What is in the air?

Imagine you are an astronaut returning to Earth from space. You have been flying through space at great speed but as you approach the planet you start to slow down. Even so, you are still flying fast when the outside of your spacecraft starts to glow and you start to feel hot. What can be causing these changes which threaten your safe return? It is the Earth's atmosphere.

Out in space there are few particles of gas so you may travel as fast as you can. As you approach the Earth you encounter more and more particles of gas. They reach such a huge number that as you move through them they rub on the sides of your spacecraft. This action creates heat, just as rubbing your hands together on a cold day helps to keep you warm. However, the speed of your spacecraft and the number of gas particles make the temperature rise much higher. Spacecraft are equipped to cope with the heat. They have special panels, which burn away without harming you or damaging the rest of the craft. This means that when you have slowed down, you can land safely.

If you had a device for detecting gases in the atmosphere you would see some changes as you came into land. At the very edge of space you would have detected hydrogen and helium. These are gases that are also found in the Sun. As you come closer to the Earth the gases in the air change. Hydrogen is no longer found. It is replaced by nitrogen and oxygen. As you get closer still, the amounts of nitrogen and oxygen rise and the amount of helium falls to almost zero. You pass through a layer of ozone then find other gases including water vapour and carbon dioxide in the last twelve kilometres above the Earth. If you let your gas detector keep on working it would record small quantities of the following gases - argon, neon, xenon and krypton.

A strange thing happens when you open the door of the spacecraft and go outside. Although your gas detector is recording nine gases you cannot see any of them. You cannot smell or taste any of them either. This makes you think that the gas detector is faulty until you breathe in deeply and them breathe out. As you breathe in you can feel the air rushing down your air passages to your lungs. As you breathe out you can feel the air blow over your raised hand. If you blew up a balloon and tried to balance it on scales with an empty balloon, the inflated balloon would sink. This shows you that the air has weight and is really there.

If you could travel back in time and visit an early Earth your gas detector would provide you with different data. The gases present then were carbon dioxide, water vapour and ammonia. In time, with the coming of living things, the atmosphere changed to the mixture of gases it is today.

What kind of gas is ammonia?
The ammonia gas is colourless but has smell, which irritates the eyes and nose. Young babies produce some ammonia in their urine. It gives their nappies a strong smell. In the past ammonia has been used as smelling salts to revive people who had fainted.

What happened to the ammonia in the atmosphere of the early Earth?
When living things developed on the planet they broke down the ammonia gas. One of the things produced by this change was nitrogen. It replaced the ammonia.

Is nitrogen an important substance?
Yes, it is. It is used by living things to build their bodies. Bacteria change the nitrogen into a mineral called nitrate. This dissolves in soil water and is taken up by plants. The plants turn the nitrates into proteins. They use the proteins to make their shoots and roots. Animals feed on plants for their proteins. When plants and animals die, their bodies rot away and proteins produce nitrates again. Some bacteria use the nitrates for food and release nitrogen back into the air. So although nitrogen is not an active gas like oxygen, that allows things to burn, it is a vital substance which allows living things to survive on the planet.

Are the gases present in small amounts any use?
Yes, they are. Helium is a very lightweight gas. It is used to make weather balloons. They rise through the air carrying a portable weather station with them. As the balloon rises the weather station sends out radio messages about the conditions in the air. Argon is used in light bulbs because it stops the filament burning out quickly. Neon is used in advertising lights because it glows with a red light when electricity passes through it. Krypton is used in airport landing lights and xenon is used in a photographer's flashgun.

Why do mountaineers wear oxygen tanks when they climb high mountains?
You should think of the air as a thick layer over the surface of the Earth. It is made of a mixture of gases, and their particles push on each other. The gases at the bottom of the atmosphere close to the Earth's surface are squashed by the 1000 kilometres of gas above them. This squashing brings the particles closer together. As you move away from the level of the seas, the squashing of the gases becomes less strong and the particles spread out more. Our bodies work when we breathe in a certain amount of oxygen in every breath. On the top of a high mountain there is less oxygen in every breath than there is at sea level because the particles are more spread out. Our bodies do not really work when the amount of oxygen in the air is low so oxygen tanks are taken to help climb the highest peaks.

Aircraft fly higher than mountains. How do people breathe in them?
The air in an aircraft is pressurised. This means that it is squashed so the particles are the same distance apart as they are at sea level. People can breathe the pressurised air in aircraft with the same ease that they can breathe air on the ground.

What is air conditioning?
Air conditioning is the changing of the air so that it is a certain temperature, contains a certain amount of moisture and moves at a certain speed through a building or vehicle like a bus or car. It does not change the amounts of gases in the air. Air conditioning is used in stores to provide a pleasant atmosphere for shopping. It is used in factories to provide the best conditions for making certain materials such as cloth. One of the first air conditioning devices was a wet mat. Egyptians hung them inside buildings and let a wind blow over them. As the water evaporated from the mats it cooled the air.