What are the sources of dissolved oxygen in water?
The following are the sources of dissolved oxygen in water
(i) Photosynthesis by aquatic plants
(ii) Due to direct contact of the water surface with air i.e., natural aeration
(iii) Mechanical aeration
What is the importance of measuring BOD of a water body?
Biological Oxygen Demand $(\mathrm{BOD})$ is the measure of level of pollution caused by organic biodegradable material. These biodegradable materials are decomposed by microorganism (bacteria) consuming dissolved oxygen.
Low value of BOD indicates that water contains less biodegradable material.
Why does water covered with excessive algae growth become polluted?
Presence of excessive algae growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc., from the surroundings. The decomposition of algae growth produces bad smell and unattractive appearance making it unfit for recreational use like swimming, boating etc. Further, decrease in dissolved oxygen may be harmful for aquatic species like fishes etc.
A factory was started near a village. Suddenly villagers started feeling the presence of irritating vapours in the village and cases of headache, chest pain, cough, dryness of throat and breathing problems increased. Villagers blamed the emissions from the chimney of the factory for such problems. Explain what could have happened. Give chemical reactions for the support of your explanation.
The symptoms observed in a village indicate that nitrogen oxide and sulphur oxide are released from the chimney of the factory. These are produced by the burning of fossil fuels such as gasoline, coal, natural gas etc. In an automobile engine, at high temperature when fossil fuel is burnt, dinitrogen and dioxygen combine to yield NO is i.e., nitric oxide.
$\begin{aligned} & \mathrm{N}_2+\mathrm{O}_2 \xrightarrow{1200-15001 \Upsilon\mathrm{C}} 2 \mathrm{NO} \\ & 2 \mathrm{NO}+\mathrm{O}_2 \xrightarrow{1100 \Upsilon\mathrm{C}} \mathrm{NNO}_2\end{aligned}$
$\mathrm{SO}_2$ is produced by burning of sulphur containing fossil fuel or by roasting of sulphide ores such a iron pyrites, copper pyrites etc.
$$\mathrm{Cu}_2 \mathrm{~S}+\mathrm{O}_2 \longrightarrow 2 \mathrm{Cu}+\mathrm{SO}_2 $$
0xidation of sulphur dioxide into sulphur trioxide in the absence of a catalyst is a slow process but this oxidation occurs easily in the atmosphere. Explain how does this happen? Give chemical reactions for the conversion of $\mathrm{SO}_2$ into $\mathrm{SO}_3$.
The oxidation of sulphur dioxide into sulphur trioxide can occur both photochemically or non-photochemically. In the near ultraviolet region, the $\mathrm{SO}_2$ molecules react with ozone photochemically.
$\mathrm{SO}_2+\mathrm{O}_3 \xrightarrow{h v} \mathrm{SO}_3+\mathrm{O}_2$
$2 \mathrm{SO}_2+\mathrm{O}_2 \xrightarrow{h v} 2 \mathrm{SO}_3$
Non-photochemically, $\mathrm{SO}_2$ may be oxidised by molecular oxygen in presence of dust and soot particles.
$$2 \mathrm{SO}_2+\mathrm{O}_2 \xrightarrow{\text { Particulates }} 2 \mathrm{SO}_3$$