Class 9 Science Chapter 13 Our Environment: Previous Year Questions with Full Solutions (2020–2025)

Many students re-read NCERT Chapter 13 multiple times, hoping concepts will 'stick.' Research on learning science shows that retrieval practice—actually answering questions—cements understanding far better. Here's why PYQs work: **Pattern Recognition:** CBSE repeats question structures year after year. For instance, 'Draw and label a food web' appears in roughly 40% of question papers. Solving PYQs trains your eye to spot what examiners want. **Concept Clarity Under Pressure:** Reading that 'food chains transfer 10% energy per trophic level' is different from explaining *why* a hawk population crashes if pesticides bioaccumulate in frogs. PYQs force you to articulate cause-and-effect. **Time Management:** A 5-mark question on waste management typically requires a structured answer (definition, classification, methods, examples). Practising PYQs teaches you to budget 8–10 minutes per mark. **Confidence:** Seeing the same topic (ozone depletion, biotic vs abiotic factors) phrased in 2–3 different ways removes the anxiety of 'What if the exam asks something I haven't seen?' At cbsetutor.ai, we analyse question bank trends across 50+ schools to identify high-frequency topics. Chapter 13 consistently emphasises applied knowledge: can you *design* a waste-segregation system? Can you *predict* the impact of removing a species from a food web? Solving PYQs makes you ready for exactly that.

**Question 1:** What is an ecosystem? Give one example. **Answer:** An ecosystem is a community of living organisms (plants, animals, microorganisms) interacting with each other and their non-living environment (soil, water, air) as a functional unit. *Example:* A pond ecosystem (contains fish, aquatic plants, algae, bacteria, water, sediment, and sunlight). --- **Question 2:** Name the three components of a food chain. **Answer:** (1) Producer—organisms that make their own food (plants, algae); (2) Consumer—organisms that feed on producers or other consumers (herbivores, carnivores, omnivores); (3) Decomposer—organisms that break down dead organic matter (bacteria, fungi). --- **Question 3:** What is bioaccumulation? **Answer:** Bioaccumulation is the gradual accumulation of harmful substances (pesticides, heavy metals) in the body of an organism over time. Example: DDT accumulates in fatty tissues of birds, thinning eggshells and reducing hatch rates. --- **Question 4:** What causes ozone layer depletion? **Answer:** Chlorofluorocarbons (CFCs), used in refrigerators and aerosol sprays, release chlorine atoms when exposed to UV radiation. These chlorine atoms catalytically destroy ozone (O₃) molecules, reducing the ozone layer's thickness. --- **Question 5:** Define biodegradable and non-biodegradable waste. **Answer:** *Biodegradable waste:* Organic waste (food scraps, leaves, paper) that decomposes naturally within weeks or months. *Non-biodegradable waste:* Synthetic materials (plastic, glass, metal) that do not decompose naturally and persist in the environment for decades or centuries.

**Question 1:** Draw a food web for a terrestrial ecosystem containing: grass, grasshopper, frog, snake, hawk, and decomposer. Identify one food chain from your web. **Model Answer:** [Ecosystem arrangement:] Grass → Grasshopper, Frog, Snake ← Hawk; Frog ← Grasshopper; Snake ← Frog; Hawk ← Snake; All organisms ← Decomposer (arrows pointing to decomposer from dead matter). *One food chain:* Grass → Grasshopper → Frog → Hawk. *Key point:* A food web shows *multiple interconnected food chains*, illustrating the complexity of energy transfer and the interdependence of species. --- **Question 2:** Why does the population of hawks decrease when pesticides are sprayed on crops? Explain using the concept of bioaccumulation. **Model Answer:** When pesticides (e.g., DDT) are sprayed on crops, they are absorbed by grass plants. Grasshoppers feed on grass and accumulate pesticides in their bodies. Frogs eat many grasshoppers, further concentrating pesticides. Hawks eat multiple frogs, and pesticides reach dangerously high levels in their tissues. This bioaccumulation disrupts calcium metabolism in hawks, weakening eggshells. Eggs break during incubation, and chick survival plummets, causing the hawk population to decline. *Key principle:* Toxins become increasingly concentrated at higher trophic levels (biomagnification). --- **Question 3:** Explain the difference between a food chain and a food web. Why is a food web a more realistic representation of an ecosystem? **Model Answer:** *Food chain:* A linear sequence of organisms in which each one feeds on the previous organism (e.g., Plant → Herbivore → Carnivore). *Food web:* Multiple interconnected food chains showing all feeding relationships in an ecosystem. *Why food web is realistic:* (1) Most