The Importance of Air

1Minutes, days, weeks

A person can survive for weeks without food and for a few days without water, but for only a few minutes without air. Air is just as important to a charcoal stove, a car engine and a power station as it is to you. Out on Lake Kivu, methane gas pumped up from the deep water is burned in air to make electricity for Rwanda's grid. Each gas in the air has its own job. Which gas does what, and what keeps their amounts steady?

2Oxygen: for breathing and burning

Respiration. Every living cell uses oxygen to release energy from food. For glucose:

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6(\mathrm{a}\mathrm{q})+6{\mathrm{O}}_2(\mathrm{g})\to 6\mathrm{C}{\mathrm{O}}_2(\mathrm{g})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{C\_6H\_\{12\}O\_6(aq)\; +\; 6O\_2(g)\; \backslash rightarrow\; 6CO\_2(g)\; +\; 6H\_2O(l)\}$$

Combustion. Fuels burn by combining with oxygen, giving out heat and light. Charcoal is mostly carbon, and methane is the main gas in Lake Kivu gas and in biogas:

$$\mathrm{C}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to \mathrm{C}{\mathrm{O}}_2(\mathrm{g})\backslash mathrm\{C(s)\; +\; O\_2(g)\; \backslash rightarrow\; CO\_2(g)\}$$

$$\mathrm{C}{\mathrm{H}}_4(\mathrm{g})+2{\mathrm{O}}_2(\mathrm{g})\to \mathrm{C}{\mathrm{O}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\backslash mathrm\{CH\_4(g)\; +\; 2O\_2(g)\; \backslash rightarrow\; CO\_2(g)\; +\; 2H\_2O(g)\}$$

Pure oxygen, separated from liquid air (Lesson 1), is given to patients in hospitals and is used with acetylene in welding torches. Senior Two looks at oxygen and its reactions in detail.

3Nitrogen: the quiet majority

Nitrogen is so unreactive that it is not used up by burning or breathing. That is useful: it dilutes the oxygen, so fires burn at a steady rate instead of fiercely. Plants need nitrogen to make proteins but cannot use the gas directly; lightning and bacteria in the roots of beans and other legumes turn it into compounds that plants can absorb. Industry turns nitrogen from the air into ammonia, which is made into fertilisers such as the urea spread on Rwandan maize and rice fields:

$${\mathrm{N}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\to 2\mathrm{N}{\mathrm{H}}_3(\mathrm{g})\backslash mathrm\{N\_2(g)\; +\; 3H\_2(g)\; \backslash rightarrow\; 2NH\_3(g)\}$$

Because it is unreactive, nitrogen is also pumped into packets of crisps and milk powder to keep out oxygen, which would make the food go stale.

4Carbon dioxide, noble gases and water vapour

Carbon dioxide is only 0.04% of air, but green plants use it to make their food by photosynthesis, releasing oxygen:

$$6\mathrm{C}{\mathrm{O}}_2(\mathrm{g})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6(\mathrm{a}\mathrm{q})+6{\mathrm{O}}_2(\mathrm{g})\backslash mathrm\{6CO\_2(g)\; +\; 6H\_2O(l)\; \backslash rightarrow\; C\_6H\_\{12\}O\_6(aq)\; +\; 6O\_2(g)\}$$

It also traps heat and keeps the Earth warm enough for life (too much of it causes global warming), puts out fires, and puts the fizz in soft drinks. Argon fills some light bulbs and shields metal from the air during welding; neon glows in advertising signs; helium fills balloons. Water vapour brings rain (Unit 9).

Table 3.1: The importance of the gases in air

5Keeping the balance

Respiration and burning remove oxygen from the air and add carbon dioxide; photosynthesis does the reverse (Figure 3.1). For thousands of years the two roughly balanced, so the amounts stayed steady. Today, burning large amounts of fuel and cutting down forests add carbon dioxide faster than plants can remove it, which is why its percentage is slowly rising.

Figure 3.1: How living things and fuels exchange oxygen and carbon dioxide with the air.