S-Cool Revision Summary

S-Cool Revision Summary

The air is always around us but we pay it little attention. It contains 78% nitrogen, 20% oxygen and 0.04% carbon dioxide and is essential for our lives.

Yet we do not seem to be too bothered about it and go on pumping all sorts of nasty things into it.

The atmosphere is a complex thing, which we do not realise is important to us. Recently we have begun to realise just how important one part of it is to us, the ozone layer.

Ozone is a molecule made up of three oxygen atoms. The oxygen molecules present in air has only 2 oxygen atoms in it. So what is so different about ozone?

If you go up to 12-30 miles above the Earth's surface you find a layer of air which contains small amounts of ozone gas. The ozone molecules absorb the ultraviolet (UV) rays that are emitted from the Sun and which are dangerous to all living things. UV rays can increase the risk of cancers forming, such as skin cancer.

So ozone gives us a protective blanket high above us that reduces the amount of UV rays that get through to the Earth's surface. Therefore ozone helps protect us.

However the atmosphere's protective effect is under threat from things we do.

CFC's are "Chloro-fluoro-carbons" to their friends. Although nowadays they don't have many friends.

We thought that they were great molecules at one time. They used to do useful things like running cool machines such as 'fridges' and air-conditioning units. They were in aerosols and polystyrene foam.

However, we now know that they break up ozone molecules and so destroy the layer that protects us from UV rays from the Sun. Not a good idea!

We all burn fossil fuels. We do it directly by burning coal or driving cars. We also indirectly burn fossil fuels when we use electricity generated using them.

Our cars and power stations are responsible for most of the fossil fuel burning. The gases released include carbon dioxide, sulphur dioxide and oxides of nitrogen (NO2, NO3, etc.). Up in the atmosphere these gases are dissolved in water and cause acid rain.

The gases also mess up the atmosphere. The extra carbon dioxide causes the greenhouse effect.

Not only do our cars burn a fossil fuel but the older "4 star" petrol contains the element lead.

This leaded petrol was used because it made engines run smoothly. But when it was burned it released the lead into the air. When breathed in it could damage our nervous system. Not nice.

Thankfully, more modern engines can burn "lead-free" petrol and so there will be less lead in the air.

The greenhouse effect is where the temperature of the Earth increases. This happens as less heat is radiated back from the Earth than is received from the Sun.

The atmosphere becomes clogged up with "greenhouse gases" such as carbon dioxide from burning fossil fuels and methane. This lets the heat from the Sun in but stops some of it from be radiated back out again.

Carbon dioxide

Normally there is a balance in the amount of carbon dioxide in the atmosphere. It is produced by respiration and used up in photosynthesis. So across the whole planet there is a beautiful balance as part of the "carbon cycle".


The other common greenhouse gas is methane. Its molecules have one carbon and four hydrogen atoms.

Marshland and bogs produce methane naturally. It bubbles up from decaying plant material.

Whenever we burn fossil fuels we release waste gases such as carbon dioxide, sulphur dioxide and various oxides of nitrogen. The main sources of these are cars and power stations.

Carbon dioxide is the main cause of the greenhouse effect.

Sulphur dioxide and the oxides of nitrogen will mix with rainwater in clouds and form acidic solutions. These then fall as acid rain.

Acid rain has severe effects on the environment and individual ecosystems within it. The acid rain will kill trees and damage buildings made from limestone.

The water will also make lakes and rivers more acidic. This will kill fish and other aquatic life.

The increased acidity of the water also allows aluminium salts to dissolve more easily. The aluminium ions are very poisonous to fish and birds.

Overpopulation occurs when there are more people than the land can sustain.

A lower death rate is a good thing. Modern medicine and farming techniques have allowed us to reduce deaths through starvation and disease.

However the birth rate has increased alarmingly. It has resulted in population rises that are out of control, many of these are in so-called 'under-developed' countries.

Farming is often under-rated nowadays. Modern farming practices have radically increased the efficiency of farming, producing more food from the available land.

In a lifetime we have moved from not having enough to eat to having tons of food.

Despite this "intensive farming" providing us with masses of cheap food there are a number of problems.

Strain can be placed upon domestic animals so that they produce the best possible food yield for us. Examples include the confinement of battery hens and veal calves.

Food chains are damaged by the use of pesticides to kill insects and animals that could damage crops. The chemicals used are indiscriminate, they kill any insect. So the links in food chains are wiped out and whole communities of animals can suffer.

Fields that are intensively farmed contain only single crop species, and the crowd of plants, insects, animals and birds that used to exist there are gone. Some face extinction.

Deforestation is the loss of trees due to demand for timber, land and so on.

It doesn't just occur in rainforests, the deforestation happens here too. We have virtually deforested our whole country over centuries, including our hedgerows.

The rainforests are the lungs of the Earth. The rapid rate of deforestation there has critical importance.

There are 4 main effects of deforestation:

  1. Decrease in rainfall: less trees means less transpiration and photosynthesis.
  2. Soil erosion: the exposed soil dries out under the Sun.
  3. Serious flooding: rainwater runs off the exposed soil rather than soaking in as before.
  4. More carbon dioxide: the trees that used to remove the gas are gone.

Pesticides are used to kill animal pests, and herbicides are used to kill weeds. Both can be useful types of chemicals. However, their use has problems.

One problem is "bioaccumulation". These chemicals are poisonous to other animal life. If the animals at the start of a food chain take up small amounts it becomes more and more concentrated higher up until it can kill the animals at the top - including humans.

Eutrophication is what happens when too much chemical fertiliser is used on crops and it washes into rivers and streams.

Organic farming is a phrase often used to describe traditional farming practices. It is becoming increasingly used and with scientific knowledge it is more efficient that it used to be but without being intensive.

Biological pest control includes any example where an animal is chosen which will eat the pest species.