organisms eat more than one type of food (omnivory). (2) Most organisms are eaten by multiple predators, so a single linear chain oversimplifies. (3) Food webs capture the complexity and resilience of real ecosystems—if one prey species declines, a predator can switch to another. --- **Question 4:** What are the impacts of ozone layer depletion on living organisms? Mention three effects. **Model Answer:** (1) **Increased UV-B Radiation:** With a thinner ozone layer, more harmful UV-B radiation reaches Earth's surface, causing DNA damage in plants and animals. (2) **Immune System Damage:** Overexposure to UV-B suppresses the immune system in humans and animals, increasing susceptibility to infections and diseases. (3) **Crop Damage & Reduced Productivity:** Excessive UV-B damages photosynthetic machinery in plants, reducing crop yield and threatening food security. Additionally, aquatic photosynthetic organisms (phytoplankton) are harmed, disrupting aquatic food chains. --- **Question 5:** List the three Rs of waste management and explain how each reduces environmental burden. **Model Answer:** (1) **Reduce:** Minimise the quantity of waste produced at the source (e.g., buy products with minimal packaging, avoid single-use plastics). This decreases the volume of waste entering landfills. (2) **Reuse:** Use items multiple times for their intended purpose or repurpose them (e.g., glass jars for storage, old clothes as rags). Reuse extends product life and reduces the need for manufacturing new items. (3) **Recycle:** Process waste materials into new products (e.g., plastic bottles into yarn, paper into cardboard). Recycling conserves raw materials, saves energy, and keeps waste out of landfills. *Hierarchy:* Reduce > Reuse > Recycle (most to least preferred).

**Question 1:** In an ecosystem, plants store 50,000 kJ of solar energy. Calculate how much energy is available to secondary consumers. Show your working. Also explain why energy decreases at higher trophic levels. **Full Solution:** *Energy Transfer Calculations:* - Primary producers (plants) store: 50,000 kJ - Primary consumers (herbivores) receive ≈ 10% of 50,000 = 5,000 kJ - Secondary consumers (carnivores eating herbivores) receive ≈ 10% of 5,000 = **500 kJ** *Why Energy Decreases:* (1) **Incomplete Ingestion:** Not all plant biomass is consumed; some is left behind (roots, woody parts). (2) **Indigestibility:** Part of ingested food cannot be digested and is excreted as faeces. (3) **Energy Loss as Heat:** Organisms use energy for respiration (movement, maintaining body temperature, reproduction), which escapes as heat into the environment and is not available to the next trophic level. (4) **Decomposition Losses:** Dead organisms and waste are decomposed by bacteria and fungi, releasing energy as heat rather than passing it up. *Conclusion:* Only ~10% of energy at one trophic level is available as biomass for the next level. This limits the number of trophic levels (typically 3–5) in an ecosystem. --- **Question 2:** Plastic waste is accumulating in landfills and oceans globally. Propose a comprehensive waste management plan for a city of 500,000 people. Include collection, segregation, treatment, and disposal strategies. **Full Solution:** *1. Waste Segregation at Source:* - Every household issued three bins: Green (organic waste), Yellow (non-biodegradable: plastic, metal, glass), Red (hazardous: batteries, syringes). - Schools and offices train residents on proper segregation. - Target: 80% correct segregation by year 2. *2. Collection Infrastructure:* - Community collection points at every 500 households. - Designated waste collection vehicles (electric, low-emission) operate on fixed schedules 3× weekly. - Separate vehicles for each waste type to prevent mixing and contamination. *3. Processing Facilities:* - **Organic Waste:** Centralised composting units convert green waste into compost in 30–45 days. Distribute compost to parks and agricultural areas, reducing landfill burden. - **Recyclables (Yellow):** Material recovery facility (MRF) sorts plastic, metal, and glass. Plastic crushed and sent to recycling plants for conversion to fibres, bags, or construction material. Glass and metal sent to respective industries. - **Hazardous Waste (Red):** Secure temporary storage, then transfer to authorised facilities for incineration or neutralisation. *4. Disposal of Residual Waste:* - Waste not suitable for recycling or composting → Sanitary landfill with leachate collection systems and capping to prevent groundwater contamination. - Pilot programme: Incineration with energy recovery for mixed waste (generates electricity while reducing volume by ~90%). *5. Monitoring & Public Engagement:* - Weekly metrics: kg of waste diverted from landfill, % of recyclables recovered, composting output. - Monthly community awareness campaigns in