NCERT Solutions Class 7 Science Chapter 1 the Ever Evolving World of Science

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 7 · SCIENCE

NCERT Solutions

Chapter 1: The Ever-Evolving
World of Science

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

1–6 6 22 English

Solutions, notes, sample papers & more at 19 pages

Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

CLASS 7 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 1: The Ever-Evolving World
of Science
Complete NCERT Solutions for Class 7 Science Chapter 1 The Ever-Evolving World of Science from the NCERT
textbook Curiosity. This is the opening chapter of the book, so every question in it is asked inside the running
text — plus the one task, Activity 1.1: Question the Answer. All of them, from page 1 to page 6, are
answered here with the reason behind the answer and everyday Indian examples.

TEXTBOOK BOOK PAGES

Curiosity (Class 7) 1–6

SECTIONS QUESTIONS

6 22

MEDIUM

English

In-text Questions — Page 1
Chapter opening — a butterfly, a paper plane and the flight of curiosity

Q1 Did you know that something as simple as a paper plane inspired real scientific
explorations of flight?

Yes — the science of flying really did begin with toys, kites and careful watching of birds. A
paper plane has no engine and no pilot, yet it stays up for several seconds. Asking why it stays
up is exactly the kind of question that led to the aeroplane.
A thrown paper plane shows all the ideas of real flight at once:

Throw (thrust) — your hand gives it a forward push. A real aircraft gets this from its engine.
Lift — as air streams past the slanted wings, the wings push the air downward and the air
pushes the wings upward. That upward push is lift.
Weight — gravity pulls the plane down all the time.
Drag — the air rubs against the plane and slows it down, which is why the flight ends.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Lift

Drag Throw (thrust)

Weight

The four pushes and pulls acting on a paper plane. A real aeroplane feels exactly the same four.

Why it happens: a paper plane is a tiny glider. It has no engine, so it slowly trades
height for forward movement. Early inventors noticed the same thing in gliding
birds — Leonardo da Vinci filled notebooks with sketches of bird wings, Sir George
Cayley built model gliders after studying them, and in 1903 the Wright brothers
added a light engine to a glider and made the first powered flight. Even today,
engineers first test small paper and card models in a wind tunnel before building a
real aircraft.

Try This: Fold three paper planes from the same sheet — one with narrow wings,
one with broad wings, one with the nose folded heavy. Throw each three times from
the same spot and measure how far it goes. Which design flies farthest, and can you
say why? You have just done an experiment.

In-text Questions — Page 2

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Science is a process — the deeper questions of Grade 7

Q1 How do things work?

This is the first of the three deeper questions Grade 7 asks. A “how” question asks for the
mechanism — the chain of steps by which something happens.
The way to answer it is always the same:

1. Break the thing into parts. A torch = cell + switch + wires + bulb.
2. Find out what each part does. The cell pushes the charge, the wires carry it, the bulb’s thin
filament glows.
3. Follow the chain from the first part to the last, and check it by changing one part at a time.

Some “how” questions waiting for you in this book:

How does a bulb glow when it is joined to a cell?
How does heat travel from the flame to the handle of a steel kadhai?
How does the food you eat reach every corner of your body?
How does water from the sea end up in a well in your village?

Why this question matters: once you know how something works, you can predict
it, repair it and improve it. A cycle mechanic who knows how a brake works can fix
any brake; someone who has only memorised the parts cannot.

Q2 Why do events happen the way they do?

A “why” question asks for the cause — the reason an event turned out this way and not some
other way. “How” describes the steps; “why” explains the reason behind them.

EVENT THE “HOW” (STEPS) THE “WHY” (CAUSE)

Ice in a glass melts Heat moves from the warm air Heat always flows from the hotter body to
into the ice the colder one

A shadow is shortest at The shadow shrinks all morning, The Sun is highest in the sky at noon
noon then grows

Mangoes ripen faster in The fruit softens and sweetens Changes speed up when things are warmer
summer sooner

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Class 7 Science Chapter 1 The Ever-Evolving World of Science
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Why it happens: a “why” answer is useful only when it can be tested. If the reason is

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Q3 And what can we learn from the patterns that we see in nature?

