1. Chapter at a Glance
Wet clothes dry in the sun. Water drops appear on the outside of a glass of cold water. An ice cube melts in a cup. A puddle vanishes after the rain. All these are part of water’s amazing journey through its three states — solid (ice), liquid (water) and gas (water vapour). This chapter explains how water changes from one state to another and how this gives us clouds, rain and the water cycle.
- Three states of water and their properties.
- Evaporation, condensation, melting and freezing.
- Factors that make evaporation faster or slower; humidity.
- Cooling effect of evaporation — the pot-in-pot cooler.
- How clouds form and give rain; the water cycle.
2. Water’s Disappearing Act
Spread a little water on a plate and leave it. After some time the plate is dry. The water has not seeped through the steel plate — it has changed into invisible water vapour and mixed with the air. This change of liquid water into vapour is called evaporation. It happens all the time, even at room temperature — clothes dry, mopped floors dry, and sweat dries from our skin.
3. Another Mystery — Drops on a Cold Glass
Put ice cubes in water in a dry glass tumbler. Soon tiny drops appear on its outer surface. Did water leak through the glass? No — if it did, coloured water inside would make coloured drops outside. Also, weighing the tumbler shows its mass slightly increases. The drops come from water vapour in the air that touched the cold surface and turned into liquid. This change of vapour into liquid is called condensation. Dew drops on grass in winter mornings form the same way.
4. The Different States of Water
| Property | Ice (solid) | Water (liquid) | Water vapour (gas) |
|---|---|---|---|
| Shape | Fixed — keeps its own shape | Takes the shape of the container | No fixed shape |
| Volume | Fixed | Fixed | Fills all available space |
| Flow | Does not flow | Flows | Spreads in all directions |
| Can we hold it? | Yes | Only in a container | No — invisible |
Other substances also exist in these states — wax and ghee can be solid or liquid; oxygen and carbon dioxide are gases at room temperature.
5. How Can We Change the States of Water?
- Melting — solid to liquid by heating (ice to water, wax melting near a flame).
- Freezing — liquid to solid by cooling (water to ice in a freezer; coconut oil becoming solid in winter).
- Evaporation — liquid to vapour.
- Condensation — vapour to liquid on a cold surface.
6. Making Evaporation Faster or Slower
| Factor | Effect on evaporation | Example |
|---|---|---|
| Exposed surface area | Larger area → faster | Water on a plate dries before water in a bottle cap; clothes are spread out to dry |
| Sunlight / heat | More heat → faster | Clothes dry faster in sun than in shade |
| Wind | More air movement → faster | Clothes dry faster on a windy day or under a fan |
| Humidity | More water vapour in air → slower | Clothes take longer to dry in the rainy season |
Humidity is the amount of water vapour in the air.
Chart — time for the same amount of water to dry (illustrative):
Bar length shows drying time; shorter bar = faster evaporation.
7. Cooling Effect of Evaporation
When water evaporates, it takes heat from the surface it is on, so the surface cools. That is why sweating cools our body, wet hands feel cold under a fan and earthen pots (matkas) keep water cool — water seeps through tiny pores and evaporates from the outer surface.
Place a small earthen pot inside a larger one and fill the gap with wet sand. Cover the top with a wet cloth. Water evaporating from the sand cools the inner pot, keeping vegetables and fruits fresh for longer without electricity.
Natural grass feels cooler than plastic grass because real grass gives out water vapour (it evaporates water from leaves), while plastic does not.
8. How Do Clouds Give Us Rain?
Water evaporates from oceans, rivers, lakes, soil and plants. Water vapour is lighter than air, so it rises. Higher up, the air is cooler, so the vapour condenses into tiny droplets around dust particles; millions of droplets form a cloud. Droplets join to become bigger and heavier and fall as rain, or as hail or snow when it is very cold. The water flows back through rivers to the oceans and seeps into the ground as groundwater.
This continuous circulation of water between the Earth’s surface and the atmosphere is the water cycle. It is driven by the Sun’s heat. Water is precious: water is our responsibility before it is our right.
9. Master Table: Key Terms
| Term | Meaning |
|---|---|
| Evaporation | Liquid water → water vapour (at any temperature) |
| Condensation | Water vapour → liquid on a cold surface |
| Melting | Solid → liquid on heating |
| Freezing | Liquid → solid on cooling |
| Humidity | Amount of water vapour in air |
| Cooling effect | Evaporation takes heat away and cools the surface |
| Water cycle | Circulation of water between Earth’s surface and atmosphere |
10. Master Summary Table
| Observation | Process | Reason |
|---|---|---|
| Wet clothes dry | Evaporation | Water turns to vapour |
| Drops outside cold glass | Condensation | Air’s vapour cools on glass |
| Ice cube becomes water | Melting | Gains heat |
| Coconut oil solid in winter | Freezing | Loses heat |
| Matka water cool | Evaporation | Cooling effect |
| Clouds and rain | Condensation, precipitation | Vapour cools at height |
11. Mind Map
- Ice, water, vapour
- Shape, flow, volume
- Melting, freezing
- Evaporation, condensation
- Area, sun, wind faster
- Humidity slower
- Cooling effect
- Clouds on dust
- Rain, hail, snow
12. Tips and Tricks
- “Evaporate = Escape into air; Condense = Come back as drops.”
