A Journey through States of Water — NCERT Solutions

Given: Equal amounts of water are taken in two identical bottle caps — one placed in sunlight and one in shade.

Expected Observation: The water in the cap kept in sunlight evaporates faster (disappears in less time) than the water in the cap kept in shade.

Conclusions:

1. Effect of temperature/sunlight: Water evaporates faster in sunlight because the sun provides more heat energy to the water molecules, enabling them to escape into the air more quickly. Higher temperature increases the rate of evaporation.

2. Effect of wind: Clothes dry faster on a windy day because moving air carries away the water vapour from the surface, allowing more water to evaporate. With increased air movement, evaporation is faster.

3. Effect of humidity: On a rainy day, the air already has high humidity (more water vapour), so water evaporates slowly. Clothes dry slowly on rainy days.

Summary: Evaporation is faster when:

• Temperature is higher (sunny/hot day)
• Air movement is more (windy day)
• Humidity is lower (dry day)
• Surface area is larger

Answer: Water stored in an earthen pot (matka) is cooler than water stored in a stainless steel pot because of the cooling effect of evaporation.

Explanation:

1. An earthen pot is made of clay and has tiny pores (small holes) on its surface.
2. Water slowly seeps through these tiny pores to the outer surface of the pot.
3. This water on the outer surface evaporates continuously.
4. During evaporation, the water absorbs heat from the water inside the pot (as heat is needed for evaporation).
5. This removal of heat from the water inside the pot makes the water cool.

Stainless steel pot: A stainless steel pot has no pores, so water cannot seep through. There is no evaporation from the outer surface, and hence no cooling effect.

Other examples of cooling effect of evaporation:

• Sprinkling water on the floor or roof during summer cools it down.
• We feel cooler when we sit under a fan — the fan speeds up evaporation of sweat from our body.
• Rubbing sanitiser on hands gives a cooling sensation.

Explanation:

1. Our body produces sweat when we feel hot. Sweat is mostly water.
2. When we sit under a fan, the moving air (wind created by the fan) flows over our skin.
3. This moving air increases the rate of evaporation of sweat from our skin.
4. Evaporation requires heat energy. This heat is taken from our body (skin).
5. As heat is removed from our body during evaporation of sweat, we feel cooler.

Key concept: Evaporation causes a cooling effect because it absorbs heat from the surroundings (in this case, from our body).

This is also why:

• Water in an earthen pot is cool (evaporation through pores cools the water).
• We feel cold when we apply sanitiser (it evaporates quickly, taking heat from our hands).

Answer: When we rub sanitiser on our hands, we feel a cooling sensation.

Reason:

1. Hand sanitiser contains alcohol, which evaporates very quickly.
2. When we rub sanitiser on our hands, it spreads over the skin and evaporates rapidly.
3. During evaporation, the sanitiser absorbs heat from our hands.
4. This removal of heat from the skin gives us a cooling effect.

This is the same principle as the cooling effect of evaporation — evaporation absorbs heat from the surface, making it feel cool.

Answer: On a rainy day, humidity is high and evaporation is slow. To dry clothes faster on a rainy day, we can:

1. Use a fan or place clothes in a well-ventilated area: Increased air movement speeds up evaporation by carrying away water vapour from the surface of the clothes.

2. Place clothes near a heater or in a warm room: Higher temperature increases the rate of evaporation.

3. Spread clothes out fully: Increasing the exposed surface area of the wet clothes allows more water to evaporate at the same time.

4. Use a clothes dryer: It provides heat and air movement simultaneously, making evaporation very fast.

5. Place clothes in a room with a dehumidifier: Reducing humidity in the room speeds up evaporation.

Key principle: Evaporation is faster at higher temperatures, with more air movement, and at lower humidity.

Observation:

• Without burnt newspaper: When the bottle is squeezed and released, little or no haziness is observed above the water.
• With burnt newspaper: When the bottle is squeezed and released, haziness (like clouds) is observed above the water in the bottle.

