| Solution 27 |
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| In the stratosphere, from 20 km to 50 km height from the earth's surface ozone layer is present. Ozone layer plays a very important role in our life in the following ways: |
(i) |
Due to the presence of ozone layer, harmful ultraviolet rays and infrared rays from the sun cannot reach the earth directly |
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| (ii) |
Ozone layer acts as a natural shield. It absorbs most of the ultraviolet rays. This helps in reducing the risk of mutation and harm to the plants and animals. |
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| Causes of depletion : |
In the presence of sunlight, pollutant like CFC gets transformed into hydrogen chloride gas which in the presence of the ice crystals breaks into chlorine molecules. The chlorine molecule dissociates into chlorine atom. This reacts with ozone (O3) and dissociates it into O2. One chlorine atom can destroy at least 10,000 ozone molecules. Major pollutants for this depletion are chlorofluorocarbons (CFCs), nitrogen oxide (coming from fertilizers) and hydrocarbons. |
Human activities prove to be greatest cause of destroying the delicate balance of production and destruction of ozone by releasing ozone depleting substances. Volcanic eruptions may eject chemical substances such as chlorine and methane gases that react and breaks ozone apart. |
One of the main oxygen depleting substances are chlorofluro carbons (CFC's) which are most commonly known as Freon. They are mainly used as refrigerants, propellants, foaming agents in plastic manufacturing, fire extinguisher etc. The sources of oxides of nitrogen are thermonuclear weapons and of carbon tetrachloride are dry cleaning sprays, spray adhesives, correction fluid etc. |
| Alternative technologies : |
(i) |
Two Japanese companies jointly developed an alternative to CFCs. The device called ice cleaning is a semiconductor washing device which uses fine particles of ice and frozen alcohol at temperature below –500 C. |
(ii) |
The chemical compound , experimentally used instead of CFC–11 and CFC–12, are HCFC–123 a, HCFC–123 and HCFC–141b |
(iii) |
CFCs are generally used for spraying of pesticides but now spray of pesticides at high pressure has been recommended |
