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

Question: How is water treated at the waterworks to make it potable?

Answer: Tap water supplied to our homes is made potable in the waterworks. The processes used are as follows:

1. Sedimentation and decantation is first done to remove larger particles of solid impurities.
2. Alum is then added to the water to separate the smaller particles by making them heavier so that they settle down.
3. The water is then filtered by passing it through sand filters.
4. Germs are killed by adding chlorine to the water, that is, by chlorination.

Question: List the main pollutants of air, and give the ill effects of each.

Answer: The main pollutants of air and their effects are as follows:

1. Suspended Particulate Matter (SPM): These are minute solid particles suspended in air. The finer of these particles cannot be filtered by the hair in our nose and hence are a major cause of lung diseases.
2. Carbon monoxide: Carbon monoxide is one of the most dangerous pollutants. It is extremely poisonous. It mixes with the blood and prevents it from carrying oxygen.
3. Excess carbon dioxide: Excess carbon dioxide in the atmosphere traps the heat of the sun. This is believed to cause an increase in the temperature of the earth. This can cause large scale melting of ice in the polar regions and also expansion of ocean water. This ultimately can lead to flooding of coastal areas by sea water.
4. Sulphur dioxide and Nitrogen dioxide: Sulphur dioxide is poisonous. It affects our lungs. This gas reacts with water to form sulphuric acid, and this gives rise to acid rain. Nitrogen dioxide is also poisonous. It irritates the eyes and affects the lungs.
5. Chlorofluorocarbons (CFCs): These can damage the ozone layer in the atmosphere, causing an increase in the amount of ultraviolet rays from the sun reaching the earth. This light causes damage to our eyes and skin.
6. Lead: Lead compounds cause various problems such as mental disorder and brain damage.

Question: Do you think acid rain also affects crops?

Answer: Yes, acid rain also affects crops.

Question: Air pollution in Delhi improved after CNG was introduced instead of diesel in public transport since CNG gives out less smoke. Introduction of the Metro in Delhi is also expected to reduce pollution. How?

Answer:Metro in Delhi will reduce use of number of buses and other vehicles, thus help in the using of less fuels which cause pollution. Also the Metro trains run on electricity and do not cause pollution.

Question: What kind of groundwater is more likely to be polluted—that found near the surface of the earth or that found deeper?

Answer: Groundwater found near the surface of the earth is more likely to be polluted than that found deeper.