Although only a small percentage of the electromagnetic radiation that is emitted by the Sun is
ultraviolet (UV) radiation, the amount that is emitted would be enough to cause severe damage to
most forms of life on Earth were it all to reach the surface of the earth. Fortunately, all of the Sun's
ultraviolet radiation does not reach the earth because of a layer of oxygen, called the ozone layer,
encircling the earth in the stratosphere at an altitude of about 15 miles above the earth. The ozone
layer absorbs much of the Sun's ultraviolet radiation and prevents it from reaching the earth.
Ozone is a form of oxygen in which each molecule consists of three atoms (03) instead of the
two atoms (02) usually found in an oxygen molecule. Ozone forms in the stratosphere in a process
that is initiated by ultraviolet radiation from the Sun. UV radiation from the Sun splits oxygen
molecules with two atoms into free oxygen atoms, and each of these unattached oxygen atoms then
joins up with an oxygen molecule to form ozone. UV radiation is also capable of splitting up ozone
molecules; thus, ozone is constantly forming, splitting, and reforming in the stratosphere. When UV
radiation is absorbed during the process of ozone formation and reformation, it is unable to reach
Earth and cause damage there.
Recently, however, the ozone layer over parts of the earth has been diminishing. Chief among
the culprits in the case of the disappearing ozone, those that are really responsible, are the
chloroflurocarbons (CFCs). CFCs meander up from Earth into the stratosphere, where they break
down and release chlorine. The released chlorine reacts with ozone in the stratosphere to form
chlorine monoxide (CIO) and oxygen (02). The chlorine then becomes free to go through the cycle
over and over again. One chlorine atom can, in fact, destroy hundreds of thousands of ozone
molecules in this repetitious cycle.
1. According to the passage, ultraviolet radiation from the Sun .
2. Look at the word encircling in paragraph 1. This word is closest in meaning to .
3. It is stated in the passage that the ozone layer .
5. Look at the word 'free' in paragraph 2. The word in paragraph 2 that could best be replaced by free.
6. Ultraviolet radiation causes oxygen molecules to
7. Look at the word 'it' in paragraph 2. The word 'it' refers to .
8. Look at the word 'culprits' in paragraph 3. This word is closest in meaning to .
9. Click on the drawing that shows what happens after a chlorine molecule reacts with an ozone molecule.
10. Click on the sentence in paragraph 3 that explains how much damage chlorine can do. The block is in the begining of the sentence.
