Science Class 8 Chapter 18 Questions and Answers - Free PDF Download
FAQs on NCERT Solutions For Class 8 Science Chapter 18 Pollution Of Air And Water - 2025-26
1. According to the NCERT solutions for Class 8 Science Chapter 18, what are the primary causes of air pollution?
As per the CBSE 2025-26 syllabus, the NCERT solutions for Chapter 18 explain that air pollution is caused by both natural and man-made sources. The main causes you should mention in your answer are:
Vehicular Emissions: Vehicles release high levels of pollutants like carbon monoxide, carbon dioxide, and nitrogen oxides.
Industrial Emissions: Factories and power plants emit harmful gases such as sulphur dioxide and smoke particles.
Use of Fossil Fuels: The burning of coal, wood, and dung cakes for domestic purposes releases suspended particulate matter.
Natural Sources: Events like forest fires and volcanic eruptions release large amounts of smoke and dust into the atmosphere.
2. How should one correctly answer the question on the 'Greenhouse Effect' as explained in NCERT Chapter 18?
To correctly answer this question, you should explain that the Greenhouse Effect is a natural process essential for keeping the Earth warm. Your answer should state that certain gases in the atmosphere, called greenhouse gases (like CO₂, methane, and water vapour), trap the sun's heat that is reflected from the Earth's surface. This trapped heat warms the planet. However, an excess of these gases due to pollution leads to an enhanced greenhouse effect, which causes global warming.
3. What is a common mistake students make when solving questions about the difference between the Greenhouse Effect and Global Warming?
A common mistake is treating them as the same thing. The correct approach is to explain that the Greenhouse Effect is a necessary, natural phenomenon where gases trap heat to warm the Earth. Global Warming, on the other hand, is the unnatural and excessive warming of the Earth caused by the increased concentration of greenhouse gases from human activities. In your answer, you must clearly state that one is a natural process and the other is the harmful consequence of its intensification.
4. How do the NCERT solutions explain the damage to the Taj Mahal from air pollution?
The NCERT solutions for Class 8 Science explain that the yellowing and corrosion of the Taj Mahal's white marble is a case study of the effects of air pollution. The primary cause is acid rain. Pollutants like sulphur dioxide and nitrogen dioxide, released from industries and refineries near Agra, react with water vapour in the atmosphere to form sulphuric acid and nitric acid. This acid falls with the rain, reacting with and corroding the calcium carbonate of the marble, a phenomenon often called 'marble cancer'.
5. What is the step-by-step method to explain water pollution as per the NCERT Class 8 textbook?
The correct step-by-step method involves defining water pollution and then listing the major pollutants (or contaminants). First, define water pollution as the contamination of water bodies by substances that make the water unusable for drinking, cooking, swimming, and other purposes. Second, list the key sources of pollutants as per the chapter:
Sewage: Untreated waste from homes and industries.
Industrial Chemicals: Toxic wastes like arsenic, lead, and fluorides released by factories directly into rivers.
Agricultural Runoff: Excess fertilisers and pesticides washed from fields into water bodies.
6. Why is Carbon Dioxide (CO₂), which is vital for photosynthesis, considered a pollutant in Chapter 18?
This is a conceptual point. While CO₂ is essential for plant life, it is considered a pollutant when its concentration in the atmosphere becomes excessive. The NCERT chapter implies that a substance becomes a pollutant when its presence in excess harms the environment. The over-accumulation of CO₂ from activities like burning fossil fuels enhances the greenhouse effect, leading to global warming. Therefore, it's the quantity and imbalance that make it a pollutant, not the substance itself.
7. What is potable water, and what are the methods of purification mentioned in the NCERT solutions?
Potable water is water that is safe and fit for drinking. The NCERT solutions for Chapter 18 highlight that not all clean-looking water is potable, as it may contain dissolved impurities and microorganisms. The common purification methods explained are:
Filtration: A physical method using filters (like a candle-type filter) to remove suspended impurities.
Boiling: A common household method that kills harmful germs present in the water.
Chlorination: The chemical method of adding chlorine tablets or bleaching powder to water to disinfect it and kill microorganisms.
8. Beyond the textbook exercises, how can the principles from Chapter 18 be applied to reduce pollution at an individual level?
Applying the concepts from the NCERT solutions in daily life is key. At an individual level, you can reduce pollution by:
Conserving Energy: Switching off lights and appliances when not in use reduces the demand on power plants that burn fossil fuels.
Using Public Transport: Opting for public transport, carpooling, or cycling reduces vehicular emissions.
Following the 3 R's: Practising Reduce, Reuse, and Recycle minimizes waste, which in turn reduces both land and water pollution.
Planting Trees: Planting trees helps absorb excess carbon dioxide from the atmosphere, fighting the greenhouse effect.
