A Journey through States of Water – Case-based Questions with Answers

CBSE Class 6 Science - Chapter 8: A Journey through States of Water — 20 Case-Based Q&A

Content Bank — Focus Areas

Key topics: states of water, water cycle, evaporation, condensation, precipitation, melting/freezing, sublimation, latent heat, density and classroom experiments.

20 Case-Based Questions — Read each short case carefully and answer

Each case is followed by a model answer that explains the scientific concept clearly and concisely.

Case 1 — Morning Dew

Early in the morning you notice water droplets on the grass while the sky is clear.

Q: Explain how dew forms and why it appears on clear nights.

A: Dew forms by condensation when surfaces like grass cool overnight below the dew point. Moist air near the ground cools and water vapour condenses into droplets on the cooler surface. Clear nights allow more rapid cooling (no cloud insulation), so dew is more likely to form.

Case 2 — Ice Floating

A pond has a layer of ice during winter while fish survive below.

Q: Why does ice float and how does this help aquatic life?

A: Ice is less dense than liquid water due to its open crystalline structure, so it floats. The floating ice insulates the water below, slowing heat loss and allowing aquatic life to survive in liquid water beneath the ice layer.

Case 3 — Boiling on a Mountain

At a hill station, water is observed to boil at a temperature lower than 100°C.

Q: Explain why boiling point is lower at high altitudes.

A: Atmospheric pressure is lower at high altitudes. Boiling occurs when vapour pressure equals external pressure, so at lower external pressure the vapour pressure needed is reached at a lower temperature, causing water to boil below 100°C.

Case 4 — Puddle Drying

After a light rain, puddles disappear faster on a sunny day than on a cloudy day.

Q: Explain the factors that cause faster drying on sunny days.

A: Higher temperature and sunlight increase evaporation by providing energy to water molecules. Sunlight and warmer air increase kinetic energy of surface molecules and wind can remove humid air, both accelerating evaporation compared to cloudy, cooler conditions.

Case 5 — Fog at Dawn

A low-lying valley experiences thick fog in the early morning while surrounding hills are clear.

Q: Why does fog form in valleys and not on the hilltops under these conditions?

A: Cold air is denser and settles in valleys, cooling the air near the ground below the dew point and causing condensation into fog. Hilltops may remain warmer and above the dew point, so fog tends to form in low-lying areas where air cools and moisture accumulates.

Case 6 — Steam Burns

A child gets a more severe burn from touching steam than from touching boiling water briefly.

Q: Explain scientifically why steam can cause more severe burns than boiling water.

A: Steam contains latent heat; when it condenses on the skin it releases additional energy (latent heat of vaporisation) plus its sensible heat, transferring more energy to the skin than boiling water at the same temperature, causing more severe burns.

Case 7 — Salt on Roads

After snowfall, salt is spread on roads and the ice melts faster near salted areas.

Q: How does salt help melt ice on roads?

A: Salt dissolves in the thin film of water on ice and lowers the freezing point (freezing point depression), making ice melt at lower temperatures. This helps break up ice and prevents re-freezing at the same temperature.

Case 8 — Cloud Seeding Proposal

A town considers cloud seeding to increase rainfall during drought.

Q: Briefly describe how cloud seeding works and give one possible environmental concern.

A: Cloud seeding introduces particles (like silver iodide) into clouds to act as condensation nuclei, encouraging droplet formation and precipitation. A concern is unintended alteration of local weather patterns or ecological effects from seeding chemicals; efficacy and fairness of rain distribution are debated.

Case 9 — Dry Ice Demo

A teacher demonstrates dry ice turning into gas without leaving liquid behind.

Q: What process is being shown and why does it occur?

A: The process is sublimation — direct change from solid to gas. Dry ice (solid CO₂) sublimates at atmospheric pressure because its vapour pressure is sufficiently high at room temperature to bypass the liquid phase.

Case 10 — Evaporative Cooling

After exercise, a person feels cooler as sweat evaporates from the skin.

Q: Explain how evaporation of sweat cools the body.

A: Evaporation requires energy (latent heat) which is taken from the skin as sweat molecules escape into vapour. This loss of energy reduces the skin's temperature, producing a cooling sensation.

