Big Ideas (Why this chapter matters)
- Ice, water, water vapour are the same substance (H₂O) in three states: solid, liquid, gas.
- State decides shape, flow, spread: Ice (solid): fixed shape, doesn't flow. Water (liquid): no fixed shape, flows, keeps volume. Water vapour (gas): no fixed shape/volume, spreads to fill space.
- Two master processes drive most changes: Evaporation (liquid → vapour) and Condensation (vapour → liquid).
🧠 Memory Hook — "S–L–G: Shape–Flow–Spread"
Solid: Shape fixed → Liquid: Flows → Gas: Spreads.
Ice vs Water: Same or Different?
Try this activity:
Keep an ice cube in a cup and watch it melt.
Inference: Ice → water without anything "added", so they're two states of the same substance.
Daily signs:
- • Water splashes, ice doesn't
- • Water flows, ice doesn't
The "Disappearing Water" Mystery → Evaporation
Puddles shrink after sunshine; utensils dry after washing; a mopped floor dries. Water on a hot pan "vanishes" as steam (water vapour; visible "steam" is tiny droplets formed from vapour).
Definition:
Evaporation = liquid water changing into water vapour. It happens even at room temperature.
Factors that change the rate of evaporation (S–W–A–H):
Surface area ↑
→ faster (thin plate dries quicker than bottle cap with the same water).
Warmth/temperature ↑
(sunlight, hot day) → faster.
Air movement ↑
(wind, fan) → faster.
Humidity ↓
(drier air) → faster. Humid/rainy days → slower drying.
🧪 Mini-tests:
- • Put equal water in a cap and a plate → plate evaporates faster (area effect).
- • Keep equal water in sun vs shade; repeat on windy vs still days.
🌬️ Cooling by evaporation:
- • Matka/surahi stay cool because a little water seeps through clay and evaporates, removing heat.
- • Hand sanitiser feels cold: fast evaporation pulls heat from skin.
- • Fan makes sweat evaporate faster → cooling.
Water Droplets on a Cold Glass → Condensation
A tumbler with ice-cold water gets droplets outside. Where from? Not seepage (water level inside doesn't fall).
Explanation:
Water vapour in air condenses on the cold surface → droplets.
Definition:
Condensation = water vapour changing into liquid water.
Examples:
- • Dew on leaves in the morning
- • Droplets on a lid over boiling water
- • Used in AWG machines (cool moist air → water)
🧪 Weighing test idea:
Cover the cold tumbler and weigh every few minutes: mass increases as droplets collect outside (from air).
Changing State with Heating/Cooling
Melting:
solid → liquid (ice → water; wax melts on heating).
Freezing:
liquid → solid (water → ice; coconut oil can solidify in winter).
Evaporation:
liquid → gas (water on hot surface).
Condensation:
gas → liquid (dew, cold glass droplets).
🧠 Memory Hook — "Mi–Fre–Ev–Con"
Milting, Freezing, Evaporation, Condensation.
Properties of States (Quick Compare)
| Property | Ice (Solid) | Water (Liquid) | Water Vapour (Gas) |
|---|---|---|---|
| Shape | Fixed | No fixed shape; takes container's shape | No fixed shape |
| Flow | No | Yes | — |
| Spread | No | Spreads over a surface (volume constant) | Fills any available space |
| Seen at room temp | Yes (as ice if cooled enough) | Yes | Yes (present in air though invisible) |
From Puddles to Monsoon — Water Cycle
1. Evaporation:
from oceans, lakes, rivers, soil, plants.
2. Condensation:
cooling of moist air → tiny droplets around dust particles → clouds.
3. Precipitation:
many droplets merge → rain (or hail/snow in special conditions).
4. Return flow:
runoff via rivers/groundwater back to oceans.
🧪 Bottle-cloud demo:
Add a tiny burnt-paper piece (dust) to a bottle with some water; squeeze–release → hazy "cloud" appears (droplets condense on dust).
Important: Only a small fraction of Earth's water is directly usable; conserve and avoid pollution.
Investigations You Can Do (Class-Ready)
- 1Seepage or not? Mark the inside water level; if outside droplets form but level inside doesn't drop, it's condensation.
- 2Which dries faster? Change one factor at a time (area, heat/sunlight, airflow, humidity); keep others same; time to dry is your measure.
- 3Cooling seat trick: On a hot day, place a wet cloth on a hot scooter seat + fan/airflow → faster evaporation → cooler seat.
- 4Where is evaporation at work at home? Sweat drying, cooking aromas spreading, clothes drying, floor mopping, ink drying.
HOTS / Exam-Style Practice
- 1
Two tumblers: one with ice water, one with room-temp water. Only the cold one "sweats". Explain with condensation.
- 2
A student says "water on the cold tumbler seeped out". Design a fair test to rule out seepage.
- 3
Clothes dry slowly on a rainy day though fans are ON. Use humidity and airflow to explain.
- 4
In two identical rooms, same wet cloth area: one sunlit, one shaded. Predict and justify which dries first and why.
- 5
Explain matka cooling with evaporation; compare with feeling cold after sanitiser use.
Quick Recap
- Same substance (H₂O) can be solid/liquid/gas with different behaviours.
- Evaporation (liquid→gas) speeds up with bigger area, heat, wind, lower humidity; it cools surfaces.
- Condensation (gas→liquid) forms dew and droplets on cold surfaces; it's central to clouds & rain.
- Melting/Freezing switch between solid and liquid via heating/cooling.
- The water cycle is evaporation → condensation → precipitation → return flow.
- Use water wisely—only a small part is readily usable.