Class 9 Science Chapter 8 A Journey through States of Water: Previous Year Questions with Solutions

When you solve previous year questions, you engage in active recall—your brain retrieves facts and applies them to real exam contexts. Reading the NCERT chapter again offers diminishing returns after your first careful read-through. Past papers reveal exactly what examiners prioritize: the specific definitions (e.g., melting point of ice vs. boiling point of water), diagram labels (latent heat during phase transitions), and real-world applications (why clothes dry faster on hot days due to faster evaporation, or how water conservation relates to the hydrological cycle). Every question in this collection mirrors the style and difficulty level of actual CBSE papers, so you're practising with authentic material. By attempting these before your final revision, you'll identify weak spots—maybe you struggle with condensation vs. sublimation, or struggle to explain why evaporation is an endothermic process. Targeted practice on these gaps beats passive reading.

One-mark questions test direct recall and quick application. Here are five types that recur frequently: **Q1: At what temperature does ice melt?** Answer: 0 °C (at standard atmospheric pressure). This is the melting point of ice, a fixed temperature at which solid ice transitions to liquid water without temperature change during melting. **Q2: Define evaporation.** Answer: Evaporation is the process by which liquid water changes into water vapour (gaseous state) at any temperature below the boiling point. It occurs at the surface of liquids and is an endothermic process (absorbs heat). **Q3: Which process is the reverse of condensation?** Answer: Evaporation. Condensation converts gas to liquid; evaporation converts liquid to gas. **Q4: What is the boiling point of water at standard atmospheric pressure?** Answer: 100 °C. At this temperature, water molecules have enough energy to escape the liquid entirely and form steam (water vapour). **Q5: Name the process in which ice directly converts to water vapour without melting.** Answer: Sublimation. Example: dry ice (solid CO₂) and frozen clothes on a clothesline in winter. These five questions cover the core definitions and phase transitions that examiners test repeatedly. Memorize them word-for-word, as 1-mark answers demand precision.

Three-mark questions require explanation, reasoning, and sometimes a simple diagram or example. Here are five classic patterns: **Q1: Explain why evaporation is an endothermic process. Use the concept of intermolecular forces.** Answer: During evaporation, liquid molecules at the surface overcome intermolecular attractive forces to escape as gas. This requires energy input (heat), which is why evaporation cools the surroundings. Example: Sweat on your skin absorbs body heat to evaporate, cooling you down. This is why wet clothes feel cold initially, or why acetone on your hand feels cool. **Q2: Draw and label the water cycle, identifying evaporation, condensation, precipitation, and collection.** Answer: [Diagram description] The sun heats water in oceans/lakes → evaporation produces water vapour → rises and cools in upper atmosphere → condensation forms clouds → precipitation falls as rain → water collects in bodies of water, completing the cycle. Label each stage clearly. **Q3: Why does ice at 0 °C melt faster if you sprinkle salt on it, yet the temperature remains 0 °C during melting?** Answer: Salt lowers the melting point (freezing point depression), but during the phase transition itself, the temperature stays constant because all absorbed heat energy goes into breaking intermolecular bonds, not raising temperature. This is why latent heat of fusion applies. **Q4: Distinguish between boiling and evaporation.** Answer: Boiling occurs at a fixed temperature (100 °C for water) with vigorous bubble formation throughout the liquid; evaporation occurs at any temperature only at the surface and is slower. Both are endothermic, but boiling is faster and requires more energy. **Q5: Explain water conservation in the context of the water cycle and human use.** Answer: Although water cycles naturally, human activities (agriculture, industry, domestic use) deplete freshwater faster than replenishment. Conservation means reducing waste through efficient irrigation, fixing leaks, and treating wastewater for reuse—ensuring the water cycle remains sustainable.

