Course 8.2Lesson 1 of 3
Ozone, O3, is a molecule of three oxygen atoms. Near the ground it is a poison, but high in the atmosphere it forms a layer that absorbs most of the Sun's ultraviolet rays. Chemicals called CFCs, once used in refrigerators and fire extinguishers, thinned this layer. In 1987 countries agreed to freeze their production.
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We are part of the environment, not outside it. What we make, use and throw away changes the air, the water and the soil around us. In this course we look at two examples: the ozone layer high above us, and the garbage we make every day.
The oxygen we breathe is made of molecules with two oxygen atoms, written . Ozone is a molecule with three oxygen atoms, written .
The two behave very differently.
Ozone forms high up in the atmosphere, where UV light is strong. It happens in two steps.
Count the atoms. One split molecule gives two atoms, and each atom uses one more . So three oxygen molecules become two ozone molecules, and all six atoms are kept.
Move the slider. You should see that the oxygen used is always one and a half times the ozone made: split molecules use molecules of and make of .
UV radiation is harmful to living things. In human beings it can cause skin cancer. Too much of it can also harm the eyes and weaken the body's defences against disease. It damages plants and the tiny organisms in the sea that start many food chains.
The ozone layer absorbs most of this radiation before it reaches the ground. Only a little gets through.
In the 1980s, scientists found that the amount of ozone in the atmosphere had begun to drop sharply. Over the South Pole the layer became so thin each spring that people spoke of an ozone hole.
The main cause was a group of man-made chemicals called chlorofluorocarbons, or CFCs.
In 1987, the United Nations Environment Programme (UNEP) brought countries together in an agreement. They agreed to freeze the production of CFCs at the level of 1986. Later agreements went further and phased them out.
Today, new refrigerators are made CFC-free. The ozone layer is slowly recovering. This shows that people can repair harm to the environment when they act together.
The program follows the two steps for one to five split molecules, and counts the atoms before and after.
You should see that the atoms before and after are always equal: and , then and , and so on.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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