NCERT Class 6 Science Chapter 8: A Journey Through States of Water
These easy-to-understand notes cover all the important topics from NCERT Class 6 Science Chapter 8, including evaporation, condensation, states of water, melting, freezing, humidity, evaporative cooling and the water cycle.
1. Investigating Water's Disappearing Act — Evaporation
Evaporation: The process by which a liquid changes into vapour from its surface, usually below its boiling point, is called evaporation.
Examples of evaporation:
- Wet clothes dry in sunlight.
- Water in an open container slowly decreases.
- Sweat evaporates from our skin.
- Puddles of water disappear after rain.
Why does water disappear?
- Water particles are continuously moving.
- Some surface particles gain enough energy to escape into the air as water vapour.
- This process is called evaporation.
Important points:
- Evaporation is a surface phenomenon.
- Evaporation causes cooling because energetic particles escape from the liquid.
- Its rate depends on temperature, humidity, wind speed and surface area.
2. Another Mystery — Condensation
Condensation: The process by which water vapour changes into liquid water on cooling is called condensation.
Examples:
- Water droplets form on the outside of a cold glass.
- Dew forms on grass in the early morning.
- A bathroom mirror becomes misty after a hot shower.
When moist air touches a sufficiently cold surface, water vapour loses energy and changes into tiny liquid water droplets.
Remember: Condensation is the reverse of evaporation. The droplets on a cold glass come from water vapour in the air, not from inside the glass.
3. What Are the Different States of Water?
Water exists in three main physical states: solid, liquid and gas.
A. Solid State — Ice
- Has a definite shape and volume.
- Particles are closely packed and vibrate about fixed positions.
- Examples: ice cubes and snow.
B. Liquid State — Water
- Has a definite volume but no definite shape.
- Takes the shape of its container.
- Particles can move past one another.
- Examples: river water and drinking water.
C. Gaseous State — Water Vapour
- Has no definite shape or volume.
- Particles move freely and are far apart.
- Water vapour is invisible; the white mist above hot water mainly consists of tiny liquid droplets.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | Not definite | Not definite |
| Volume | Definite | Definite | Not definite |
| Particles | Closely packed | Can slide past one another | Far apart |
4. How Can We Change the States of Water? — Melting and Freezing
Melting
Melting is the process by which a solid changes into a liquid on heating. For example, ice melts into water.
At normal atmospheric pressure, pure ice melts at 0°C.
Freezing
Freezing is the process by which a liquid changes into a solid on cooling. For example, water freezes into ice.
At normal atmospheric pressure, pure water freezes at 0°C.
Other Changes of State
- Vaporisation: Liquid changes into gas.
- Condensation: Gas changes into liquid.
- Sublimation: Solid changes directly into gas.
- Deposition: Gas changes directly into solid.
These are physical changes. The substance remains water when ice, liquid water and water vapour change from one state to another.
5. How Can Water Be Evaporated Faster or Slower? — Humidity
Humidity: The amount of water vapour present in the air is called humidity.
- High humidity: Evaporation generally becomes slower.
- Low humidity: Evaporation generally becomes faster.
Factors affecting evaporation:
| Factor | Effect |
|---|---|
| Higher temperature | Increases evaporation |
| Larger surface area | Increases evaporation |
| Higher wind speed | Generally increases evaporation |
| Higher humidity | Generally decreases evaporation |
Difference Between Evaporation and Boiling
| Evaporation | Boiling |
|---|---|
| Occurs at the surface. | Occurs throughout the liquid. |
| Can occur at many temperatures. | Occurs at the boiling point for a given pressure. |
| Usually a slow process. | Usually a rapid process. |
| Does not require bubbles. | Bubbles form throughout the liquid. |
6. Cooling Effect — Pot-in-Pot Cooler
A pot-in-pot cooler is a simple cooling device that works on the principle of evaporative cooling.
Construction
- Take two unglazed earthen pots of different sizes.
- Place the smaller pot inside the larger pot.
- Fill the space between them with moist sand.
- Keep the sand moist by adding water.
- Place fruits or vegetables inside the smaller pot and cover with a damp cloth.