Explanation:

1. Squeezing the bottle increases pressure inside, and releasing it causes a sudden drop in pressure.
2. This drop in pressure causes the air inside to cool slightly.
3. The burnt newspaper provides very tiny, invisible dust particles (smoke particles).
4. Water vapour present in the bottle condenses around these dust particles, forming tiny water droplets.
5. These tiny water droplets appear as haze or clouds above the water in the bottle.

Role of dust particles: Dust particles act as condensation nuclei — tiny surfaces around which water vapour condenses to form water droplets. This is similar to how clouds form in the atmosphere, where water vapour condenses around dust particles present in the air to form tiny water droplets, which together appear as clouds.

Labelling of the Water Cycle Diagram (Fig. 8.9):

Storage locations:

• Ocean — Large water body where most of Earth's water is stored.
• Lake — Inland water body on the Earth's surface.
• River — Flowing water body that carries water from land to the ocean.
• Groundwater — Water stored underground, below the Earth's surface.
• Cloud — Water stored in the atmosphere as tiny water droplets.

Processes (shown with arrows):

• Evaporation — Arrow pointing upward from Ocean/Lake/River to the atmosphere. Water from the surface converts into water vapour and rises into the atmosphere.
• Condensation — Arrow showing water vapour in the atmosphere converting into tiny water droplets to form clouds.
• Rain — Arrow pointing downward from clouds to the Earth's surface (liquid water falling).
• Snow — Arrow pointing downward from clouds to mountains/cold regions (solid water falling).

Description of the Water Cycle:
$$\text{Ocean/Lake/River} \xrightarrow{\text{Evaporation}} \text{Water Vapour in atmosphere} \xrightarrow{\text{Condensation}} \text{Clouds} \xrightarrow{\text{Rain/Snow}} \text{Earth's surface} \rightarrow \text{Rivers/Groundwater} \rightarrow \text{Ocean}$$

This continuous circulation of water between the Earth's surface and the atmosphere is called the water cycle.

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

Justification: Condensation is defined as the process by which water vapour (gaseous state) converts into liquid water upon cooling. This happens when water vapour comes in contact with a cold surface (e.g., water droplets on the outer surface of a cold glass tumbler, dew drops on plants in the morning). Options (i) and (ii) describe evaporation, and option (iii) describes cloud formation, which is a result of condensation but not the definition of condensation itself.

Answer:

(i) Colouring: Evaporation is very important when colouring with (b) water colours.

Reason: Water colours are dissolved in water. When we apply water colours on paper, the water in the paint needs to evaporate for the colour to dry and set on the paper. Crayons and pencil colours are solid and do not involve water, so evaporation is not important. Acrylic colours dry partly by evaporation of water but also by a chemical process (polymerisation), but water colours rely primarily on evaporation.

(ii) Writing on paper: Evaporation is very important when writing with (b) ink pen.

Reason: Ink used in ink pens contains a liquid (water or solvent) in which the dye is dissolved. After writing, the liquid in the ink needs to evaporate for the ink to dry on the paper. Pencil and ball point pen use solid graphite and thick oil-based ink respectively, where evaporation plays little or no role.

Answer: The space around natural grass feels cooler than the space around plastic grass because of the cooling effect of evaporation (also called transpiration in plants).

Explanation:

1. Natural grass is a living plant. It absorbs water from the soil through its roots.
2. This water evaporates from the surface of the grass leaves (a process called transpiration).
3. During evaporation, heat is absorbed from the surroundings, which cools the air around the natural grass.
4. Additionally, natural grass also absorbs sunlight for photosynthesis rather than reflecting all of it as heat.

Plastic grass:

1. Plastic grass is not a living plant and does not contain water.
2. There is no evaporation or transpiration from plastic grass.
3. Plastic absorbs and retains heat from sunlight, making the surrounding area warmer.