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They let us predict. Day follows night, the Moon waxes and wanes in about 29–30 days, the
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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

SOUR FOOD ACID IT CONTAINS

Lemon, orange, mausambi Citric acid

Tamarind (imli) Tartaric acid

Curd, buttermilk Lactic acid

Vinegar (sirka) Acetic acid

Amla Ascorbic acid (vitamin C)

The same fruit also changes its sourness as it grows. A raw green mango is sharply sour; the
ripe one is sweet. As a fruit ripens, much of the acid is used up and starch is converted into
sugar — so sourness falls and sweetness rises.

Why it happens: sourness is a property of the material, exactly like shine or
hardness. Because acids behave in a fixed way — they turn blue litmus red and lose
their sourness when mixed with a base — sourness can be used to sort substances
into groups. That is why the very next chapter of this book begins with sour and
bitter things.

Check it yourself: Squeeze a little lemon juice into a spoon of water and taste it.
Now stir in a pinch of baking soda. The fizz and the loss of the sour taste tell you an
acid has met a base.

Q5 What happens when we wash a haldi stain on our school uniform?

The yellow haldi stain turns reddish-brown the moment soap or detergent touches it — the
stain seems to get worse. Then, when you rinse it well with plenty of water, the red colour fades
back to yellow, and with more washing the stain finally goes.

Haldi + water → stays yellow

Haldi + soap solution (a base) → turns red-brown

Red-brown stain + water or lemon juice (an acid) → yellow again

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Why it happens: turmeric contains curcumin, which is a natural indicator — a
substance that shows one colour with acids and neutral things, and a different
colour with bases. Soap and detergent solutions are basic, so they flip the curcumin
to red-brown. Water or a drop of lemon juice brings the yellow back. Nothing has
been spoiled; the colour is simply reporting what kind of substance touched it.

Try This: Soak a strip of blotting paper in a little turmeric paste, dry it, and you have
a home-made haldi indicator strip. Touch it with soap water, lemon juice, curd and
baking-soda water and note the colours. You will use exactly this trick in the chapter
on acids and bases.

In-text Questions — Page 3
Batteries, lamps and wires; the changes around us

Q1 What kind of materials do we need to make a lamp glow?

We need materials that let electricity pass through them. These are called conductors — and
almost all of them are metals.
Look at the photograph on page 3 of your book: a cell, two wires, a small bulb in a holder — and
a steel spoon joined into the loop. The bulb glows because the spoon is metal, so the circuit is
complete all the way round.

Cell Bulb

object tested

Bulb glows only if the object is a conductor

The simple tester you will build: cell, wires, bulb and a gap. Put the object in the gap and watch the
bulb.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

BULB GLOWS — CONDUCTORS BULB DOES NOT GLOW — INSULATORS

Copper wire, iron nail, steel spoon, aluminium foil, Plastic scale, rubber band, dry wood, glass, chalk, paper,
coin cloth

Why it happens: the bulb needs a complete, unbroken path for the current. If the
object in the gap is a conductor, the path is closed and the bulb lights; if it is an
insulator, the path stays broken and nothing happens. Notice what this simple test
really gives you — a way to sort materials by a property. Sort enough of them and
two clear families appear: metals and non-metals.

Tip: If the bulb refuses to glow even with a spoon, check the boring things first — a
loose wire end, a dead cell, or a painted spoon handle. Good scientists rule out the
simple causes before inventing clever ones.

Q2 Batteries run out, ice melts into water, fruits ripen, rocks break into pebbles… what
kind of changes are these?

They are all changes, but they fall into two families: some can be reversed and some cannot.

CHANGE CAN IT BE WHY
REVERSED?

Ice melts into Reversible Cool the water and you get ice back — it is the same
water substance, only its state has changed

A torch battery Irreversible The chemicals inside have changed into new substances; no
runs out amount of shaking brings them back

A fruit ripens Irreversible Starch has turned to sugar and the acids are used up; a ripe
mango never becomes raw again

Rocks break into Irreversible The material is still rock, but the broken pieces cannot be
pebbles joined back into the original rock

Two more things the chapter wants you to notice:

Heat changes the speed. Many of these happen only on heating, or happen much faster
when heated — mangoes ripen faster in May than in December, and ice melts faster in a
warm room.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Fast and slow. Ice melting takes minutes; a rock crumbling into pebbles takes thousands of
years. Both are changes.

Why it happens: in a reversible change only the form or state of the material
changes, and the original material is still there. In an irreversible change new
substances are formed, or the pieces can no longer be put back, so there is no way
home.

