Oxygen, the Active Part of Air

1Which part of the air does the work?

A charcoal stove needs air to burn. An iron roof sheet rusts only where air and rain reach it. You breathe air in and out all day. Yet four-fifths of the air seems to do nothing at all. Chemists call oxygen the active part of air, because it is the part that takes part in burning, rusting and breathing. Can we prove that it makes up about one-fifth of air, and measure that fraction ourselves?

2Experiment 1 (teacher demonstration): copper and gas syringes

1. Connect two 100 cm³ gas syringes to a silica tube packed with copper turnings (Figure 2.1). Draw exactly 100 cm³ of air into syringe A; syringe B is pushed in.
2. Heat the copper strongly. Push the air slowly from A to B and back again, many times, over the hot copper.
3. Stop when the volume no longer changes. Turn off the burner, let the apparatus cool to room temperature, push all the gas into one syringe and read its volume.

Figure 2.1: Measuring the oxygen in air with hot copper, before and after.

3What happens

The pink-brown copper turns black as it combines with the oxygen of the air to form copper(II) oxide, a solid:

$$2\mathrm{C}\mathrm{u}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2\mathrm{C}\mathrm{u}\mathrm{O}(\mathrm{s})\backslash mathrm\{2Cu(s)\; +\; O\_2(g)\; \backslash rightarrow\; 2CuO(s)\}$$

Because the oxygen becomes part of a solid, the volume of gas falls. The nitrogen and the other gases do not react with hot copper, so they are left. The fall in volume is the volume of oxygen that was in the air:

$$\text{percentage of oxygen}=\frac{\text{initial volume}-\text{final volume}}{\text{initial volume}}\times 100\backslash text\{percentage\; of\; oxygen\}\; =\; \backslash frac\{\backslash text\{initial\; volume\}\; -\; \backslash text\{final\; volume\}\}\{\backslash text\{initial\; volume\}\}\; \backslash times\; 100$$

A burning splint put into the gas that is left goes out at once: it is mainly nitrogen.

4Experiment 2 (class practical): iron wool over water

Wet a small ball of iron wool (steel wool without soap) and push it to the closed end of a graduated tube. Stand the tube upside down in a beaker of water and read the volume of air in it, with the water levels inside and outside the tube equal. Leave it for a week, then read the volume again (Figure 2.2). Iron rusts, using the oxygen, and water rises up the tube to take the oxygen's place:

$$4\mathrm{F}\mathrm{e}(\mathrm{s})+3{\mathrm{O}}_2(\mathrm{g})+2\mathrm{x}{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\cdot \mathrm{x}{\mathrm{H}}_2\mathrm{O}(\mathrm{s})\backslash mathrm\{4Fe(s)\; +\; 3O\_2(g)\; +\; 2xH\_2O(l)\; \backslash rightarrow\; 2Fe\_2O\_3\backslash cdot\; xH\_2O(s)\}$$

Figure 2.2: Iron wool rusting in a tube of air over water, at the start and one week later.

Table 2.1: The two methods compared

5Why not a candle in a jar?

A common demonstration stands a burning candle in water and covers it with a jar; the water rises and the candle goes out. It is not a good way to measure oxygen. The candle goes out before all the oxygen is used; burning makes carbon dioxide and water vapour, which take up space; and the hot gas expands and bubbles out under the jar at the start, so the water rises further when it cools. The result can be far from 21% for reasons that have nothing to do with oxygen.

6Oxygen in Rwanda's air

The air in Kigali, about 1,500 m above sea level, is still 21% oxygen. But the air pressure there is only about 85% of the pressure at sea level, so each breath holds fewer oxygen molecules. That is why visitors from the coast feel short of breath when they first climb Kigali's hills, and why athletes sometimes train at altitude. Rusting is the same reaction as Experiment 2: an iron roof sheet corrodes where its protective coating is scratched, because air and rain reach the bare metal.