dav class 8 science chapter 20 Question Answers – Pollution of Water
Water looks simple, but keeping it clean is one of the biggest challenges any town or city faces. Chapter 20 of the DAV Class 8 Science book explains what pollutes our water, why that is dangerous, and how water is made safe to drink. Below you will find the answers to every exercise in the chapter, written in plain language so you can understand the idea behind each answer and put it in your own words in the exam.
Quick revision before you start
Any unwanted material that spoils the quality of water is called a water pollutant. These come from several sources. Domestic sewage carries waste from homes. Industrial waste carries chemicals from factories. Agricultural waste, mainly fertilisers and pesticides, washes off the fields into streams and rivers. Even everyday habits like bathing and washing clothes in rivers and ponds add to the problem.
Water that is safe to drink is called potable water. Before water is supplied for drinking, it has to meet a few conditions: it should be colourless, odourless, tasteless, free from suspended impurities, free from disease-causing microorganisms and free from harmful chemicals. Water can be cleaned by physical methods such as filtration, and by chemical methods such as chlorination and treating it with alum.
A. Fill in the blanks
1. Chlorination is a chemical method of purifying water.
2. Fertilisers and pesticides make up agricultural wastes.
3. Domestic sewage in water encourages the growth of algae and bacteria, which use up a lot of the oxygen dissolved in the water.
4. Water that is safe for drinking is called potable water.
5. One purpose of purifying water is to remove the suspended impurities present in it.
B. State true or false
| Statement | Answer |
|---|---|
| 1. A bad taste in water shows that it contains pollutants. | True |
| 2. We should always bathe and wash clothes in rivers. | False |
| 3. Potable water can cause water-borne diseases. | False |
| 4. We should always wash our vehicles with tap water using a pipe. | False |
| 5. Waste water from industrial units must be treated before it is released into rivers. | True |
A quick reason for each. Bad taste is a warning sign that something has got into the water. Washing in rivers adds soap, dirt and germs to the water. Potable water is safe by definition, so it does not cause water-borne diseases. Washing a vehicle with a running pipe wastes a lot of water, and a bucket does the job just as well. Untreated industrial waste carries harmful chemicals that would damage rivers and the creatures living in them.
C. Tick the correct option
1. Domestic sewage released into rivers causes: the growth of algae.
2. Water that is safe for drinking is known as: potable water.
3. A method that helps destroy harmful germs in water is: chlorination.
4. Adding alum to make suspended particles heavier is called: loading.
5. A method that helps conserve water is: half flush.
Look for these answers among the options in your own book. Loading works because the heavier particles settle at the bottom, so the clear water can be drawn off from above. A half flush uses a smaller amount of water when a full flush is not needed.
D. Answer the following in brief
1. State any two ways in which water can get contaminated.
Waste from factories flows into rivers and ponds and pollutes them. Human activities such as bathing and washing clothes in water bodies also add dirt and germs to the water.
2. What is meant by water pollutants?
Water pollutants are any unwanted foreign substances that enter water and spoil its quality.
3. Name two diseases caused by drinking contaminated water.
Jaundice and diarrhoea. Other examples are cholera and typhoid.
4. Name two chemicals that help in purifying water.
Alum and chlorine, which is often used in the form of bleaching powder or chlorine tablets.
5. Which growths in water use up its dissolved oxygen?
The growth of algae and bacteria. When there is a lot of sewage in the water, they multiply quickly and consume the oxygen that fish and other water animals need.
E. Answer the following questions
1. List the main conditions water must satisfy before it is supplied for drinking.
Before drinking water is supplied, it must pass a few tests. It should be colourless and have no smell. It should be tasteless. It should be free from suspended impurities such as sand and mud. It should be free from the microorganisms that cause diseases. And it should be free from harmful chemicals.
2. Describe one physical and one chemical method of water purification.
Physical method: filtration. Water is passed one after another through layers of sand, gravel and charcoal. These layers trap the suspended impurities and let the cleaner water pass through. It is a simple method that can be used even when advanced equipment is not available.
Chemical method: chlorination. Chlorine tablets or bleaching powder are added to water to kill the germs in it. This method has to be used carefully, because too much chlorine makes the water unsafe, so the recommended quantity should never be exceeded.
3. State the measures that should be taken to check water pollution by sewage.
Sewage should be treated before it is released into any water body. The large solid waste should be broken down and removed first. Alum can be added to the water so that the tiny suspended particles clump together and settle. Finally, the water should be disinfected with chlorine, which kills the harmful microbes. Treated water can then be released or reused safely.
4. Suggest some simple steps to conserve water.
Water conservation starts at home, and small habits add up. Repair leaking taps straight away, because a slow drip can waste many litres in a day. Turn off the tap while brushing your teeth. Do not keep water running while washing dishes by hand. Wash a vehicle with a bucket and a cloth, not a hose. Use a bucket and mug for bathing instead of a shower. These steps save a lot of water without costing anything.
Common mistakes students make in this chapter
Many students mix up the physical and chemical methods of purification. Filtration only removes particles, so it is physical. Chlorination kills germs through a chemical action, so it is chemical.
Another common slip is writing “chlorination” for the process of making particles heavier. That process is called loading, and it uses alum.
In the conservation question, many answers say only “save water”. Marks are given for specific, practical steps, so name at least four.
Finally, remember that potable means safe to drink. It is not the same as pure water. Potable water can still contain dissolved minerals, as long as it is harmless.
Frequently asked questions
What is potable water?
Potable water is water that is safe for drinking. It is clear, odourless, tasteless and free from harmful germs and chemicals.
Why does sewage in water harm fish and other water animals?
Sewage encourages algae and bacteria to grow very fast. They use up the oxygen dissolved in the water, and animals living in the water do not get enough oxygen to survive.
What is the role of alum in purifying water?
Alum makes the tiny suspended particles stick together and become heavier, so they settle at the bottom. This process is called loading.
Is chlorination safe?
Yes, when the right amount is used. Too much chlorine can make the water harmful, so the quantity should be carefully measured.
Written for students following the DAV Science book for Class 8. Please cross-check the answers with your own textbook and teacher.