Check it yourself: Curd from milk, a burnt matchstick and a rusted nail — reversible
or irreversible? All three are irreversible. Now try: wet clothes drying, and a rubber
band stretched. Both reversible.

In-text Questions — Page 4
Life processes in animals and plants; time and speed

Q1 Especially around the middle-school years, our bodies are changing rapidly! Why?

Because this is the age at which the body begins to grow up into an adult body. Chemical
messengers called hormones, released by glands inside the body, switch on a burst of growth
and other changes.

A growth spurt — height and weight increase faster now than at any time since you were a
baby. Bones lengthen, so hands and feet often grow first.
Body proportions change — shoulders, hips and muscles develop, and the voice deepens in
boys.
Feelings change too — mood swings and new interests are a normal part of the same
process.

Why it happens: growth is one of the life processes. To build new bone and muscle
so quickly the body needs a lot of raw material and energy, which is why appetite
rises at this age. All of it is normal, it happens to everyone, and it happens at a
different pace for each person — there is no “right” speed.

Tip: Rapid growth needs a balanced diet (dal, milk or curd, green vegetables, fruit,
eggs or nuts), 8–9 hours of sleep and daily physical activity. Skipping meals during
these years costs more than at any other age.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science
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stops you short: plants are living too, so plants must be doing all of this as well.

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LIFE PROCESS IN ANIMALS
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Nutrition Eat food made by others Make their own food in green leaves

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Why we miss it: we do not see a plant eating, so it feels as if it lives on nothing but
water. In fact a plant is doing something far cleverer than eating — it is
manufacturing its food out of air, water and sunlight, right where it stands.

Q4 How do they get their food?

Green plants make their own food by the process of photosynthesis, which happens mainly in
the leaves.

Carbon dioxide + Water —sunlight, chlorophyll→ Food (glucose) + Oxygen

Where each ingredient comes from:

Carbon dioxide — from the air, through tiny pores in the leaf called stomata.
Water and minerals — pulled up from the soil by the roots and carried to the leaves
through tubes in the stem.
Sunlight — the energy source.
Chlorophyll — the green colouring matter of the leaf, which traps the sunlight. This is why
the leaf is the “food factory” of the plant.

A few plants cannot do this and get food in other ways: amarbel (Cuscuta), the yellow thread-like
plant, sucks food from the plant it twines around; the pitcher plant traps insects to make up for
poor soil.

Why it matters to us: photosynthesis is where nearly all the food on Earth begins.
Whether you eat rice or fish, the energy in your meal was first captured from
sunlight by some green leaf. The oxygen you are breathing right now came from the
same process.

Q5 Do they also breathe? How?

Yes. Plants respire all day and all night, in every living cell. They simply have no lungs and no
nose, so the exchange of gases happens through small openings spread over the plant body.

Leaves — through stomata, tiny pores on the leaf surface that open and close.
Stems — through lenticels, small breaks in the bark of woody stems.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Roots — the root surface takes in air from the spaces between soil particles.

RESPIRATION PHOTOSYNTHESIS

When All the time, day and night Only in light

Where Every living cell Only in green parts

Gas taken in Oxygen Carbon dioxide

Gas given out Carbon dioxide Oxygen

Why plants seem not to breathe: in bright daylight a green leaf uses up carbon
dioxide for photosynthesis much faster than its respiration produces it, so from
outside it looks as though the leaf only gives out oxygen. Respiration has not
stopped — it is simply hidden behind the larger process.

Check it yourself: Over-water a potted plant for a few days and it wilts. The water
has filled the air spaces in the soil, so the roots cannot get oxygen. Proof that roots
really do need to breathe.

Q6 Ah, but what is time?

Time is what we measure between two events. It answers two different questions: which
event came first, and how long something lasted.
Time is not a thing you can hold, weigh or put in a box — but it can be measured, and that is
enough for science. We measure it exactly the way we measure length: by choosing a fixed unit
and counting how many of them fit in.

1 minute = 60 seconds

1 hour = 60 minutes = 3600 seconds

1 day = 24 hours = 24 × 3600 = 86 400 seconds

The second (s) is the SI unit of time. Notice that our everyday units all come from repeating
events in the sky — a day is one spin of the Earth, a month is roughly one round of the Moon, a
year is one round of the Earth about the Sun.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Why it happens: a clock is possible only because some events repeat with the same
duration every time. If sunrise came after a short gap one day and a long gap the
next, no calendar could ever have been made.

