Chapter 8: A Journey Through States of Water

 Chapter 8: A Journey Through States of Water

1. Water Exists in Different States

Water is found in three states:

State             Form of Water                     Main Property

• Solid                 Ice                                     Has a fixed shape
• Liquid                 Water                         Flows and takes the shape of its container
• Gas                     Water vapour                 Spreads throughout the available space

Ice and water are different forms of the same substance.     

2. Evaporation:-When water changes from the liquid state to the gaseous state, the process is called evaporation.

Examples:- 

- Wet clothes become dry.

- A mopped floor dries.

- Sweat disappears from our body.

- Water in puddles gradually disappears.

- Water sprinkled on a hot pan disappears.

Important: Evaporation can occur even at room temperature.     

Water Vapour

Water vapour is the gaseous form of water. It is normally invisible and is present in the air around us.    

3. Condensation:- When water vapour changes into liquid water, the process is called condensation.

Examples

- Water droplets appear on the outside of a glass containing ice-cold water.

- Dew drops form on plants.

- Water droplets form on the inner side of a lid placed over boiling water.

Water vapour in the air comes in contact with a cold surface and changes into water droplets.     

Important Experiment

When a glass tumbler containing ice-cold water is kept covered and its water level does not decrease, but water droplets appear outside, the droplets are formed mainly due to condensation of water vapour from the air.   

4. Properties of the Three States of Water

Solid – Ice

- Has a fixed shape.

- Does not flow.

- Does not spread.

- Retains its shape when transferred between containers.

Liquid – Water

- Has no fixed shape.

- Takes the shape of its container.

- Flows from one place to another.

- Can spread.

- Its volume remains constant.

Gas – Water Vapour

- Has no fixed shape.

- Spreads throughout the available space.

- Water vapour is present in air even at room temperature.

- It is generally invisible.     

5. Change of States of Water:- Water can change from one state to another when it is heated or cooled.

Melting

Solid → Liquid

The conversion of a solid into a liquid is called melting.

Example:

Ice → Water

Freezing

Liquid → Solid

The conversion of a liquid into a solid is called freezing.

Example:

Water → Ice

Evaporation

Liquid → Gas

Water → Water vapour

Condensation

Gas → Liquid

Water vapour → Water

 

Easy Flow Chart

Ice

↓ Melting / Heating

Water

↓ Evaporation / Heating

Water Vapour

And in the reverse direction:

Water Vapour

↓ Condensation / Cooling

Water

↓ Freezing / Cooling

Ice

6. Factors Affecting Evaporation:- Evaporation can be made faster or slower depending on different conditions.

(a) Exposed Surface Area:- A larger exposed surface allows faster evaporation.

Example: Water spread on a plate evaporates faster than the same amount of water kept in a small bottle cap.    

(b) Temperature:- Higher temperature makes evaporation faster.

Example: Clothes dry faster on a hot, sunny day.

(c) Movement of Air:- Moving air increases evaporation.

Example: Clothes dry faster on a windy day.    

(d) Humidity:- Humidity means the amount of water vapour present in the air.

When humidity is high, evaporation becomes slower.

Example: Clothes dry slowly on a rainy day because the air already contains more water vapour.    

Remember

Evaporation is faster when:

- Surface area is large

- Temperature is high

- Air movement is greater

- Humidity is low

7. Cooling Effect of Evaporation:- Evaporation produces a cooling effect.

When a liquid evaporates, it takes heat from its surroundings, causing cooling.

Examples

- Sweat evaporates and cools our body.

- Sanitiser feels cool when rubbed on the hands.

- Water sprinkled on the floor or roof during summer provides cooling.

- Wind from a fan helps sweat evaporate faster, making us feel cooler.     

8. Why Is Water in a Matka Cool?:- An earthen pot (matka) has tiny pores through which a small amount of water can reach the outer surface.

This water evaporates from the surface. Evaporation produces a cooling effect, making the water inside the pot cooler.    

A surahi also uses this principle to keep water cool.

9. Pot-in-Pot Cooler:- A simple electricity-free cooler can be made using:

- Two earthen pots of different sizes

- Sand

- Water

- A lid or wet cloth

Water keeps the sand moist and evaporates from the earthen pot, producing a cooling effect.     

