Class 6 Science Chapter 8 Notes
💧
(New NCERT "Curiosity" | 2025-26)
📌 Chapter Overview
This chapter explores how water exists in three different states — solid, liquid, and gas — and how it changes from one state to another. Through fun activities and real-life observations, students learn about evaporation, condensation, and the water cycle. The chapter builds scientific thinking by encouraging students to observe, question, and experiment.
💡 Opening Story: Aavi and Thirav are enjoying shikanji (lemonade) on a bright summer afternoon. Thirav looks at the ice cubes and wonders — "Are ice and water the same substance or different?" This curiosity starts the entire journey of the chapter!
🔑 Key Points
- Water exists in three states — Solid (ice), Liquid (water), Gas (water vapour)
- Water changes state by heating or cooling
- Evaporation = liquid water → water vapour (by heating)
- Condensation = water vapour → liquid water (by cooling)
- Melting = solid ice → liquid water (by heating)
- Freezing = liquid water → solid ice (by cooling)
- The continuous movement of water between Earth and atmosphere = Water Cycle
- Changes in states of water are reversible (physical) changes
🔎 8.1 Three States of Water
| State | Form | Properties | Examples |
|---|---|---|---|
| Solid | Ice | Fixed shape, fixed volume | Ice cube, glacier, snowflake |
| Liquid | Water | No fixed shape, fixed volume | River, rain, glass of water |
| Gas | Water Vapour | No fixed shape, no fixed volume | Steam, clouds, humidity |
💡 Key Insight: Ice, water, and water vapour are all the same substance — H₂O — just in different states! Thirav was wrong — ice and water are NOT different substances.
Other substances also show three states:
- Wax → solid (candle) melts to liquid when heated
- Ghee → liquid in summer, solid in winter
- Coconut oil → solidifies in cold weather
🔎 8.2 Evaporation — The First Mystery
Story: On a rainy morning, Aavi and Thirav saw puddles on their school playground. By evening, many puddles had disappeared! Where did the water go?
They found two answers:
- Water seeped into the ground
- Water turned into water vapour and went into the air → Evaporation
✅ What is Evaporation?
- Evaporation is the process by which liquid water slowly turns into water vapour (gas) and goes into the air
- It happens at all temperatures (not just at boiling point)
- It is a slow process that occurs only at the surface of liquid
✅ Common Examples of Evaporation:
- Wet clothes drying in the sun
- Mopped floor becoming dry
- Hand sanitizer disappearing when rubbed
- Water sprinkled on hot dosa pan disappearing
- Puddles disappearing after rain
✅ Factors That Affect Rate of Evaporation:
| Factor | Effect on Evaporation |
|---|---|
| Temperature | Higher temperature → Faster evaporation |
| Surface Area | Larger surface area → Faster evaporation |
| Wind / Air movement | More wind → Faster evaporation |
| Humidity | Higher humidity → Slower evaporation |
📌 Activity 8.7: Water in a plate evaporates faster than water in a bottle cap — because the plate has a larger surface area. Clothes dry faster on a sunny, windy day — because of high temperature + air movement.
🔎 8.3 Cooling Effect of Evaporation
- Evaporation absorbs heat from the surroundings → causes cooling effect
- That is why:
- Earthen pot (matka) keeps water cool — water seeps through tiny pores and evaporates, cooling the remaining water
- We feel cool when sweat evaporates from our skin
- Pot-in-pot cooler works on this principle — used in rural areas as a natural refrigerator without electricity!
✅ Pot-in-Pot Cooler (Activity 8.9):
- Place a small earthen pot inside a larger one
- Fill the gap between them with wet sand
- Water evaporates from the sand → cooling effect keeps food/water inside cool
- Eco-friendly and sustainable — no electricity needed!
🔎 8.4 Condensation — The Second Mystery
Story: While making lemonade, Aavi and Thirav noticed water droplets forming on the outer surface of the cold glass. Where did this water come from? Did it seep through the glass?