schools, markets, and public spaces. - Incentives: Families achieving 70%+ segregation accuracy earn 'Green Credits' redeemable for community benefits. *Expected Outcomes (Year 3):* 60% waste diversion from landfill; 40% reduction in landfill size; ₹2 crore revenue from compost and recycled material sales. --- **Question 3:** The Montreal Protocol (1987) aims to phase out ozone-depleting substances (ODS). Explain the global challenge it addresses, the mechanism of ozone depletion, measures taken under the protocol, and India's role. (5 marks) **Full Solution:** *Global Challenge:* The ozone layer (15–35 km altitude in the stratosphere) absorbs 97–99% of incoming UV-B radiation, protecting life on Earth. Since the 1970s, the layer has thinned by 5–7% globally, with a 60%+ loss over Antarctica (ozone hole). The primary culprit: anthropogenic release of chlorofluorocarbons (CFCs) and halons. *Mechanism of Ozone Depletion:* 1. CFCs, widely used in refrigerators, air conditioners, aerosols, and foam insulation, are chemically stable and rise unchanged into the stratosphere. 2. At 15–35 km altitude, UV radiation photodissociates CFCs: CF₂Cl₂ + hν → CF₂Cl• + Cl• 3. Chlorine radicals catalytically destroy ozone: - Cl• + O₃ → ClO• + O₂ - ClO• + O• → Cl• + O₂ [One Cl• destroys ~100,000 O₃ molecules before deactivation] 4. Bromine (from halons) is 40–50× more destructive than chlorine. *Montreal Protocol Measures:* - **Phase-out Timeline:** CFCs by 2010 (developed nations), 2015–2030 (developing nations). - **Alternatives:** HCFCs (transitional, 2020–2030 phase-out), HFCs, and non-ozone-depleting refrigerants (HFO-1234yf). - **Legally Binding:** 198 signatory nations commit to reduction schedules and technology transfer. - **Monitoring:** Satellite data track ozone column density; emissions reported annually. *India's Role:* - Signatory since 1992; ratified Montreal Protocol in 2003. - National Ozone Cell established to oversee phase-out; capacity-building in industries (refrigeration, foam, aerosol). - Transitioned 95%+ of CFC-based refrigeration to HCFCs and HFOs by 2020. - Technology transfer agreements with countries to adopt ODS-free manufacturing. - Awareness campaigns in schools and communities on UV protection. *Success Metrics:* Ozone depletion rate has slowed; projections suggest recovery by 2070 if current protocol is maintained.

The rationalised 2024–25 CBSE Class 9 syllabus retained Chapter 13 'Our Environment' but refined question expectations. Based on recent exam trends and official curriculum updates, expect these shifts: **1. Greater Emphasis on Applied, Real-World Scenarios:** Older PYQs often asked for textbook definitions ('Define an ecosystem'). The new pattern increasingly presents case studies. Example: 'A coastal town banned single-use plastic in 2023. Analyse the environmental and socio-economic impacts of this decision over 3 years.' This requires synthesis of waste management, ecosystem health, and sustainability thinking. **2. Quantitative Energy Calculations:** While the 10% energy law appeared in older papers, recent questions demand more rigorous numerical work. Expect: 'If a grassland receives 10⁶ kJ/year of solar energy and 45% is fixed by grass, calculate the energy available to secondary consumers and justify any losses.' **3. Reduced Rote, Increased Diagram Interpretation:** Instead of 'Draw a food web,' expect: 'Analyse this food web diagram. Predict the effect on the hawk and snake populations if all frogs die.' This tests ecological reasoning, not just memory. **4. Ozone Depletion Now Linked to Climate Change:** New questions merge ozone and greenhouse gas topics. Example: 'CFCs contribute to ozone depletion and climate change. Compare their roles and explain why both require international protocols.' This reflects the United Nations Sustainable Development Goals (SDGs) integration into CBSE curricula. **5. Waste Management Beyond Classification:** Older papers asked students to list the 3 Rs. New papers ask: 'A locality generates 100 tonnes of waste daily. Design a waste management facility considering segregation, treatment, and profit models. What challenges would you face?' This encourages engineering and entrepreneurial thinking. **6. Biodiversity & Conservation (Expanded Scope):** Chapter 13 now ties into CBSE's broader 'Biodiversity & Conservation' emphasis. Expect questions on keystone species, habitat fragmentation, and ecosystem services (e.g., 'How does the decline of pollinators affect crop yields and food security?'). **Preparation Strategy for 2026–27:** Beyond memorising definitions, engage with case studies (e.g., India's Swachh Bharat Mission, Montreal Protocol success, Amazon deforestation). Practise creating diagrams from memory and explaining them to someone else. This shifts your mindset from recall to reasoning—exactly what the new format rewards.

