NCERT Solutions Curiosity Chapter 8 and 162Chapter exercises — Let us enhance our learning

Book page 161 Updated on2026-09-05

Q1.

Which of the following best describes condensation? (i) The conversion of water into its vapour state. (ii) The process of water changing from a liquid into gaseous state. (iii) The formation of clouds from tiny water droplets. (iv) The conversion of water vapour into its liquid state.

Answer

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

This is the chapter's own definition — the process of conversion of water vapour into its liquid state is called condensation.

Option	Why it is right or wrong
(i) Conversion of water into its vapour state	Wrong — this is evaporation, the opposite process
(ii) Water changing from a liquid into gaseous state	Wrong — this is again evaporation, only in different words
(iii) Formation of clouds from tiny water droplets	Wrong — this is a result of condensation, not condensation itself. Condensation is what makes the droplets in the first place; the cloud is what those droplets look like
(iv) Conversion of water vapour into its liquid state	Correct — gas → liquid, which happens whenever vapour meets a colder surface

Tip for eliminating options: read the direction of the change. Condensation must go gas → liquid. Options (i) and (ii) both go liquid → gas, so they can be struck out at once. Between (iii) and (iv), pick the one that describes the process rather than its outcome.

Q2.

Identify in which of the given processes, evaporation is very important— (i) Colouring with (a) crayons (b) water colours (c) acrylic colours (d) pencil colours; (ii) Writing on paper with (a) pencil (b) ink pen (c) ball point pen

Answer

(i) Colouring — the answer is (b) water colours.

Colouring medium	Is evaporation involved?
(a) Crayons	No — a crayon is solid wax rubbed on to paper. Nothing has to dry
(b) Water colours	Yes, very important — the pigment is mixed with water. The picture is finished only when the water evaporates and leaves the colour on the paper
(c) Acrylic colours	Evaporation of water does take place, but the colour also sets by the acrylic hardening. Water colours are the clearer answer
(d) Pencil colours	No — solid coloured leads, nothing wet at all

(ii) Writing on paper — the answer is (b) ink pen.

Writing tool	Is evaporation involved?
(a) Pencil	No — solid graphite leaves a mark straight away, with nothing to dry
(b) Ink pen	Yes, very important — fountain-pen ink is a thin, water-based liquid. The writing smudges until the water in the ink evaporates, which is why blotting paper was once needed
(c) Ball point pen	Very little — ball pen ink is a thick, oily paste that dries almost at once and hardly depends on evaporation

The rule to apply: ask whether the material is put on the paper as a liquid that has to dry. If yes, evaporation is essential; if the material is already solid, evaporation plays no part.

Check it yourself: Write one line with a fountain pen and one with a ball pen, and touch both after five seconds. The fountain-pen line smudges; the ball-pen line does not.

Q3.

We see green coloured plastic grass at many places these days. Space around natural grass feels cooler than space around the plastic grass. Can you find out why?

Answer

Because natural grass is full of water and keeps losing it by evaporation, while plastic grass has no water at all.

	Natural grass	Plastic grass
Water present	Yes — in the leaves, in the soil below and in the dew on it	None at all; the ground beneath is usually a sealed mat
Evaporation	Constant — water escapes from the leaves and from the moist soil	None
Effect on heat	Evaporation takes heat from the leaves and the air, cooling both	All the heat absorbed stays there and is passed on to the air
Behaviour in sunlight	Stays fairly cool; you can walk on it barefoot at noon	Gets very hot — plastic absorbs sunlight and heats up quickly

The science in one line: evaporation is a cooling process. Every gram of water leaving the grass carries heat away with it, so a natural lawn behaves like a huge, silent, electricity-free cooler. Plastic grass has nothing to evaporate, so it simply absorbs sunlight, becomes hot and warms the air above it.

Check it yourself: On a sunny afternoon, place a laboratory thermometer just above a real lawn and then just above a plastic mat, waiting two minutes each time. Note both readings in °C — the difference is often more than 10 °C.

Q4.

Give examples of liquids other than water that evaporate.

Answer

Many liquids evaporate, and several of them do so much faster than water.

Liquid	Where you meet it	How fast it evaporates
Spirit / alcohol	Hand sanitiser, the swab before an injection	Very fast — the skin feels cold at once
Acetone	Nail-polish remover	Very fast, with a strong smell
Petrol	At the petrol pump	Very fast — you can smell it several metres away
Kerosene and diesel	Lamps, stoves, vehicles	Fast
Perfume, deodorant, attar	Applied on the skin	Fast — which is exactly why we can smell it
Turpentine and thinner	With oil paints	Fast
Milk, buttermilk, tea	In the kitchen	Slow — only the water in them evaporates, solids are left behind
Vinegar, lemon juice	In cooking	Slow
Coconut oil, mustard oil	In every home	Very slow, but they do evaporate over months

Why different liquids evaporate at different rates: the particles of some liquids are held together far less strongly than those of water, so they escape more easily at the same temperature. Such liquids are said to be volatile — spirit, acetone and petrol are the everyday examples. That is also why their bottles must always be kept tightly closed and away from flames.