Experiment: They placed a sealed cold glass on a weighing balance. The weight increased over time! This proved water came from outside the glass — from the air around it!
✅ What is Condensation?
- Condensation is the process by which water vapour (gas) in the air cools down and turns into liquid water droplets
- It is the opposite of evaporation
- It occurs when water vapour touches a cold surface
✅ Common Examples of Condensation:
- Water droplets on outer surface of cold glass/tumbler
- Dew on grass and leaves in early morning
- Water droplets on the inner side of a lid covering boiling water
- Fog forming on cold window panes
- Water droplets inside a cold water bottle taken outside
🔎 8.5 Evaporation vs Boiling
| Feature | Evaporation | Boiling |
|---|---|---|
| Speed | Slow process | Fast process |
| Where it happens | Only at the surface of liquid | Throughout the entire liquid |
| Temperature | Occurs at any temperature | Occurs only at boiling point (100°C) |
| Example | Clothes drying | Water bubbling on a stove |
🔎 8.6 The Water Cycle
The continuous circulation of water between the Earth's surface and the atmosphere is called the Water Cycle.
✅ Stages of the Water Cycle:
Step 1 — Evaporation: Sun heats water in oceans, rivers, lakes → water turns into water vapour → rises into the atmosphere
Step 2 — Rising Vapour: Water vapour is lighter than air → rises high up in the atmosphere
Step 3 — Condensation: High up, air is very cold → water vapour condenses around tiny dust particles → forms tiny liquid droplets → these droplets cluster together to form clouds
Step 4 — Precipitation: Tiny droplets in clouds join together → form bigger, heavier drops → too heavy to stay in air → fall down as rain, snow, or hail
Step 5 — Collection: Water collects back in oceans, rivers, lakes, and underground → cycle repeats!
💡 Cloud Formation Activity (8.10): A burnt piece of newspaper in a bottle provides dust particles → water vapour condenses around them → a hazy cloud appears inside the bottle!
✅ Why is the Water Cycle Important?
- Replenishes rivers, lakes, and oceans continuously
- Provides fresh water for all living things
- Regulates Earth's temperature and climate
- Without evaporation → no clouds → no rain → no life on Earth!
🔎 8.7 Changes of State Summary
- All these changes are reversible — the chemical nature of water (H₂O) does NOT change
- They are physical changes, not chemical changes
📖 Important Definitions
| Term | Definition |
|---|---|
| Evaporation | Process where liquid water turns into water vapour at any temperature |
| Condensation | Process where water vapour cools and turns back into liquid water |
| Melting | Process where solid ice turns into liquid water on heating |
| Freezing | Process where liquid water turns into solid ice on cooling |
| Water Cycle | Continuous movement of water between Earth's surface and the atmosphere |
| Precipitation | Water falling from clouds as rain, snow, or hail |
| Humidity | The amount of water vapour present in the air |
| Evaporative Cooling | Cooling effect produced when liquid evaporates, absorbing heat |
| Reversible Change | A change that can be undone; original substance can be recovered |
| Pot-in-Pot Cooler | Eco-friendly cooler using evaporation from wet sand to keep food cool |
❓ NCERT Exercise Questions & Answers
Q1. Which of the following best describes condensation? (i) Conversion of water into vapour (ii) Water changing from liquid to gas (iii) Formation of clouds from droplets (iv) Conversion of water vapour into liquid
Answer: (iv) Conversion of water vapour into its liquid state is called condensation.
Q2. In which of these processes is evaporation very important? (b) Water colours drying after painting (c) Salt production from sea water (d) Drying of clothes
Answer: Evaporation is important in — (b) water colours drying, (c) salt production from sea water (water evaporates, salt remains), and (d) drying of clothes (water evaporates from wet cloth).
Q3. Why do water droplets appear on the outer surface of a glass containing cold water?
