Ncert Exercises & Solutions

Which of the following conditions shows the polluted environment?

a.	pH of rainwater is 5.9
b.	Amount of carbon dioxide in the atmosphere is 0.03%
c.	Biochemical oxygen demand 10 ppm
d.	Eutrophication

Choose the correct option
1. (a, b, c)
2. (b, c, d)
3. (c, d)
4. (a, b, d)

Polluted water may contain nutrients for the growth of algae, which voers the water surface and reduces the oxygen concentration in water. This leads to anaeobic condition, accumulation of obnoxious decay and animal death. This is the process of eutrophication.

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample of water, is called Biochemical Oxygen Demand. Clean water would have BOD value of 5 ppm whereas highly polluted could have BOD value of 17 ppm or more.

Normally rain water has pH of 6 due to H⁺ ion formed by reaction of rain water with carbond dioxide in the atmosphere. When the pH of the rain water drops below 5.6, it is called acid rain.

280 ppm is the ideal level of CO₂ for human life, but was 416 parts per million (ppm) or 0.04% in 2021.Carbon dioxide About 0.03% of the atmosphere by volume is safe.

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