Conclusion: Natural grass cools the surrounding area through evaporation of water, while plastic grass heats up the surrounding area by absorbing and retaining heat. This is why natural grass areas feel cooler.

Examples of liquids other than water that evaporate:

1. Alcohol (ethanol): Used in hand sanitisers; evaporates quickly giving a cooling sensation.
2. Petrol: Evaporates at room temperature; that is why we can smell petrol near fuel stations.
3. Acetone (nail polish remover): Evaporates very quickly.
4. Perfume/Cologne: The liquid perfume evaporates and spreads its fragrance in the air.
5. Kerosene: Evaporates slowly at room temperature.
6. Coconut oil: Evaporates slowly when heated.
7. Turpentine: Used in paints; evaporates and helps paint dry.

Common feature: All these liquids evaporate because their molecules gain enough energy to escape from the liquid surface and enter the gaseous state. Some evaporate faster (alcohol, acetone) and some slower (kerosene, coconut oil).

Answer: This is an interesting observation. Let us understand it clearly.

Why fans create a cooling sensation:

• When a fan blows air over our skin, it speeds up the evaporation of sweat.
• Evaporation requires heat, which is taken from our skin, making us feel cooler.
• The fan does not actually lower the temperature of the air — it just increases the rate of evaporation of sweat.

Why fans also dry wet clothes:

• Wet clothes have water on their surface. For this water to evaporate, it needs heat energy.
• The heat energy comes from the surroundings (the air, the room temperature).
• When a fan blows air over wet clothes, it continuously replaces the humid air (air with water vapour) near the clothes with drier air.
• This allows more water to evaporate from the clothes into the air.
• The fan does not make the clothes colder — it speeds up evaporation by increasing air movement.

Key understanding: A fan speeds up evaporation by increasing air movement. Whether it is sweat on our skin or water on wet clothes, faster evaporation occurs. In the case of our skin, we feel cooler because the heat is taken from our body. In the case of clothes, the heat comes from the surrounding air, so the clothes dry faster without us feeling that the fan is 'warming' the clothes.

Conclusion: Fans dry wet clothes by increasing the rate of evaporation through increased air movement, not by providing heat directly.

Answer: Leaving sludge in heaps next to the drain for 3–4 days before transporting it is a practical approach that uses the principle of evaporation.

How it reduces transportation cost:

1. Fresh sludge removed from drains contains a large amount of water.
2. When left in heaps for 3–4 days, the water in the sludge evaporates due to sunlight, heat, and air movement.
3. As water evaporates, the sludge becomes drier and lighter.
4. Dry sludge weighs much less than wet sludge.
5. Transporting lighter, drier sludge requires less fuel, fewer trips, and smaller vehicles, thereby reducing transportation costs significantly.

How it enhances safety of individuals handling it:

1. Fresh sludge from drains contains harmful bacteria, pathogens, and toxic gases (like methane and hydrogen sulphide).
2. When left to dry for 3–4 days, exposure to sunlight and air helps to:

• Kill many harmful microorganisms (UV rays from sunlight have a disinfecting effect).
• Allow toxic gases to escape into the atmosphere.

3. Dry sludge is also less slippery and easier to handle than wet sludge.
4. Workers handling dry sludge are at lower risk of infection and accidents compared to handling wet, fresh sludge.

Conclusion: Allowing sludge to dry through evaporation makes it lighter (reducing cost) and safer (reducing health risks) — a simple but effective application of the principle of evaporation.

Activities in the house that involve evaporation:

1. Drying wet clothes after washing — water evaporates from the clothes.
2. Mopping the floor — water on the floor evaporates and the floor dries.
3. Cooking — water evaporates from food while cooking (boiling, frying).
4. Sweating — sweat evaporates from our body, keeping us cool.
5. Applying sanitiser — alcohol in sanitiser evaporates quickly.
6. Storing water in an earthen pot — water seeps through and evaporates, keeping the water cool.
7. Drying hair after a bath — water evaporates from hair.
8. Drying dishes after washing — water evaporates from the surface of utensils.
9. Using perfume — the liquid perfume evaporates and spreads fragrance.
10. Sprinkling water on the roof in summer — water evaporates and cools the roof.