- To prove condensation, use coloured water inside the glass — the outer drops are colourless.
- Evaporation questions: check the four factors — area, heat, wind, humidity.
- Thinking evaporation happens only at 100 °C — it happens at all temperatures; boiling happens at 100 °C.
- Saying the drops on a cold glass leaked from inside.
- Writing that a fan makes clothes dry because it cools them — it increases air movement, which speeds evaporation.
13. Important One-liners
- Water exists as solid, liquid and gas.
- Ice has a fixed shape; water takes the shape of its container.
- Water vapour fills all available space and is invisible.
- Evaporation happens at room temperature too.
- Condensation forms dew and drops on a cold glass.
- A larger surface area, sunlight and wind increase evaporation.
- High humidity slows evaporation.
- Evaporation causes cooling.
- Clouds form when water vapour condenses around dust particles.
14. Exam-pattern Questions with Detailed Answers
The change of water vapour into liquid water is called:
- Evaporation
- Melting
- Condensation
- Freezing
Show answer
Vapour cooling into liquid is condensation; evaporation is the reverse; melting and freezing involve the solid state.
Answer: (C) Condensation
Name the process by which wet clothes dry.
Show answer
Water in the clothes changes into vapour and goes into the air.
Answer: Evaporation
Give two examples of melting from daily life.
Show answer
Ice cream melting in summer; candle wax melting near the flame.
Answer: Ice cream, candle wax
Why do clothes dry faster on a windy day?
Show answer
Wind carries away the water vapour near the clothes, so more water can evaporate quickly. More air movement means faster evaporation.
Answer: Wind removes vapour, speeding evaporation
How would you show that the drops on the outside of a glass of ice water do not come from inside?
Show answer
Add colour to the water inside. If the outer drops are colourless, they come from water vapour in the air (condensation), not from leakage.
Answer: Use coloured water — outer drops stay colourless
Why does water stay cool in an earthen pot in summer?
Show answer
Water seeps through tiny pores of the pot and evaporates from its outer surface. Evaporation takes heat from the pot and the water inside, cooling them.
Answer: Evaporation from pores causes cooling
Why do clothes take longer to dry in the rainy season?
Show answer
The air already has a lot of water vapour (high humidity), so it cannot take up more vapour quickly. Evaporation slows down.
Answer: High humidity slows evaporation
Where is water found in solid form in nature?
Show answer
As snow and glaciers on high mountains, as ice in polar regions, and as hailstones during some storms.
Answer: Snow, glaciers, polar ice, hail
A fan gives a cooling feeling, yet clothes dry faster under a fan. How can both be true?
Show answer
A fan does not cool the air; it moves it. Moving air speeds up evaporation of water (from our skin or clothes). On our skin, faster evaporation takes away heat, so we feel cool; on clothes, faster evaporation dries them quickly. Both are results of increased evaporation.
Answer: Fan increases evaporation — causing both cooling and drying
How can you cool a two-wheeler seat that has become very hot in the sun?
Show answer
Sprinkle a little water or wipe it with a wet cloth. As the water evaporates it takes heat from the seat, cooling it. Parking in the shade also helps.
Answer: Wet it — evaporation cools the seat
Explain how a bottle with a little water and some smoke from burnt paper can show cloud formation.
Show answer
The smoke gives tiny particles (like dust). When the squeezed bottle is released, the air inside cools, and water vapour condenses on the particles, making the bottle look hazy — a mini cloud. Real clouds form in the same way around dust particles.
Answer: Vapour condenses on smoke particles when air cools
Why does natural grass feel cooler than plastic grass on a hot day?
Show answer
Real grass contains water and loses it as vapour from its leaves. This evaporation cools the grass and the air near it. Plastic grass has no water to evaporate, so it heats up.
Answer: Evaporation from real grass cools it
15. Quick Revision Summary
- Three states: ice (fixed shape), water (flows, fixed volume), vapour (fills space).
- Melting and evaporation need heat; freezing and condensation happen on cooling.
- Evaporation is faster with more area, sunlight and wind; slower with high humidity; it causes cooling.
- Pot-in-pot cooler and matka use evaporative cooling.
- Clouds form by condensation on dust; rain, hail and snow complete the water cycle.