Q5.

Fans move air around, creating a cooling sensation. It might seem strange to use a fan to dry wet clothes since fans usually make things cooler, not warmer. Normally, when water evaporates, it requires heat, not cold air. What do you think about this?

Answer

There is no contradiction here at all. The puzzle disappears once we see that a fan does not supply the heat for evaporation, and it does not need to.

Sorting out the two ideas:

Where does the heat come from? Not from the fan. The heat needed for evaporation is taken from the wet cloth itself and from the surrounding air, which are already at room temperature — say 30 °C or more. That is more than enough; evaporation does not need boiling at 100 °C.
What does the fan actually do? As water evaporates, a layer of very humid air collects right against the cloth and blocks further evaporation. The fan keeps sweeping this saturated layer away and brings fresh, drier air to the surface. So evaporation can carry on at full speed instead of stalling.
Why does a fan then feel cool on our skin? For exactly the same reason. It makes the sweat on our body evaporate faster, and the evaporating sweat takes heat from our skin. The air blown by the fan is not cold — a thermometer held in front of a fan shows the same room temperature.

Fan → removes the humid layer of air → faster evaporation
Faster evaporation from cloth → clothes dry sooner
Faster evaporation from skin → heat taken from the body → we feel cool

The single idea behind both: the fan speeds up evaporation, nothing else. Whether that shows up as "drying" or as "cooling" depends only on what the water is evaporating from — a shirt or your skin.

Check it yourself: Hang a wet handkerchief in front of a fan and an identical one in a still corner of the same room. The one in front of the fan dries first, although the air blowing on it is not warmer.

Q6.

Usually, when sludge is removed from drains, it is left in heaps next to the drain for 3–4 days. Afterward, it is transported to a garden or a field where it can be used as manure. This approach reduces transportation cost of the sludge and enhances the safety of individuals handling it. Reflect upon it and explain how.

Answer

The whole benefit comes from the water in the sludge evaporating during those three or four days.

How the transportation cost falls:

Fresh sludge is mostly water — often three-quarters of its weight.
Spread in heaps in the open, it is exposed to sun and wind, and a large part of that water evaporates.
The heap becomes much lighter and smaller in volume.
So the same amount of useful solid matter can be carried in fewer trips, in smaller vehicles, using less fuel — a direct saving in cost. Carrying water to a field, when the field does not need that water, is simply wasted money.

How the safety of the workers improves:

Dried sludge is firm and crumbly instead of a slippery, dripping slurry. It can be lifted with a spade and does not splash on to the skin, eyes or clothes.
There is much less liquid to spill or leak from the cart on the way.
Drying in the sunshine kills a large number of germs, and the strong smell reduces a great deal as the wet, rotting matter dries.
A lighter load means less physical strain and less chance of an accident.

The science in one line: the sun and the wind do the drying free of cost, using nothing but evaporation. A few days of waiting replaces fuel, money and risk — this is the same idea that lies behind drying grain, papads, chillies and cow dung cakes in the sun.

Tip: The heaps also need to be kept away from drinking-water sources and covered before rain — otherwise the rain would soak them again and undo all the drying.

Q7.

Observe the activities in your house for a day. Identify the activities that involve evaporation. How does understanding the process of evaporation help us in our daily activities?

Answer

Activities in an ordinary Indian home that involve evaporation:

Time of day	Activity	What is evaporating
Morning	Wet hair and a wet towel drying after a bath	Water from hair and cloth
Morning	Mopping the floor; the floor drying	Water from the floor
Morning	Washing utensils and leaving them on the rack	Water on the steel
Morning	Hanging the washing on the line	Water from the clothes
Noon	Boiling milk down for kheer or khoya	Water from the milk
Noon	Drying papads, chillies, mango slices or grain on the terrace	Water inside the food
Noon	Water in the matka staying cool; the level slowly falling	Water through the pores of the clay
Afternoon	Sprinkling water on the courtyard or the roof	Water from the floor
Any time	Sweating and then sitting under a fan; using sanitiser	Sweat and alcohol from the skin
Evening	Painted or whitewashed walls drying; ink drying on your notebook	Water from the paint and the ink

How understanding evaporation helps us:

We dry clothes better — spread out, in a breeze, in the sun, never bunched in a closed bathroom.
We keep water cool without electricity using a matka, a surahi or a wet cloth around a bottle.
We cool a room by sprinkling water on the floor or by hanging wet khus curtains, and we understand why a desert cooler works badly in the monsoon.
We preserve food by drying it — dried fish, papad, badi and dehydrated vegetables keep for months because germs cannot grow without water.
We store liquids correctly — keeping petrol, spirit and perfume tightly closed so they do not evaporate away.
We stay comfortable and safe — we know why we sweat, why loose cotton clothes feel better in summer, and why a humid day feels far more oppressive than a dry day at the same temperature.

