Background information on the formation and depletion of ozone and the role of chlorine in the process;
Things to Emphasize
Note: It is not expected that students at this level will understand complex chemical reactions. Given graphic representations and concrete demonstrations though, they should be able to understand the basic concepts pertaining to the role of chlorine in the atmosphere.
Ultraviolet-C (uv-C) uses ultraviolet light to form ozone from oxygen molecules.
Ultraviolet-A and ultraviolet-B (uv-A&B) use ultraviolet light to photo-dissociate ("break apart") ozone and form oxygen molecules.
uv-A&B is a bit slower at destroying ozone than uv-C is at making it.
As a result there is always a surplus of ozone in the atmosphere. This surplus ozone forms the Earth's ozone layer. It is responsible for absorbing almost all the uv-A and uv-B light from the sun and protects plants and animals from this potentially harmful radiation.
When chlorine is introduced into the picture things become very different. Chlorine speeds up the conversion of ozone into molecular oxygen. In effect, chlorine "steals" the ozone from the atmosphere. Moreover, the chlorine reaction doesn't have the same reaction rate (speed control) limitations as the uv-A&B process. Chlorine converts ozone into molecular oxygen at a rate faster than uv-C can produce ozone. With no ozone left in the atmosphere (because it is efficiently depleted by reactions with chlorine), ultraviolet A&B has no absorbing ozone molecules available. With no molecules available in the atmosphere to absorb uv-A&B radiation, the radiation reaches the Earth's surface.
CFCs (chlorofluorocarbons) undergo a series of reactions which lead to the dispersal of ClO molecules in the stratosphere. These molecules contain a single chlorine atom and a single oxygen atom. It is this molecule (from its precursor CFCs) which reduces the amount of ozone in the upper atmosphere.
Teacher explanation of the formation and depletion of ozone with the introduction of chlorine. See Fact Sheet - The Role of Chlorine for explanation and transparency masters.
uv Absorption by Oxygen
The formation and depletion of ozone with the introduction of chlorine. Red arrows indicate the path of ozone molecules and blue arrows show the path of oxygen molecules. The air in within the glass jar represents the Earth's atmosphere. Transparency Master
Student demonstration of how chlorine reacts in the atmosphere.
flavoured mini marshmallows (at least two colours are necessary)
Divide the class into pairs of students.
Distribute 9 mini marshmallows of one colour, (to represent oxygen atoms), 3 mini-marshmallows of a second colour (to represent chlorine atoms), and 6-9 toothpicks per pair.
Ask students to combine the "oxygen atoms" with the toothpicks to create "oxygen molecules" and "ozone molecules."
Ask students to introduce a "free chlorine atom" and demonstrate how it attacks an "ozone molecule" to form ClO and O2.
Allow students sufficient time to experiment with and discuss possible combinations and recombinations of chlorine and oxygen atoms.
Discuss sources of ozone destroying compounds in our daily lives. Refer to Fact Sheet.
Ask students to bring in and share advertisements for "ozone-friendly" products.