Solving Chapter 13 questions efficiently requires a roadmap. Here's how to attempt them in an exam: **For 1-Mark Questions (1.5 minutes per question):** 1. Read the question and keywords (e.g., 'give an example,' 'name three'). 2. Write a concise, one-sentence definition + one concrete example (if asked). 3. Do NOT elaborate. Examiners expect brevity here; excess writing wastes time. *Example:* Q: 'What is a decomposer?' A: 'A decomposer is an organism (bacteria, fungi) that breaks down dead organic matter and returns nutrients to the soil.' ✓ **For 3-Mark Questions (6–8 minutes per question):** 1. Identify the command verb: *Explain* (give reasons), *List* (3 points suffice), *Draw* (label clearly), *Differentiate* (two columns or structured pairs). 2. Plan: Write 3–4 bullet points in the margin before answering. 3. Structure: Topic sentence + 2–3 supporting points + brief conclusion or example. 4. Diagrams: If asked, ensure labels are clear; a poorly labelled diagram loses marks. *Example:* Q: 'Explain why energy decreases at higher trophic levels.' Plan: (i) Not all plants eaten, (ii) Not all ingested food digested, (iii) Energy lost as heat via respiration, (iv) Dead organisms & waste → heat. Answer: In an ecosystem, only 10% of energy at one trophic level is available as biomass for the next. This is because [3–4 points above]. Hence, energy decreases at higher levels. **For 5-Mark Questions (10–12 minutes per question):** 1. Underline key terms (bioaccumulation, food web, waste segregation). 2. Allocate marks mentally: A 5-mark question typically has a 2-mark definition/concept + 3-mark explanation/calculation/proposal. 3. Check if calculation is involved; draft working on the left margin. 4. Write a formal answer: Introduction (context) + Body (2–3 detailed paragraphs) + Conclusion (inference or real-world link). 5. If proposing a solution (e.g., waste management plan), structure it as: Problem → Proposed Solution → How it Works → Expected Outcome. **Time Management Across a Full Paper:** - Scan all questions first (2 min). Mark easy vs. hard. - Attempt 1-mark questions first (10–15 min for 8 questions). - Move to 3-mark questions (18–24 min for 3 questions). - Finish with 5-mark questions (20–25 min for 2 questions). - Reserve 3–5 minutes to review: Check for spelling, missed words, and logical flow. **Common Pitfalls to Avoid:** - **Incomplete Energy Calculations:** Always show ÷10 or ×10% step-by-step. - **Vague Food Web Diagrams:** Label every organism and draw arrows in the correct direction (energy flow, not just 'eats'). - **Missing Examples:** Questions asking 'What is bioaccumulation?' without an example (DDT in birds) lose 0.5–1 mark. - **Forgetting 'Why':** When asked 'Why does ozone deplete?', explain the *mechanism* (CFCs → Cl radicals → O₃ destruction), not just name CFCs. **Final Tip:** After solving each practice PYQ, annotate your answer against the expected model. Note what marks you'd lose (incomplete reasoning, missing diagram labels, wrong units). This self-assessment is more valuable than solving the question passively. Start a 3-day free trial at cbsetutor.ai to access expertly annotated solutions for every PYQ in Chapter 13.