Class 9 Science Chapter 6: Materials Around Us – Complete Important Questions & Solutions

The CBSE Class 9 Science board exam structure emphasizes competency-based assessment. Chapter 6 Materials Around Us typically appears across 8–12 marks in a 80-mark paper, split between factual recall (1–2 marks) and application-based reasoning (3–5 marks). Examiners test your ability to: (1) classify materials using physical properties (hard/soft, transparent/opaque, lustrous/non-lustrous), (2) explain why certain materials suit specific purposes, and (3) solve numerical problems on density and solubility. The 2026-27 pattern continues emphasizing case-study and real-world scenarios—for instance, 'Why is copper preferred over wood for electrical wiring?' or 'Compare the suitability of leather vs. rubber for shoe soles using material properties.' Questions in this guide are reverse-engineered from past CBSE question banks, NCERT textbook exemplars, and official sample papers. They target the exact cognitive levels your examiner assesses: remembering (definitions), understanding (property comparisons), applying (material selection), and analyzing (multi-property reasoning). By practicing these patterns now, you'll develop the analytical muscle needed to tackle unfamiliar material-selection scenarios on exam day without panic.

One-mark multiple-choice questions test factual recall and straightforward concept comprehension. These appear in Section A of most CBSE Science papers. **Q1.** Which of the following materials is a poor conductor of electricity? (a) Copper (b) Aluminium (c) Rubber (d) Iron **Answer: (c) Rubber** — Metals conduct electricity due to free electrons; rubber is a non-metal insulator. **Q2.** A substance that dissolves completely in water is called: (a) Insoluble (b) Soluble (c) Opaque (d) Malleable **Answer: (b) Soluble** — Solubility is the maximum mass of a substance that dissolves in a fixed mass of solvent at a given temperature. Salt and sugar are soluble in water. **Q3.** The property of a material to be drawn into thin wires is called: (a) Brittleness (b) Ductility (c) Hardness (d) Lustre **Answer: (b) Ductility** — Gold, copper, and aluminium are ductile metals. Brittle materials like glass snap rather than stretch. **Q4.** Which material has high lustre (shine)? (a) Wood (b) Paper (c) Silver (d) Stone **Answer: (c) Silver** — Metals like silver, gold, and copper exhibit lustre due to their polished surface reflecting light. Non-metals typically lack lustre. **Q5.** If an object sinks in water, its density is: (a) Less than 1 g/cm³ (b) Equal to 1 g/cm³ (c) Greater than 1 g/cm³ (d) Zero **Answer: (c) Greater than 1 g/cm³** — Objects with density > 1 g/cm³ (density of water) sink. Iron, lead, and glass sink; cork, wood, and plastic float.

Two-mark questions require brief explanations, examples, and one-line reasoning. These test understanding and direct application. **Q1. Differentiate between transparent and opaque materials. Give one example of each.** **Answer:** Transparent materials allow light to pass through completely, making objects visible on the other side (e.g., glass, water). Opaque materials do not allow light to pass through; you cannot see through them (e.g., wood, iron, paper). Semi-transparent materials (translucent) allow partial light transmission, like frosted glass. **Q2. Why is copper preferred over wood for making electrical wires?** **Answer:** Copper is a good conductor of electricity (high electrical conductivity) and has high ductility, allowing it to be drawn into thin wires without breaking. Wood is an insulator (poor conductor) and cannot safely transmit electrical current. Additionally, copper has low resistance, minimizing energy loss. **Q3. Define solubility. How does it differ from dissolvability?** **Answer:** Solubility is the maximum mass (in grams) of a solute that can dissolve in 100 g of solvent at a specific temperature. Dissolvability refers to whether a substance dissolves at all (qualitative). For example, sugar has high solubility in water (~200 g per 100 mL at 20°C), while sand has zero solubility—it cannot dissolve, only disperse temporarily. **Q4. Two materials have the same mass but different volumes. Which is denser? Explain with an example.** **Answer:** The material with smaller volume is denser. Density = Mass ÷ Volume. For equal mass, smaller volume means higher density. Example: 1 kg of iron occupies ~127 cm³ (density ≈ 7.87 g/cm³), while 1 kg of water occupies 1000 cm³ (density = 1 g/cm³). Iron is denser than water. **Q5. Why is cotton suitable for summer clothing but wool is better for winter? Use material properties in your answer.** **Answer:** Cotton is light, breathable (porous), and absorbs moisture (hygroscopic), allowing sweat to evaporate and keeping the body cool. Wool is dense, warm-insulating (traps air pockets), and has high thermal resistance, preventing heat loss—ideal for cold weather. Cotton's low thermal conductivity is an advantage in summer (doesn't absorb body heat); wool's high thermal insulation is an advantage in winter.