Q7 …but have you thought about how we measure time?

By counting a repeating event. Any event that repeats after exactly the same interval can be
turned into a clock; the whole history of timekeeping is a hunt for events that repeat more and
more reliably.

DEVICE THE REPEATING EVENT IT COUNTS

Sundial (shadow clock) The Sun’s daily journey moving the shadow across a dial

Water clock, sand clock (hourglass) A steady trickle of water or sand emptying a fixed vessel

Pendulum clock The to-and-fro swing of a pendulum

Quartz watch, mobile phone The vibrations of a tiny quartz crystal

Atomic clock Vibrations inside atoms — the most reliable of all

The oldest method is the one mentioned on this page of your book: long before electric clocks,
people watched the shadow of an object in the Sun and read the time from where the shadow
fell. The Jantar Mantar observatories at Jaipur and Delhi still carry giant stone sundials built on
exactly this idea.

Why a pendulum works so well: for a given length, a pendulum takes the same
time for every swing, whether the swing is wide or narrow. Count 60 such equal
swings and you have a fixed interval you can trust — which is why pendulum clocks
ruled for nearly 300 years.

Try This: Tie a small stone to a 1-metre thread and let it swing. Count the time for 20
complete swings and divide by 20 to get the time for one. Repeat with a half-metre
thread. The shorter pendulum swings faster — you have just built and tested a clock.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Q8 And how fast does something happen?

“How fast” is a question about rate — how much of something happens in one unit of time. For
a moving object we call it speed.

Speed = Distance travelled ÷ Time taken

A runner covers 100 m in 20 s

Speed = 100 ÷ 20 = 5 m/s

In km/h: 5 × 3600 ÷ 1000 = 18 km/h

Comparing speeds is what makes a race meaningful. Of two runners, the faster one is the one
who covers the same distance in less time, or a greater distance in the same time.
“How fast” applies to changes too, not only to motion:

A matchstick burns in a few seconds — a fast change.
An iron gate rusts over months — a slow change.
A rock crumbles into pebbles over thousands of years — an extremely slow change.

Why it happens: to say how fast, you must always divide by the time taken. That is
why “fast” is meaningless on its own: a bus covering 60 km is neither fast nor slow
until you are told whether it took one hour or one day.

Check it yourself: Time a friend walking the length of your school corridor and
measure the corridor with a metre scale. Divide length by time — that is your friend’s
speed in metres per second. Ordinary walking works out to about 1.2–1.5 m/s.

In-text Questions — Page 5

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Class 7 Science Chapter 1 The Ever-Evolving World of Science
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Shadows, eclipses and the movements of the Earth and the Moon
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things we can never touch, never visit and never bring into a laboratory, and still find the
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Look at what this one page has justaclaimed:

The Earth spins on its axis, which gives us day and night. Half the globe faces the Sun, half
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is in its own shadow — exactly what the photograph on page 5 shows.

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Why it happens: curiosityais the starting point of every discovery. The chain always
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science; it is where science begins. That is why the book’s title is Curiosity.
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s m you know? Indian astronomer Aryabhata, writing about 1500 years ago, said
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Activity 1.1: Question the Answer — Pages 5–6
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Section 1.1 Happy Exploring!

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ACTIVITY
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This is the worked example the book gives, to show that one answer can belong to many
completely different questions. The textbook itself offers four:

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

“How do we ensure getting equal shares of cake?”
“My essay is too long.”
“I can’t fit this in the envelope.”
“I cannot dance to such a long song.”

A few more of our own, to show how far the same answer can stretch:

“This recipe is for eight people, but only four of us are eating.”
“The rangoli design is too big for this doorstep.”
“My school bag is too heavy to carry every day.”
“The speech is 10 minutes long but I have been given only 5.”

Why the book starts here: in an exam the question is fixed and only one answer is
right. In real science it is the other way round — the answer is often in front of you
and the hard part is asking the right question about it. A fossil, a stain, a reading on
a meter: all are answers waiting for someone to ask what they mean.

Tip: Play this game in pairs. One person says an answer, the other has 30 seconds to
invent three questions for it. You will find that the funnier the answer, the better the
questions.

Q2 Question: ______________________________________________? Answer: Just add some milk.

