Chemistry Reference
In-Depth Information
E1.12.
Composition of Air
Problem: The students use the terms “good air” and “stale air” in their
everyday language, but they do not think about the oxygen content. For this
reason it is important to run experiments demonstrating the composition of
air. It has to be explained to the students why a pure metal or phosphorous is
being used: both are reacting with oxygen to form solid oxides, they take the
oxygen out of the air and release other gases of the air mixture.
Material: Two 100 mL gas syringes, combustion tube, glass bowl, small gas
cylinder with cover glass, splint, bell jar, combustion spoon with plug, ruler;
iron wool, red phosphorus (F).
Procedure: (a) A combustion apparatus is to be built (see picture). The
trapped air with a volume of 100 mL is to be lead over the heated iron wool
a couple of times. The volume of the remaining gas is to be measured. The
remaining gas is to be collected in the small gas jar pneumatically and tested
with the burning splint.
(b) A bell jar stands in the water of the glass bowl (picture). A spatula tip of
phosphorus is to be put into the combustion spoon and ignited; the spoon is
to be fixed inside the bell jar. The rising water level can be observed in the
gas jar, and the remaining part of gas can be estimated.
Observation: (a) Iron wool glows and forms a black product, the gas volume
decreases to 80 mL, and the remaining gas smothers a burning splint.
(b) Phosphorus burns for some time, a white smoke forms, the flame goes
out, the water level in the bell jar rises, and the volume of the remaining gas
is about 80% from before.