How understanding evaporation helps in daily activities:

1. Drying clothes faster: We know to spread clothes in sunlight and windy areas to speed up evaporation.
2. Keeping cool in summer: We use fans, wet cloth on forehead, and earthen pots — all based on cooling effect of evaporation.
3. Cooking efficiently: We understand why food dries out if left uncovered and why covered vessels retain moisture.
4. Water conservation: Understanding evaporation helps us cover water containers to prevent unnecessary water loss.
5. Health and hygiene: We understand why sanitisers work and why sweat helps regulate body temperature.

Answer: Water is present in the solid state in nature in the following forms:

1. Ice: Water freezes into ice when the temperature drops to $0°C$ or below. Ice is found in:

• Polar ice caps: Thick layers of ice cover the North Pole (Arctic) and South Pole (Antarctica).
• Glaciers: Large, slow-moving masses of ice found in mountain regions (e.g., Himalayan glaciers like Gangotri).
• Frozen lakes and rivers: In very cold regions, the surface of lakes and rivers freezes in winter.

2. Snow: When water vapour in clouds freezes, it forms snowflakes that fall as snow. Snow covers mountain peaks and cold regions.

3. Hail: Under special atmospheric conditions, water droplets in clouds freeze into small balls of ice called hailstones, which fall to the ground.

4. Frost: Water vapour directly freezes on cold surfaces (like leaves, grass, windows) forming a thin layer of ice crystals called frost.

Importance: Glaciers and polar ice caps store a large amount of fresh water. Glaciers are an important source of fresh water for rivers (e.g., the Ganga river originates from the Gangotri glacier).

Reflection on the statement: 'Water is our responsibility before it is our right.'

This statement is very meaningful and thought-provoking. Let us understand it:

Water as a Right:

• Every living being — humans, animals, and plants — needs water to survive.
• Access to clean, safe water is considered a basic human right.
• We have the right to use water for drinking, cooking, bathing, agriculture, and other needs.

Water as a Responsibility:

• While we have the right to use water, we also have the responsibility to use it wisely and not waste it.
• Only a small fraction of Earth's water is fresh water suitable for use. Most water is in the oceans (salty) and cannot be used directly.
• The demand for water is increasing due to growing population, while the supply remains limited.
• Water bodies are being polluted by industrial waste, sewage, and garbage, making water unsafe.

My thoughts:

1. We must use water carefully and avoid wasteful habits (like leaving taps running, overwatering plants).
2. We should keep water bodies (rivers, lakes, ponds) clean and free from pollution.
3. We should practise rainwater harvesting to conserve water.
4. We should repair leaking taps and pipes promptly.
5. We should educate others about the importance of water conservation.

Conclusion: Water is precious and limited. Before claiming our right to use water, we must fulfil our responsibility to conserve it, use it wisely, and protect it from pollution — for ourselves and for future generations.

Answer: We can cool down the hot seat of a two-wheeler by sprinkling or pouring water on it.

Explanation:

1. When water is sprinkled on the hot seat, it starts to evaporate because the seat is very hot.
2. During evaporation, water absorbs heat from the seat surface.
3. This removal of heat from the seat causes it to cool down.
4. After a short while, the seat becomes cool enough to sit on.

Principle used: This works on the cooling effect of evaporation — evaporation absorbs heat from the surface, thereby cooling it.

Other similar examples:

• Sprinkling water on the floor or roof during summer to cool it.
• Wet cloth placed on the forehead to reduce fever.
• Water stored in an earthen pot stays cool due to evaporation through its pores.

Additional tip: We can also cover the seat with a cloth or park the two-wheeler in the shade to prevent it from getting too hot in the first place.