10. How Do Clouds Form?:- The formation of clouds involves evaporation and condensation.

Steps

1. Evaporation:- Water from oceans and Earth's surface evaporates and becomes water vapour.

2. Rising of Water Vapour:- Water vapour rises into the atmosphere because it is lighter than air.

3. Cooling:- As air rises higher, it becomes cooler.

4. Condensation:- Water vapour changes into tiny water droplets, generally around dust particles.

5. Cloud Formation:- Many tiny droplets come together and form clouds.

6. Rainfall:- The droplets join together to form larger, heavier drops. When they become sufficiently heavy, they fall as rain.

Under special conditions, precipitation may also occur as hail or snow.    

11. Role of Dust Particles in Cloud Formation:-Tiny water droplets can form around very small particles such as dust.

The textbook activity using a bottle demonstrates that water vapour can condense around tiny particles and produce a hazy, cloud-like appearance.     

12. Water Cycle:- The continuous movement of water between the Earth's surface and atmosphere is called the water cycle.

Main Processes

Water bodies → Evaporation → Water vapour → Condensation → Clouds → Rain/Snow/Hail → Rivers/Groundwater → Oceans

The water from oceans and Earth's surface evaporates, returns to Earth as precipitation, and eventually flows back towards the oceans.     

Important Terms

- Evaporation: Liquid → Gas

- Condensation: Gas → Liquid

- Melting: Solid → Liquid

- Freezing: Liquid → Solid

- Humidity: Amount of water vapour in air

- Water cycle: Continuous circulation of water between Earth and atmosphere

13. Water and Our Responsibility:- Only a small portion of Earth's water is suitable for direct use by plants, animals and humans. Most water is present in oceans and cannot be used directly.

Increasing population increases the demand for water and can cause water shortages.

Therefore, we should:

- Use water wisely.

- Avoid wasting water.

- Keep rivers, lakes and other water bodies free from pollution.

- Understand that conserving water is everyone's responsibility.     

Answers to “Let Us Enhance Our Learning”

1. Which of the following best describes condensation?

Answer: (iv) The conversion of water vapour into its liquid state.

Condensation is the process in which water vapour changes into liquid water.     

2. Identify in which of the given processes evaporation is very important.

(i) Colouring with:

Answer: (b) Water colours

Water in the colours evaporates, allowing the colour to dry.

(ii) Writing on paper with:

Answer: (b) Ink pen

The liquid part of the ink evaporates, leaving the writing on the paper.

3. Why does the space around natural grass feel cooler than around plastic grass?

Answer: Natural grass contains moisture. Water from the grass and surrounding soil evaporates and takes heat from the surroundings. This produces a cooling effect. Plastic grass does not have the same evaporation of water, so it does not provide this cooling effect.     

4. Give examples of liquids other than water that evaporate.

Answer: Some liquids that evaporate are alcohol, petrol, perfume and nail-polish remover.

Note: The chapter establishes that liquids other than water can evaporate; these are common examples.

5. How does a fan help in drying wet clothes even though it makes things feel cooler?

Answer: A fan increases the movement of air around the wet clothes. Moving air helps water evaporate faster. Evaporation requires heat, which is taken from the surroundings and the wet clothes. Therefore, the clothes dry faster while the surroundings may feel cooler.     

6. Why is sludge left in heaps for 3–4 days before transportation?

Answer: Sludge contains a considerable amount of water. When it is left in heaps, water evaporates and the sludge becomes drier and lighter. This reduces its volume and weight, making transportation easier and reducing transportation costs. It also makes handling safer. This is an application of evaporation.    

7. What activities at home involve evaporation? How does understanding evaporation help us?

Answer: Activities involving evaporation include:

- Drying wet clothes.

- Drying a mopped floor.

- Drying washed utensils.

- Sweat evaporating from our body.

- Drying water from surfaces.

Understanding evaporation helps us know how to dry things faster and understand the cooling effect of evaporation. For example, spreading clothes out and allowing air to move around them helps them dry faster.    

8. How is water present in the solid state in nature?

Answer: Water is present in the solid state as ice, snow and hail. These are frozen forms of water.    

9. “Water is our responsibility before it is our right.” Share your thoughts.

Answer: Water is essential for all living beings. We should use it wisely, avoid unnecessary wastage and prevent water pollution. Since usable water is limited, everyone has a responsibility to conserve and protect it for present and future generations.     

10. How can you cool a hot two-wheeler seat parked in the sun?

Answer: Sprinkle or pour a small amount of water on the hot seat and allow it to evaporate. During evaporation, water takes heat from the seat and surroundings, producing a cooling effect. Air movement can make evaporation faster.