Answer: The air around the cold glass contains water vapour. When this warm water vapour comes in contact with the cold outer surface of the glass, it cools down and condenses into tiny liquid water droplets. This is condensation. The water does NOT come from inside the glass.
Q4. Why do clothes dry faster on a sunny, windy day compared to a cloudy, calm day?
Answer: On a sunny, windy day:
- High temperature increases evaporation rate
- Wind carries away water vapour, allowing more to evaporate quickly On a cloudy, calm day, both temperature and wind are lower, so evaporation is slower. That is why clothes take longer to dry.
Q5. Explain why an earthen pot keeps water cool.
Answer: An earthen pot has tiny pores. Water slowly seeps through these pores to the outer surface of the pot. This water on the outer surface evaporates. Evaporation absorbs heat from the water inside the pot, causing cooling. This is called evaporative cooling. That is why water stored in a matka feels cooler than water in a plastic or metal bottle.
Q6. What is the water cycle? Explain its stages.
Answer: The water cycle is the continuous natural process by which water circulates between the Earth's surface and the atmosphere. Its stages are:
- Evaporation — Sun heats water in oceans/rivers → turns into water vapour
- Rising — Water vapour rises high into the atmosphere
- Condensation — Water vapour cools and condenses around dust particles → forms clouds
- Precipitation — Water droplets join and fall as rain, snow, or hail
- Collection — Water collects back in oceans, rivers, lakes → cycle repeats
Q7. Ice is solid, water is liquid, and steam is gas — are they the same substance? Give a reason.
Answer: Yes, all three — ice, water, and steam — are the same substance (H₂O — water). They just exist in different states due to different temperatures. When heated, ice melts to water and water evaporates to steam. When cooled, steam condenses to water and water freezes to ice. The chemical composition does not change — only the state changes. These are reversible, physical changes.
✅ MCQ Questions
1. The process where water turns into water vapour is called: a) Condensation b) Freezing c) Evaporation ✅ d) Precipitation
2. Water droplets on the outer surface of a cold glass are due to: a) Evaporation b) Condensation ✅ c) Melting d) Precipitation
3. Evaporation happens fastest when: a) Surface area is small b) Temperature is low c) Humidity is high d) Surface area is large and temperature is high ✅
4. The SI unit of temperature at which water boils: a) 0°C b) 37°C c) 100°C ✅ d) 212°C
5. Which of these is a reversible change? a) Burning of wood b) Cooking rice c) Melting of ice ✅ d) Rusting of iron
6. Earthen pot keeps water cool because of: a) Condensation b) Freezing c) Evaporative cooling ✅ d) Precipitation
7. Clouds are formed when water vapour condenses around: a) Water drops b) Ice crystals c) Dust particles ✅ d) Salt crystals
8. Water falling from clouds as rain/snow/hail is called: a) Evaporation b) Condensation c) Collection d) Precipitation ✅
🧠Quick Revision Table
| Process | Change | Cause |
|---|---|---|
| Melting | Solid → Liquid | Heating |
| Freezing | Liquid → Solid | Cooling |
| Evaporation | Liquid → Gas | Heating / Any temperature |
| Condensation | Gas → Liquid | Cooling |
| Precipitation | Cloud → Rain/Snow/Hail | Heavy droplets fall |
🌟 Ancient Indian Connection!
The earthen pot (matka) cooling system has been used in India for thousands of years — long before refrigerators were invented! This traditional technology uses the science of evaporative cooling. The modern Pot-in-Pot Cooler (also called Zeer pot) is inspired by this ancient Indian wisdom and is still used in rural areas today as an eco-friendly, zero-electricity solution! 🇮🇳
📌 Exam Tips
- Most asked question — "Where do water droplets on a cold glass come from?" → Always answer: condensation of water vapour from the air
- Know all 4 processes — melting, freezing, evaporation, condensation — with examples
- Water cycle diagram with all 5 stages is very commonly asked
- Remember: Evaporation ≠ Boiling — know the differences clearly
- Earthen pot question = always mention evaporative cooling 🎯