JSSS
Teacher support material
Air
OC21,22
Until the 17th century air was considered to be an element. Karl Scheele, in Sweden in 1771,
separated oxygen from the air. Daniel Rutherford, in Edinburgh in 1772, was the first to
recognise nitrogen as a distinct gas.
Air in the lower atmosphere is now recognised to be a mixture of nitrogen (approximately 78%),
oxygen, carbon dioxide, water and other gases (including the noble gases, helium, neon, argon
(the most abundant), krypton, xenon and radon). The constituents of air may be separated by
liquefying it, at – 1900 C if not under pressure, and boiling off the individual gases at different
temperatures. This method is used to produce oxygen for industrial and medical uses.
Nitrogen
Because of the triple bond between the two nitrogen atoms, the nitrogen molecule is very stable.
Because of this, nitrogen does not react easily to form compounds. This explains why, despite
the presence of so much nitrogen in the air, nitrogen compounds need to be spread on land to
improve crop yields. Some bacteria are capable of fixing nitrogen, i.e. taking the nitrogen from
the air and converting it to compounds usable by plants. Most of these bacteria will only fix
nitrogen in a very low oxygen environment and so tend to be associated with the roots of plants.
Legumes, including peas, lupins and an African tree, called the camel thorn have nodules in their
roots which contain bacteria that fix nitrogen. The remains of these legumes can then increase
the nitrogen content of the soil. Some non legumes, including alder and bog myrtle have similar
arrangements with other nitrogen fixing bacteria.
Oxygen
Misconception
The percentage of oxygen in the air can not be accurately measured by the traditional method of
burning a floating candle in a closed jar over water.
When candle wax is burned more than 60% of the oxygen molecules are replaced by carbon
dioxide molecules. If candle wax is taken as C21H44, then the chemical equation is
C21H44 + 32 O2 → 21 CO2 + 22 H2O.
The only reason that the volume of the air appears to be reduced by about one fifth is due to
expansion of the air as it is heated. Some of this air escapes before the bottle is sealed. This can
easily be shown using plastic bottles with “sporty” tops as it is possible to seal these before the
air in the bottle has time to expand.
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JSSS
Teacher support material
Show that approximately one fifth of the air is oxygen OC14
Method 1
This can be done by using an oxygen sensor connected to a data logger or a computer. This will
immediately show the concentration of oxygen in the air. To make it more investigative the
logger can be programmed to record the percentage of oxygen in the air over a period of time
Method 2
Fact When iron rusts it uses up oxygen. When rusting of iron takes place in a closed container,
in which there is sufficient iron and water present, rusting will continue until all of the oxygen in
the air is used up.
Apparatus Steel wool, plastic syringe and Leur-Lock.
Procedure

Remove the plunger from the syringe and put some moist steel wool into it.

Attach a Luer-Lock to the syringe.

Invert the syringe into a trough of water and wait for the levels of water inside and
outside the syringe to equalise.

Seal the syringe using the Luer-Lock.

Leave for some time.
1. What happens to the level of the water in the syringe? _________________________
2. What does this indicate about the air? ______________________________________
3. What fraction of the air is used up? ________________________________________
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JSSS
Teacher support material
Method 3 - teacher demonstration.
Oxygen reacts with heated copper to produce copper oxide. If air is repeatedly passed over
heated copper powder, all the oxygen will be consumed.
Apparatus
Two 100 cm3 gas syringes, glass tube, Bunsen burner and copper powder.
Copper powder
Silica tube
Gas syringe 2
Gas syringe 1
Bunsen burner
Procedure

Use the plunger to remove all the air from one syringe

Pull the plunger so that there is 100 cm3 of air in the other syringe.

Put some copper powder into the glass tube and block it loosely at each end with some
wire wool.

Connect the 2 gas syringes to either end of the glass tube to ensure an air tight seal.

Heat the glass tube and copper with the Bunsen.

Pass the air over the hot copper, for about ten minutes, by pushing it from one syringe
to another.

Remove the Bunsen and allow the apparatus to cool.

Note the change in the colour of the copper.

Push the plunger on one of the syringes so that it is empty and read the volume of air
in the other syringe. Record your result.

The reduction in the volume of air in the syringe is the volume of oxygen that was in
the air.
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