Case 11 — Boiling in Pressure Cooker

A pressure cooker cooks food faster at the same heating because contents cook quicker than in an open pot.

Q: Explain the physics behind faster cooking in a pressure cooker.

A: A pressure cooker increases internal pressure, raising the boiling point of water above 100°C. Higher boiling temperature means food cooks at higher temperatures, speeding up chemical reactions and softening food faster than in open cooking.

Case 12 — Cloud Formation Over Sea

Warm moist air rises from the ocean and forms clouds offshore.

Q: Describe how moisture from the sea contributes to cloud formation.

A: Sun-heated seawater evaporates, adding moisture to the air. Warm moist air rises, cools at higher altitudes, and the water vapour condenses on condensation nuclei to form cloud droplets, producing offshore cloud formation.

Case 13 — Rapid Drying with Fan

Wet clothes dry faster when placed in front of a fan compared to still air.

Q: Explain why airflow speeds up evaporation.

A: Airflow removes saturated air near the clothing surface and replaces it with drier air, maintaining a concentration gradient that allows more water molecules to escape from the liquid surface; this increases the rate of evaporation.

Case 14 — Freezing Point Depression Experiment

Students add salt to an ice-salt mixture and observe temperature drop below 0°C.

Q: Explain why temperature drops when salt is added and how this is useful for making ice cream.

A: Dissolving salt lowers the freezing point of water, causing ice to melt while absorbing heat from the surroundings, which lowers the mixture's temperature below 0°C. This cold mixture is used in traditional ice cream making to freeze the mix without a freezer.

Case 15 — Rain After Heat

On a hot summer afternoon, sudden showers occur nearby.

Q: Explain how daytime heating can lead to sudden rainfall.

A: Strong solar heating causes air to rise, carrying moist air upward where it cools and condenses into clouds. Rapid uplift and cooling can lead to heavy cloud formation and sudden convective rainfall, common in summer thunderstorms.

Case 16 — Puddle Ice in Shade

After a cold night, puddles in shaded areas remain frozen while those in the road melt.

Q: Why do shaded puddles freeze and stay frozen longer than puddles exposed to sunlight?

A: Shaded areas receive little direct sunlight, so they absorb less heat and remain colder; ice there does not receive enough energy to melt quickly. Road surfaces warm faster in sunlight and transfer heat to puddles, causing them to melt sooner than those in shade.

Case 17 — Groundwater Recharge

A region implements rainwater harvesting to replenish groundwater.

Q: Explain how rainwater harvesting helps recharge groundwater and benefits local water supply.

A: Collected rainwater is directed into recharge pits or percolation wells where it percolates through soil layers, replenishing aquifers. This increases groundwater levels, improves well water availability, reduces dependence on surface water, and mitigates drought effects.

Case 18 — Formation of Hail

During a storm, hailstones are observed damaging crops.

Q: Describe how hail forms within thunderclouds.

A: Strong updrafts in thunderclouds carry water droplets upward into freezing regions; droplets freeze and are repeatedly carried up and down, accumulating layers of ice until heavy enough to fall as hail. Rapid vertical motion and freezing cycles create hailstones.

Case 19 — Condensation Inside a Car

On a rainy day, car windows fog up inside.

Q: Explain why condensation forms on the inside of car windows and how to reduce it.

A: Warm moist air inside the car contacts the cooler glass surface, cooling below the dew point and condensing into droplets on the glass. To reduce it, increase ventilation, use the demister or air conditioning to remove moisture, or warm the glass to raise its temperature above the dew point.

Case 20 — Urban Flooding After Heavy Rain

A city experiences rapid flooding after intense rainfall despite moderate total rainfall for the season.

Q: Explain how urbanisation increases flood risk and suggest one mitigation strategy.

A: Urbanisation increases impermeable surfaces (roads, pavements, buildings) that reduce infiltration and increase rapid surface runoff, overwhelming drainage systems and causing floods. A mitigation strategy is to increase green spaces and install permeable pavements and rainwater harvesting systems to enhance infiltration and reduce runoff.

Note: These case-based questions and answers follow the NCERT Class 6 Science syllabus for Chapter 8 and are ideal for CBSE exam practice and application-based understanding.