Three-mark questions demand explanation, comparison, and reasoned answers. They bridge concept understanding and real-world application. **Q1. A jeweller has three metals: gold, silver, and aluminium. Using properties of malleability, ductility, and lustre, explain why gold and silver are preferred for jewellery over aluminium.** **Answer:** Gold and silver are both highly malleable (can be hammered into thin sheets) and ductile (drawn into fine wires), allowing intricate designs. Both exhibit high lustre (natural shine), making jewellery attractive without heavy polishing. Aluminium, though malleable, has lower lustre and can oxidize (tarnish), reducing its aesthetic value. Gold and silver also resist corrosion, maintaining appearance over years. Thus, malleability + ductility + high lustre + corrosion resistance make them ideal for jewellery. **Q2. A builder needs to choose between glass and polycarbonate sheets for a greenhouse. Compare using transparency, hardness, and durability. Which is better and why?** **Answer:** Both are transparent, allowing sunlight entry. Glass is harder (high hardness, rating 5.5 on Mohs scale) but brittle—breaks easily under impact. Polycarbonate is less hard but highly impact-resistant (ductile), lasting 10+ years vs. glass's fragility. In a greenhouse exposed to weather and potential hail, polycarbonate is superior: it remains transparent, withstands mechanical stress, and has lower replacement costs despite slightly lower transparency than pure glass. **Q3. At 20°C, sugar's solubility in water is 200 g per 100 mL. Calculate how much sugar can dissolve in 500 mL of water at the same temperature. If you heat the solution to 50°C, will more sugar dissolve? Explain.** **Answer:** Solubility proportion: (200 g ÷ 100 mL) × 500 mL = 1000 g sugar. Yes, more sugar will dissolve at 50°C because sugar solubility increases with temperature (endothermic dissolution). At 50°C, sugar's solubility ≈ 487 g per 100 mL, so 500 mL can dissolve ~2435 g—far exceeding the 20°C amount. This demonstrates that solubility is temperature-dependent, a critical concept for crystallization and industrial processes. **Q4. A metal X sinks in water but floats in mercury. Explain this using density. If the density of mercury is 13.6 g/cm³ and water is 1 g/cm³, what can you infer about X's density?** **Answer:** Since X sinks in water, its density > 1 g/cm³. Since X floats in mercury, its density < 13.6 g/cm³. Therefore, 1 < density of X < 13.6 g/cm³. This range includes metals like iron (7.87 g/cm³), copper (8.96 g/cm³), and nickel (8.90 g/cm³). This demonstrates that density determines whether objects float or sink relative to specific liquids—a core principle in material selection for maritime and aerospace applications.

