An Overview of Important Questions Class 8 Science Chapter 18
FAQs on Important Questions Class 8 Science Chapter 18
1. What types of questions (1-mark, 3-mark, 5-mark) are important for the CBSE Class 8 Science exam from Chapter 18, Pollution of Air and Water?
For the 2025-26 CBSE Class 8 Science exam, you can expect a variety of important questions from this chapter. These include:
- 1-mark questions: Often in MCQ or fill-in-the-blanks format, focusing on key definitions like 'pollutant', 'potable water', or naming a specific greenhouse gas.
- 3-mark questions: These typically require short explanations, such as describing the Greenhouse Effect, explaining the causes of acid rain, or listing the sources of water pollution.
- 5-mark questions: These are detailed questions that might ask you to explain a case study like the effect of pollution on the Taj Mahal, describe the objectives of the Ganga Action Plan, or differentiate between related concepts.
2. What is acid rain and how is it responsible for the 'marble cancer' affecting the Taj Mahal?
Acid rain is the term for rainfall made acidic by atmospheric pollution. Industries and vehicles release pollutants like sulphur dioxide (SO₂) and nitrogen dioxide (NO₂) into the atmosphere. These gases react with water vapour to form sulphuric acid and nitric acid, which then fall to the earth with rain. The Taj Mahal is built primarily of white marble (calcium carbonate). When acid rain falls on it, the acids react with the marble, corroding it. This phenomenon is often called 'marble cancer', which causes the monument to discolour and deteriorate.
3. What are the main causes of water pollution and what were the key objectives of the Ganga Action Plan?
The main causes of water pollution include the discharge of untreated sewage from cities, harmful industrial chemicals, and agricultural runoff containing pesticides and fertilisers into water bodies. The Ganga Action Plan was launched to address the severe pollution of the River Ganga. Its primary objectives were to improve water quality by diverting and treating domestic sewage and preventing toxic industrial waste from being discharged into the river, thereby making the water safer for human use and aquatic life.
4. For exam purposes, how can a student clearly differentiate between the Greenhouse Effect and Global Warming?
This is a common point of confusion. For full marks, it's important to be precise. The Greenhouse Effect is a natural and essential process where certain gases in the atmosphere (like CO₂, methane) trap heat from the sun, keeping the Earth warm enough for life. Global Warming, on the other hand, refers to the enhancement of this natural effect due to increased concentrations of greenhouse gases from human activities like burning fossil fuels. This excess trapping of heat leads to a gradual increase in the Earth's average temperature, causing climate change.
5. From an exam perspective, what are the five major air pollutants a student must know?
According to the CBSE syllabus, five important air pollutants to remember for your exam are:
- Carbon Monoxide (CO): From incomplete combustion of fuels.
- Sulphur Dioxide (SO₂): From burning fossil fuels in power plants and industries.
- Nitrogen Oxides (NOx): From vehicle exhaust and industrial processes.
- Chlorofluorocarbons (CFCs): Previously used in refrigerators, ACs, and aerosol sprays.
- Suspended Particulate Matter (SPM): Tiny solid or liquid particles suspended in the air, like dust, soot, and smoke.
6. Why are Chlorofluorocarbons (CFCs) considered such a dangerous air pollutant despite being non-toxic to humans directly?
While CFCs are not directly harmful to breathe, they are extremely dangerous to the environment because they cause depletion of the ozone layer. The ozone layer, located in the stratosphere, protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. When CFCs reach the stratosphere, UV radiation breaks them down, releasing chlorine atoms. A single chlorine atom can destroy thousands of ozone molecules, creating 'holes' in the ozone layer and allowing harmful UV radiation to reach the Earth's surface.
7. What is potable water, and what are the important steps to purify water to make it safe for drinking?
Potable water is water that is clean, safe for drinking, and free from harmful microorganisms and impurities. Common methods for purifying water include:
- Filtration: A physical method to remove suspended impurities. A simple candle-type filter is a common household example.
- Boiling: A very effective method where boiling water for several minutes kills most harmful bacteria and viruses.
- Chlorination: A chemical method where chlorine tablets or bleaching powder are added to water to kill germs. It is widely used in municipal water treatment plants.
8. Beyond just studying for exams, what are three practical steps a student can take to help reduce pollution?
Applying knowledge is key. Three effective steps you can take are:
- Conserve Energy: Reduce electricity consumption at home by turning off lights and appliances when not in use. This reduces the burning of fossil fuels at power plants, which is a major source of air pollution.
- Use Public Transport: Encourage the use of public transport, carpooling, or cycling instead of using personal vehicles for short distances. This directly cuts down on vehicle emissions.
- Follow the 3Rs: Actively Reduce, Reuse, and Recycle. Reducing consumption, reusing items, and recycling materials like paper and plastic minimises waste, which can otherwise pollute land and water.