Working
- Water from the moist sand moves through the pores of the outer earthen pot.
- This water evaporates from the surface.
- Evaporation takes energy from the pot and its surroundings.
- The inner pot becomes cooler, helping keep the stored items relatively cool.
Why does it work better in dry weather? Low humidity allows faster evaporation, producing greater cooling.
Advantages: It is inexpensive, does not require electricity and can help keep fruits and vegetables fresh for longer under suitable conditions.
Limitation: It is less effective in humid weather because evaporation is slower.
7. How Do Clouds Give Us Rain? — The Water Cycle
Water cycle: The continuous movement of water between Earth's surface and the atmosphere is called the water cycle.
Steps of the Water Cycle
1. Evaporation
The Sun heats water in oceans, rivers and lakes. Liquid water changes into water vapour, which rises into the atmosphere.
2. Transpiration
Plants release water vapour mainly through tiny openings in their leaves. This process is called transpiration.
3. Condensation
Water vapour cools and condenses into tiny water droplets or ice crystals, forming clouds.
4. Precipitation
When water droplets or ice crystals grow sufficiently large, they may fall to Earth as rain, snow or hail.
5. Collection
Water collects in rivers, lakes, ponds and oceans. Some water seeps into the ground and becomes groundwater. The cycle continues.
Importance of the Water Cycle
- Replenishes rivers, lakes and other water bodies.
- Helps recharge groundwater.
- Supports plants, animals and humans.
- Distributes water across different parts of Earth.
- Plays an important role in weather and climate.
8. Important Definitions — Quick Revision
- Evaporation: Change of a liquid into vapour from its surface.
- Condensation: Change of water vapour into liquid water.
- Melting: Change of a solid into a liquid on heating.
- Freezing: Change of a liquid into a solid on cooling.
- Humidity: The amount of water vapour present in the air.
- Water vapour: Water in its gaseous state.
- Precipitation: Water falling from the atmosphere as rain, snow, hail or other forms.
- Transpiration: Release of water vapour from the aerial parts of plants, mainly through leaves.
- Water cycle: Continuous movement of water between Earth's surface and the atmosphere.
- Evaporative cooling: Cooling that occurs when evaporation removes energy from a surface or its surroundings.
9. Important Questions and Answers
Q1. Why do wet clothes dry faster on a sunny day?
Answer: The Sun increases the temperature of the water in the clothes, generally increasing evaporation.
Q2. Why do water droplets form on the outside of a cold glass?
Answer: Water vapour in the surrounding air cools on touching the glass and condenses into liquid droplets.
Q3. Why does evaporation cause cooling?
Answer: Energetic particles escape during evaporation, taking energy away and cooling the remaining liquid and its surroundings.
Q4. Why do clothes dry slowly on humid days?
Answer: Humid air already contains a large amount of water vapour, so evaporation is slower.
Q5. What happens to ice when it is heated?
Answer: Ice melts and changes from the solid state into the liquid state at its melting point.
Q6. What happens to water when it is cooled sufficiently?
Answer: At normal atmospheric pressure, pure water freezes at 0°C and changes into ice.
Q7. Why is an earthen pot useful for cooling water?
Answer: Water evaporates through the pores of the pot. This evaporation removes energy and cools the remaining water.
Q8. What is the role of the Sun in the water cycle?
Answer: The Sun supplies energy for evaporation and helps drive the water cycle.
Q9. How are clouds formed?
Answer: Clouds form when water vapour cools and condenses into tiny water droplets or ice crystals.
Q10. Name the three main states of water.
Answer: Solid (ice), liquid (water) and gas (water vapour).
Q11. What is the difference between evaporation and condensation?
Answer: Evaporation changes liquid water into vapour, whereas condensation changes water vapour into liquid water.
Q12. Why does a pot-in-pot cooler work better in dry weather?
Answer: Low humidity allows water to evaporate faster, producing greater cooling.
Chapter Summary
Water continuously changes between solid, liquid and gaseous states. Heating and cooling cause changes of state, evaporation produces cooling, and the water cycle continuously circulates water between Earth and the atmosphere.
These notes are designed for revision and understanding of NCERT Class 6 Science Chapter 8.
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