Any question that this could sensibly answer is correct — there are no wrong questions here.
What makes a question good is that the given answer really does solve it.
Sample answers:

“The tea has come out far too strong and bitter — what do I do now?”
“How do I turn this cocoa powder into something I can drink?”
“The kheer has thickened too much while cooling. How do I loosen it?”
“The dough for the roti is crumbling. What should I add?”
“My coffee is too hot to sip and I have to leave in a minute!”

Try This: Write a question for which “just add some milk” would be a terrible answer
— for example “How do I make this lemonade tastier?” Working out why an answer
fails one question but fits another is itself good scientific thinking.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Q3 Question: ______________________________________________? Answer: Because the cat’s teeth
were crooked.

An answer beginning with “Because…” must belong to a “Why…?” question. That single clue
makes this one easier than it looks.
Sample answers:

“Why did the mouse manage to escape from the cat?”
“Why did the cat leave half its fish untouched?”
“Why does this cat’s smile look so funny in the photograph?”
“Why could the cat not bite through the string?”
“Why did the vet ask the family to bring the cat back next week?”

Why this one is a good exercise: the answer states a cause, so you have to imagine
the effect that goes with it. Working backwards from a cause to its possible effects is
exactly what a scientist does when she asks, “If this is true, what else should we be
seeing?”

Q4 Question: ______________________________________________? Answer: Don’t panic, I have my
towel.

This answer is a calm reply to somebody who is worried — so the question must describe a
small emergency.
Sample answers:

“It has started raining and we are still half an hour from home — now what?”
“The water bottle has leaked all over the inside of the bag!”
“We forgot to pack anything for the swim after the picnic.”
“How will you dry your hair before the school photograph?”
“The bench at the sports ground is wet — where will we sit?”

Did you know? “Don’t panic” and the towel both come from the funny science-fiction
book The Hitchhiker’s Guide to the Galaxy, in which a towel is said to be the most
useful thing a space traveller can carry. Your textbook is winking at the same book
again in the next answer.

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

Q5 Question: ______________________________________________? Answer: 42

A number can answer a thousand questions — but the book asks you not to take the obvious
road (“What is 32+10?”). So look for questions where 42 is a real, interesting quantity.
Sample answers:

“How many students in our class took part in the science fair?”
“What was the highest temperature in °C recorded in our city last May?”
“How many laddoos were left after the whole class had eaten?”
“How many seconds does the school bell ring for?”
“How many pages of the notebook have we filled since April?”
“How many rows of seats are there in the school auditorium?”

What the exercise is really teaching: a number by itself means nothing. 42
becomes information only when it is attached to a question and a unit — 42
students, 42 °C, 42 seconds are three completely different facts. This is why, all
through this book, you will be asked to write the unit with every measurement.

Did you know? In The Hitchhiker’s Guide to the Galaxy, a giant computer thinks for
millions of years and declares that the answer to life, the universe and everything is
42 — but nobody remembers what the question was. That joke is exactly the point of
this activity: an answer without its question is useless.

Chapter at a glance
Curiosity Grade 7 is not a book of facts to be memorised. It is an invitation to question, to
experiment and to explore — science is a process, a way of thinking that welcomes curiosity
and is open to the unknown.
In Grade 6 we mostly asked what. In Grade 7 we go deeper and ask “How do things
work?”, “Why do events happen the way they do?” and “What can we learn from the
patterns we see in nature?”
The chapter is a guided tour of the whole book — properties of materials and sour fruits,
electricity and circuits, metals and non-metals, reversible and irreversible changes, heat,
water, life processes in animals and plants, time and speed, light and shadows, and the
motions of the Earth and the Moon.
Although the book is printed as separate chapters, the different fields of science — physics,
chemistry, biology and earth science — are all interconnected. An idea in one area often

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Class 7 Science Chapter 1 The Ever-Evolving World of Science AglaSem · NCERT Solutions

sparks a discovery, or at least a good question, in another.
Science is not only about discovery, it is also about responsibility. Human activity is linked
to what happens in nature, so science has a big role in meeting environmental challenges
and building a sustainable world.
Even the page numbers carry the message — they follow a butterfly and a paper plane,
because learning takes flight when curiosity leads the way. And, as Activity 1.1 shows, a
good scientist does not only answer questions, she asks amazing ones.

Page 18 of 19

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