The bigger point: understanding a process lets us speed it up when we want it and slow it down when we do not. That is far more useful than merely knowing its name.

Q8.

How is water present in the solid state in nature?

Answer

Water occurs naturally in the solid state — as ice — wherever the temperature falls to or below 0 °C.

Form	How it is formed	Where you find it in India
Snow	Water vapour high in the sky freezes directly into ice crystals and falls	The Himalayas — Kashmir, Himachal, Uttarakhand, Sikkim, Ladakh
Hail (ole)	Raindrops are carried up into very cold clouds and freeze into lumps of ice	During thunderstorms in the plains, especially in spring
Glaciers	Snow piling up for centuries is pressed into thick, slow-moving rivers of ice	The Gangotri and Siachen glaciers — the source of the Ganga and other rivers
Polar ice caps and icebergs	Vast permanent sheets of ice; broken pieces float in the sea	The Arctic and Antarctica, where India has research stations
Frost	Water vapour freezes directly on very cold surfaces on winter nights	On grass and crops in north India in December and January
Frozen lakes and rivers	The surface water freezes when the air stays below 0 °C	Pangong Tso and the Zanskar river in Ladakh, where the frozen river becomes the Chadar trek route
Snow-capped peaks	It is so cold at great heights that the snow never fully melts	Kanchenjunga, Nanda Devi and other high peaks all through the year

Why solid water matters to us: the snow and glaciers of the Himalayas are a natural store of fresh water. They melt slowly through spring and summer and keep the Ganga, the Yamuna, the Brahmaputra and the Indus flowing even when there is no rain. That is why the melting of glaciers is such a serious concern.

Did you know? Ice floats on water because it is lighter than the same volume of water. This is very unusual, and it is a good thing — a lake freezes only at the top, and fish can survive in the liquid water beneath.

Q9.

Reflect on the statement “Water is our responsibility before it is our right.” Share your thoughts.

Answer

This is a reflection question, so give your own view with reasons. Here is a full model answer you can adapt.

Sample answer: I agree with the statement. Every person has a right to clean drinking water — but that right can only be met if someone has first taken the responsibility of keeping the water clean and available. If everybody claims the right and nobody accepts the duty, the well runs dry and the river turns black, and then there is no right left to claim. So responsibility must come first, and it belongs to all of us, not only to the government.

The reasoning behind it, in points:

The chapter itself notes that only a small portion of the Earth's water is fit for use by plants, animals and human beings. Most of it is in the oceans and cannot be used directly.
The demand keeps rising with population, and many parts of the world already face shortage.
Water is not manufactured anywhere. The water cycle only moves the same water around. So the only thing we can control is how we use it and how clean we keep it.
What one person wastes or pollutes is taken away from someone downstream — so the right of one person depends directly on the responsibility of another.

What taking responsibility looks like in practice:

Close the tap while brushing your teeth or soaping your hands; repair a dripping tap — one drop a second wastes many litres in a day.
Use a bucket instead of a shower, and re-use the water from washing vegetables for the plants.
Do not throw plastic, oil, paint or waste into ponds, rivers and drains, and keep water bodies free from pollution.
Support rainwater harvesting at home and in school, and help recharge wells and ponds.
Water plants in the cool of the morning or evening, when less is lost to evaporation.

Why the wording of the statement is clever: putting "responsibility" before "right" reverses the way we usually think. It reminds us that a resource shared by everybody survives only if each person guards it — and India's stepwells, johads and village tanks were built exactly on that understanding, centuries ago.

Q10.

The seat of a two-wheeler parked on a sunny day has become very hot. How can you cool it down?

Answer

The quickest way is to use evaporation — wet the seat and let the water evaporate, taking heat away with it.

What to do	Why it works	How fast
Sprinkle a little water on the seat and let it dry	The water evaporates and takes heat from the seat — the cooling effect	Within a few minutes
Wipe it with a wet cloth, or spread a wet cloth on it	Same reason, and the seat is not left dripping	A few minutes
Fan the wet seat, or push the vehicle into a breeze	Moving air makes the water evaporate faster	Fastest of all
Move the vehicle into the shade and wait	The seat stops absorbing sunlight and slowly loses its heat to the air	Slow — 10 to 20 minutes
Cover the seat with a wet towel or newspaper before you park (prevention)	Sunlight never reaches the black seat at all	Best of all — the seat never gets hot

Why the seat gets so hot in the first place: the seat cover is usually black, and dark surfaces absorb almost all the sunlight falling on them and turn it into heat. The plastic and foam then hold that heat. In peak summer such a seat can reach well above 60 °C — hot enough to burn the skin.

Tip: Do not pour a lot of water over the whole vehicle. A light sprinkling on the seat is enough, and it does not let water seep into the foam or reach the electrical parts.

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