Dissolved oxygen in water is very important for aquatic life. What processes are responsible for the reduction of dissolved oxygen in water?
The process which are responsible for the reduction of dissolved oxygen in water are-use of phosphatic and nitrate fertilisers, detergents, the discharge of human sewage and organic waste from food, paper and pulp industries. The microorganisms which oxidise organic matter also utilise oxygen dissolved in water. Moreover during night, photosynthesis stops but the aquatic plants continue to respire, resulting in reduction of dissolved oxygen.
On the basis of chemical reactions involved, explain how do chlorofluorocarbons cause thinning of ozone layer in stratosphere?
Chlorofluorocarbons are introduced into the atmosphere from aerosol sprays in which they function as propellants and from refrigerating equipments in which they act as coolants. It is also used as solvents.
It has very long life and stay in atmosphere for years and ultimately reach the upper layer of atmosphere (stratosphere) where it decompose in presence of UV-radiation of the sun. On account of decomposition, it forms chlorine atom or free radicals.
$\mathrm{CF}_2 \mathrm{Cl}_2 \xrightarrow{h v}{ }^{\infty} \mathrm{CF}_2 \mathrm{Cl}+\mathrm{Cl}^{\infty}$
The active chlorine atoms then destroy the ozone layer.
$\begin{aligned} & \mathrm{Cl}^{\infty}+\mathrm{O}_3 \longrightarrow \mathrm{ClO}^{\infty}+\mathrm{O}_2 \\ & \mathrm{ClO}^{\infty}+\mathrm{O} \longrightarrow \mathrm{Cl}^{\infty}+\mathrm{O}_2\end{aligned}$
It has been found that one molecule of chlorofluorocarbon can destroy one thousand ozone molecules in the stratosphere.
What could be the harmful effects of improper management of industrial and domestic solid waste in a city?
All the solid wastes either domestic or industrial are of two types
(i) biodegradable and
(ii) non-biodegradable
If the disposal of these wastes is not properly done, these wastes may find their way into sewers and some may be eaten up by the cattle. The non-biodegradable waste like polythene bags, if swallowed by cattle, can result into their death.
During an educational trip, a student of botany saw a beautiful lake in a village. She collected many plants from that area. She noticed that villagers were washing clothes around the lake and at some places waste material from houses was destroying its beauty. After few years, she visited the same lake again. She was surprised to find that the lake was covered with algae, stinking smell was coming out and its water had become unusable. Can you explain the reason for this condition of the lake?
The process of eutrophication is responsible for this condition of lake. The domestic waste and organic compounds such as detergents can provide plant nutrients which can enhance the growth of algae and aquatic plants.
These are decomposed by the bacterial population giving disagreeable odour and spoiling the beauty of the lake.
What are biodegradable and non-biodegradable pollutants?
Biodegradable pollutants are those which are decomposed by bacteria e.g., sewage, cow dung, fruit, vegetable etc.
Non-biodegradable pollutants are those which cannot be decomposed by bacteria e.g., mercury, aluminium, lead, copper, DDT etc.