Five-mark questions require comprehensive, multi-part answers showing deep understanding, calculations, and extended reasoning. **Q1. Describe the classification of materials based on physical properties. For each classification category, provide at least two examples and explain one real-world application.** **Full Solution:** Materials are classified into five main categories: (1) **Hardness (Hard/Soft):** Hard materials (iron, glass, diamond) resist scratching and deformation; soft materials (wax, cloth, foam) deform easily. Iron's hardness makes it ideal for construction tools and structural beams; wax's softness suits it for candles and crayons. (2) **Transparency (Transparent/Translucent/Opaque):** Transparent materials (glass, water) allow clear light passage; translucent materials (frosted glass, paper) diffuse light; opaque materials (wood, metal) block light. Transparent glass is essential for windows and lenses; opaque metals are used for machine parts needing structural integrity. (3) **Lustre (Lustrous/Non-lustrous):** Lustrous materials (metals like copper, aluminium) reflect light, appearing shiny; non-lustrous materials (rubber, wood) appear dull. Lustrous metals are chosen for decorative items and mirrors; non-lustrous materials are used where glare is undesirable. (4) **Solubility (Soluble/Insoluble):** Soluble materials (salt, sugar) dissolve in solvents like water; insoluble materials (sand, rocks) do not. Salt's solubility enables its use in food and preservation; sand's insolubility makes it suitable for construction without contamination. (5) **Density (High/Low):** High-density materials (lead, iron, 7–13 g/cm³) are heavy and compact; low-density materials (cork, foam, 0.2–0.5 g/cm³) are light and buoyant. Lead's high density makes it suitable for radiation shielding; foam's low density makes it ideal for insulation and buoyancy devices. This multi-property classification allows engineers and manufacturers to match materials to specific functional requirements, ensuring safety, efficiency, and longevity. **Q2. Sameer wants to design a saucepan for cooking. Explain which material (copper, glass, plastic) is most suitable. Use at least four material properties in your reasoning (thermal conductivity, melting point, hardness, and cost). Include why the other two are unsuitable.** **Full Solution:** **Copper is the best choice for a cooking saucepan.** **Copper's Advantages:** - **High thermal conductivity (~385 W/m·K):** Distributes heat evenly across the pan, cooking food uniformly without hot spots. - **High melting point (1084°C):** Withstands high cooking temperatures (typically 100–300°C) without deforming or releasing toxic fumes. - **Moderate-to-high hardness:** Resists scratches from utensils and maintains shape under mechanical stress during cooking and cleaning. - **Cost-effectiveness:** While more expensive than plastic initially, copper's durability (20–30 years) provides long-term value. **Why Glass is Unsuitable:** - **Low thermal conductivity (~1 W/m·K):** Heats slowly and unevenly; food burns in spots while other areas remain undercooked. - **Brittleness:** Sudden temperature changes (thermal shock) cause cracking. Placing hot glass on a cold surface risks breakage. - **Risk of shattering:** Dangerous in kitchens; broken glass fragments contaminate food. **Why Plastic is Unsuitable:** - **Low melting point (60–150°C depending on type):** Melts or warps under cooking heat, releasing harmful chemicals (BPA) into food. - **Poor thermal conductivity (~0.2 W/m·K):** Inadequate for efficient cooking; handles become uncomfortably hot. - **Low hardness:** Scratches easily; damaged surfaces trap bacteria and reduce lifespan. **Conclusion:** Copper's combination of high thermal conductivity, high melting point, adequate hardness, and reasonable cost makes it the superior choice. Stainless steel is an equally excellent alternative, offering corrosion resistance alongside similar thermal properties. **Q3. At 20°C, the solubility of potassium nitrate (KNO₃) in water is 13 g per 100 mL. A saturated solution contains 260 g of KNO₃ dissolved in water. (a) Calculate the volume of water used. (b) If 50 g of KNO₃ is added to this saturated solution and the temperature is raised to 40°C (where KNO₃ solubility becomes 63 g per 100 mL), will all 50 g dissolve? Show calculations and explain.** **Full Solution:** **(a) Calculate the volume of water:** Solubility at 20°C = 13 g per 100 mL water. If 260 g KNO₃ is dissolved: Volume = (260 g ÷ 13 g) × 100 mL = 20 × 100 = 2000 mL = 2 L **(b) Will 50 g KNO₃ dissolve when added and heated to 40°C?** Current dissolved KNO₃ = 260 g in 2000 mL. Solubility at 40°C = 63 g per 100 mL. Maximum KNO₃ that can dissolve in 2000 mL at 40°C = (63 g ÷ 100 mL) × 2000 mL = 1260 g. Total KNO₃ after adding 50 g = 260 + 50 = 310 g. Since 310 g < 1260 g (maximum at 40°C), **yes, all 50 g will dissolve** and the solution remains unsaturated. **Explanation:** Solubility of KNO₃ increases significantly with temperature (from 13 to 63 g per 100 mL). Heating the saturated solution increases its capacity to dissolve more solute. The 50 g addition plus heating creates space for additional dissolution, converting the solution from saturated to unsaturated.

**Case Study: Selecting Materials for a Multi-Purpose Backpack** A manufacturer is designing a lightweight, durable backpack for students. The design must include: - A main body fabric - Zippers and metal fittings - Straps (weight-bearing) - A transparent front pocket Available material options: (A) Fabric: Cotton (200 g/m²) vs. Polyester (150 g/m²) (B) Metal fittings: Copper vs. Aluminium vs. Steel (C) Strap material: Leather vs. Nylon (D) Transparent pocket: Glass vs. Polycarbonate **Question:** Using material properties (density, hardness, thermal conductivity, ductility, transparency, and cost-effectiveness), recommend the best combination. Justify each choice with at least two property-based reasons. Identify one potential drawback of your recommendation. **Solution (Step-by-Step):** **Step 1: Analyze the Fabric Choice** - **Polyester is superior.** Reason: Lower density (1.38 g/cm³) than cotton (1.54 g/cm³) = lighter backpack. Polyester has high ductility, resisting tears; cotton is more prone to wear. Polyester is also hydrophobic, resisting water damage—critical for student use in varying weather. - Recommendation: **Polyester 150 g/m²** **Step 2: Analyze Metal Fittings** - **Aluminium is optimal.** Reason: Density 2.70 g/cm³ (lighter than steel at 7.87 g/cm³ and comparable to copper at 8.96 g/cm³), reducing overall weight. Aluminium exhibits moderate hardness and high ductility, withstanding repeated zipping and strain without cracking. It's corrosion-resistant (oxide layer) and cost-effective compared to copper/steel. - Recommendation: **Aluminium** **Step 3: Analyze Strap Material** - **Nylon is better than leather.** Reason: Nylon has lower density (~1.14 g/cm³) than leather (~0.9–1.0 g/cm³ but thicker), making straps lighter. Nylon exhibits high ductility and tensile strength (bears 3–6 kg per strap without stretching). Leather degrades with moisture; nylon resists water and UV damage. Cost-effectiveness: Nylon is 40–60% cheaper than quality leather. - Recommendation: **Nylon straps** **Step 4: Analyze Transparent Pocket Material** - **Polycarbonate is superior to glass.** Reason: Polycarbonate is highly transparent (transmits 88–92% of light) yet impact-resistant; glass is brittle and shatters under impact, creating hazardous sharp edges near students' faces/hands. Polycarbonate has lower density (~1.2 g/cm³) than glass (~2.5 g/cm³), reducing weight. Polycarbonate resists yellowing and is shatterproof—critical for student safety. - Recommendation: **Polycarbonate** **Final Recommendation:** Polyester fabric + Aluminium fittings + Nylon straps + Polycarbonate pocket. **Potential Drawback:** Polyester is less breathable than cotton, potentially causing moisture accumulation if heavy items are stored. Solution: Add ventilation holes or mesh panels in the backpack design. **Why This Approach Scores High Marks:** - Demonstrates multi-property reasoning (density, hardness, ductility, transparency, cost, durability) - Connects abstract properties to real-world function - Shows critical evaluation (trade-offs acknowledged) - Uses quantitative data (density values, percentages) - Acknowledges limitations—a hallmark of mature scientific thinking