Five-mark questions demand deeper understanding, often combining multiple concepts, calculations, or extended explanations. **Q1: A student heats ice from –10 °C to steam at 110 °C. Sketch a temperature vs. time graph and explain what happens at 0 °C and 100 °C.** Full Solution: — From –10 °C to 0 °C: Temperature rises linearly (ice warms). Slope represents heating solid ice (lower specific heat capacity). — At 0 °C: Temperature plateaus horizontally as ice melts. No temperature change; all energy breaks bonds (latent heat of fusion ≈ 334 kJ/kg for water). — From 0 °C to 100 °C: Temperature rises again (liquid water warms). Steeper slope than ice (water's specific heat capacity ≈ 4200 J/kg·K). — At 100 °C: Another plateau as water boils (latent heat of vaporization ≈ 2260 kJ/kg—much higher than fusion because gas molecules are far apart). — From 100 °C to 110 °C: Temperature rises (steam heats further). This illustrates why phase transitions require enormous energy despite no temperature change. **Q2: Explain why clothes dry faster on a hot, dry, windy day than on a cool, humid day. Use concepts of evaporation and water vapour pressure.** Full Solution: On a hot day: (a) Higher temperature = molecules move faster, escape liquid surface more readily. (b) Lower humidity = lower water vapour concentration in air, so the air can "absorb" more evaporated water (favouring net evaporation). (c) Wind removes water vapour from cloth surface, maintaining a concentration gradient that drives more evaporation. On a cool, humid day: (a) Lower temperature = slower molecular motion, fewer molecules escape. (b) High humidity = air already saturated with water vapour, so evaporation and condensation reach equilibrium (no net drying). (c) Still air traps water vapour near cloth, slowing further evaporation. This demonstrates that evaporation depends on temperature, humidity, and air movement—all three factors in the real world. **Q3: The water cycle is essential for life on Earth. Explain the role of evaporation and condensation in the cycle, and discuss two human practices that disrupt this cycle and two conservation strategies to protect it.** Full Solution: Role of evaporation: Solar energy heats oceans, lakes, and rivers, converting liquid water to vapour. Plants also release water via transpiration (evapotranspiration). This vapour rises into the atmosphere, carrying water to inland regions. Role of condensation: As water vapour rises and cools in the upper atmosphere, it condenses into water droplets, forming clouds. Precipitation (rain/snow) returns water to land and oceans. Disruptions: (1) Deforestation reduces transpiration, lowering water vapour release. (2) Pollution and industrial effluent contaminate water bodies, reducing available freshwater for the cycle. Conservation strategies: (1) Afforestation increases transpiration and groundwater recharge. (2) Wastewater treatment and recycling reduce freshwater demand, allowing more water to cycle naturally. These human actions either break or restore the balance of the hydrological cycle, directly affecting regional rainfall and water availability.

The CBSE rationalized syllabus for 2024–25 onwards has refocused on conceptual understanding and real-world applications, moving away from rote memorization. For Chapter 8, this means: (1) **More scenario-based questions**: Expect multi-part questions describing real situations (e.g., 'A village faces a drought; explain how the water cycle and conservation practices are linked'). (2) **Emphasis on energy concepts**: Latent heat, endothermic/exothermic processes, and why phase transitions require specific energy inputs are tested more rigorously. (3) **Integration with climate and sustainability**: Water conservation, climate change impacts on precipitation patterns, and environmental responsibility are now mainstream in Class 9 Science. (4) **Practical observations**: Questions may ask you to design an experiment (e.g., 'How would you demonstrate evaporation and condensation in a sealed container?'). (5) **Reduced rote definitions**: While definitions matter, they're contextualized rather than standalone. Study this chapter alongside Chapter 9 (Soil) and Chapter 10 (Gravitation) to see how water, soil, and natural systems interconnect. This holistic approach better prepares you for competitive exams later (JEE, NEET).

During your exam, time management is critical. Here's a proven strategy: **For 1-mark questions (2 minutes each):** Read the question, identify whether it asks for a definition, a temperature, or a process name. If it's a definition, use the textbook wording (precision matters). If it's a temperature or fixed fact, recall it directly. Move on; don't overthink. **For 3-mark questions (5–6 minutes each):** (a) Underline the key verb: 'Explain', 'Distinguish', 'Describe', 'State'. (b) Plan a 3-point structure in the margin: point 1, point 2, point 3. (c) Write briefly but clearly. Use a labelled diagram if needed (diagrams often secure a full mark even if explanation is partial). (d) Proofread for spelling of scientific terms (evaporation, condensation, sublimation). **For 5-mark questions (8–10 minutes each):** (a) Read the full question—it often has multiple parts (e.g., 'Explain AND discuss'). (b) Sketch a diagram first (temperature vs. time graph, water cycle, etc.). (c) Write point-by-point with sub-headings (Role of evaporation, Role of condensation, etc.). (d) Include one worked example or numerical detail if possible (e.g., latent heat values). (e) End with a concluding statement that ties back to the question. **General tips:** (1) Answer in the order questions appear—don't skip ahead unless stuck. (2) If you blank on a full answer, write partial knowledge; examiners award step-marks. (3) Keep a watch; allocate time proportionally: 1-mark = 2 min, 3-mark = 5 min, 5-mark = 8 min. (4) For diagrams, label all parts clearly—unlabelled diagrams score zero. Start a 3-day free trial at cbsetutor.ai to practice mock papers with this exact strategy and receive instant feedback on your timing and accuracy.

Chapter 8 tests your understanding of phase transitions, the water cycle, and how energy is involved in state changes. The three states of water—solid, liquid, gas—are fundamental to chemistry and physics, so mastery here builds confidence for future chapters. Evaporation and condensation are the most exam-heavy topics; ensure you can explain why evaporation is endothermic (absorbs heat, cooling surroundings) and why condensation is exothermic (releases heat). The water cycle connects all these concepts: evaporation → condensation → precipitation → collection, each step a real-world application. Water conservation is increasingly important in CBSE papers; link it to the hydrological cycle and human impact. Work through the 13 questions provided above at least twice: first under timed conditions (simulate exam pressure), then untimed (focus on depth). Refer to your NCERT textbook for any concept that remains unclear—the diagrams in NCERT (especially the water cycle and temperature vs. time graph for heating ice to steam) are gold. Finally, discuss difficult concepts with peers or a tutor; explaining to others solidifies your own understanding.