of oxygen that combines with portions of metals: iron wool reacts to solid iron
oxide and gets heavier by the mass of oxygen, and magnesium reacts to solid
magnesium oxide and the mass increases by reacting with oxygen (E1.6).
To show the same effect for candles or tea lights the masses of the gaseous
products water vapor and carbon dioxide are to be measured with an apparatus
absorbing both gases with the help of calcium oxide and sodium hydroxide (E1.7).
Both effects with metals and candles are shown in an
open apparatus
: the mass of
oxygen adds to the mass of substances before.
The most important step is to show the conservation of mass in a
closed
apparatus
. If the combustion of iron wool is carried out in a test tube and the
opening is closed with an air balloon, the educts iron and air can be weighed. After
heating the test tube and observing the formation of black iron oxide the mass is
determined a second time: the same mass can be observed (E1.8). In the case of