Scoring high in Class 9 Science Chapter 6 requires more than solving questions once; it demands **spaced repetition, adaptive feedback, and pattern recognition**—exactly what cbsetutor.ai's AI tutor delivers. **Our Drilling Methodology:** **1. Adaptive Question Generation:** Our AI generates unlimited variations of each question type. After answering "Why is copper preferred for wires?", the system auto-generates 10 parallel scenarios: "Why is silver used in circuits?", "Why is rubber used for insulation?"—training your brain to apply concepts to unfamiliar contexts (the essence of CBSE board exams). **2. Intelligent Error Diagnosis:** When you misidentify a property, our AI pinpoints whether you confused definitions (transparent vs. translucent), miscalculated density, or misapplied reasoning. It then generates targeted mini-lessons—not generic tutoring, but microsessions addressing your specific gap. **3. Spaced Repetition Schedule:** Questions are resurface at optimal intervals (Day 1, Day 3, Day 7, Day 21) to move knowledge from short-term to long-term memory. By exam day, you've reinforced these patterns 15–20 times, building unshakeable automaticity. **4. Real Exam Simulation:** Our platform includes timed mock tests simulating actual CBSE paper structures—Section A (MCQs), Section B (2-mark), Section C (3-mark), Section D (5-mark)—with authentic difficulty and time constraints. **5. Daily 15-Minute Power Sessions:** Instead of marathon study, you drill 3–5 questions daily in 15 minutes. This consistency (daily habit > weekend cramming) builds deep, retrievable learning. **6. Video Explanations by Expert Tutors:** Each question links to a 2–3 minute video from qualified CBSE teachers explaining the concept, worked examples, and common mistakes—available 24/7 for on-demand learning. **Sample Workflow:** - Monday: Answer 4 MCQs on solubility; AI identifies you confused "solubility" with "boiling point." - AI delivers a 3-minute microlesson: Solubility vs. Boiling Point (definitions, real examples). - Tuesday: Same concept resurfaces in a different question; you answer correctly. - Week 3: A 3-mark question on solubility reappears; you solve it confidently. Start a 3-day free trial at cbsetutor.ai to experience this AI-powered, pattern-drilling approach—no credit card required.

Beyond practicing questions, these evidence-backed strategies amplify your learning: **1. Create a Property Comparison Matrix:** Sketch a table with material names (copper, glass, plastic, wood, cotton, wool) vs. properties (hardness, transparency, lustre, solubility, density, thermal conductivity). Fill cells with 'High/Low/Yes/No.' This visual synthesis strengthens memory retrieval. **2. Use Density as Your Anchor:** Most solubility and floating/sinking problems hinge on density. Memorize: Water = 1 g/cm³; Iron ≈ 7.9 g/cm³; Aluminium ≈ 2.7 g/cm³; Cork ≈ 0.24 g/cm³. With these benchmarks, you instantly recognize whether an object sinks or floats. **3. Real-World Mapping:** When learning a property, immediately think of three everyday items exemplifying it. E.g., "Transparency: Window (glass), Frosted lamp (translucent), Wooden door (opaque)." This cognitive linking prevents rote memorization and aids exam recall under time pressure. **4. Teach-Back Method:** After studying a section, explain it to a parent or friend in 2 minutes without notes. Articulation reveals gaps in your understanding that re-reading alone misses. **5. Practice Backward:** Instead of answering "What is lustre?" (easy recall), reverse-engineer: "Glass and metal are lustrous; rubber and paper aren't. Why?" This forces deeper reasoning—exactly what 3 and 5-mark questions demand.