NCERT 8th Class (CBSE) Science: Pollution of Water and Air

Question: What harm can dumping of untreated sewage into a river cause?

Answer: Untreated sewage from homes is often disposed off into rivers. These wastes contain many harmful microorganisms – bacteria, viruses, fungi and parasites that cause diseases such as diarrhoea, dysentery, cholera, typhoid and jaundice.

Question: Why is hot water discharge into rivers also considered as a cause of water pollution?

Answer: The temperature of the waste water from factories that is discharged into rivers is often higher than the temperature of the river water. The resultant increase in the river water temperature is itself a form of pollution as it adversely affects aquatic plants and animals.

Question: If water is filtered, it becomes free of all suspended impurities and hence potable. Do you agree? Give reasons.

Answer: I don’t agree with statement that filtered water is potable because only solid impurities are filtered, it may still have germs in it and hence, not potable.

Question: Suggest two methods by which water can be made potable at home.

Answer: At home, water can be made potable by killing the germs in the following ways:

1. Boiling: Boiling the water for 15-20 minutes kills all germs.
2. Purifier with ultraviolet light: A special type of light called ultraviolet light kills germs. It is used in several water purifiers available in the market today.

Question: What are the main causes of water pollution?

Answer: The main causes of water pollution are:

1. Sewage
2. Industrial waste
3. Fertilizers and pesticides

Question: The use of CNG instead of diesel is being encouraged these days in vehicles. Why?

Answer: CNG (Compressed Natural Gas) reduces the emission of carbon particles and harmful gases such as carbon monoxide. It has now become compulsory for all commercial vehicles in Delhi to use CNG. This has reduced the pollution in the city.