NCERT Solutions Class 8 Science Chapter 1 Exploring the Investigative 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 8 · SCIENCE

NCERT Solutions

Chapter 1: Exploring the
Investigative World of Science

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

1–7 6 14 English

Solutions, notes, sample papers & more at 17 pages

Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 1: Exploring the
Investigative World of Science
Complete NCERT Solutions for Class 8 Science Chapter 1 Exploring the Investigative World of Science from
the NCERT textbook Curiosity. This opening chapter prints only one question set — Probe and ponder on
page 1 — and asks everything else inside the running text, right up to the puffing puri on pages 6–7. Every
one of those questions is answered here, with the mechanism behind the answer, not just the observation.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 1–7

SECTIONS QUESTIONS

6 14

MEDIUM

English

Probe and ponder — Page 1
Chapter opening — the questions that start your Grade 8 journey

PROBE AND PONDER

Q1 Why is one side of a puri thinner than the other?

Because the two faces of the puri do not set at the same instant. The face that hardens first
can no longer stretch; the face that is still soft is the one the steam pushes out, so it spreads
over a larger area and becomes thin.
Follow what happens inside the puri:

1. The rolled disc holds water bound in the dough.
2. Hot oil is at roughly 180 °C, far above the boiling point of water. Within a second or two the
outer surface of the disc dries and stiffens into a thin, sealed crust.
3. Under that crust the water flashes into steam. The same water needs more than a thousand
times as much room as steam, so the pressure inside shoots up.
4. The dough underneath is still soft and stretchy, so instead of bursting it splits into two
layers and balloons out.
5. The layer that met the oil first — the one we also hold down with the slotted spoon — has
already stiffened, so it stretches very little. The other layer is still soft, gives way, and
stretches. The same amount of dough spread over a bigger area must be thinner.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

thin, stretched layer

trapped steam

thicker layer — this face set first in the oil

Cross-section of a puffed puri. Steam trapped under a sealed crust splits the disc into two layers; only
the softer layer can still stretch, so it thins out.

Why it happens: rolling out a disc by hand never gives a perfectly even thickness
either. The thicker patches hold more water and stay soft longer, so they are the
parts that give way when the pressure builds. Thickness, temperature and how the
puri enters the oil together decide which face ends up thin.

Check it yourself: roll two puris of exactly the same thickness. Slide one in flat and
let the other fall in edge-first. Is the thin side always the side that touched the oil
last? Do this only with an elder beside you.

Did you know? The book says this plainly on page 7 — the puffing of a puri is still
not completely understood by scientists. The explanation above is our best
account, and the fine details are an open research question sitting in your own
kitchen.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Q2 Are there more grains of sand on all the beaches and deserts of the world, or more
stars in our galaxy?

There are far more grains of sand — roughly a billion times more. Our galaxy is enormous,
but a star is enormous too, so there is room for only a few hundred billion of them. A sand grain
is tiny, so a single beach already holds an uncountable number.
You can settle this yourself with an order-of-magnitude estimate — you only need the power of
10 to be right.

Take one grain ≈ 0.5 mm across → volume ≈ 0.125 mm³

1 cm³ = 1000 mm³ → 1000 ÷ 0.125 = 8000 grain-volumes

Grains never pack perfectly; about 60% of the space is filled

So 1 cm³ ≈ 8000 × 0.6 ≈ 5000 grains

1 m³ = 106 cm³ → 1 m³ ≈ 5 × 109 grains

Sand on all the world’s beaches (a widely used estimate) ≈ 7.5 × 1018 grains

Add the deserts — the Sahara alone dwarfs every beach — total ≈ 1020 grains or more

Stars in the Milky Way, our galaxy ≈ 100 to 400 billion = 1 to 4 × 1011

1020 ÷ 1011 = 109 → about a billion grains of sand for every star in our galaxy

Why it happens: the two numbers feel similar because both are “huge”, but huge is
not one size. Every step of ten multiplies the count, and sand beats stars by nine
such steps. Our brains have no feel for that gap, which is why the question is worth
asking.

Did you know? Change one word and the answer flips. If you compare sand with all
the stars in the observable universe — about 1022 to 1024, because there are roughly
two trillion galaxies — the stars win easily. That is the whole lesson: a scientific
question must state exactly what is being compared.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Tip: in science, “why” means “how did this come about”, not “for what purpose”. Nature
did not sit down and design variety; variety is what you get when inherited variation
meets many different surroundings over a very long time. Scientists call this process
natural selection.

Q4 Is there such a question that makes you curious about the world? Write it here!
_________________________________

Write your own question in the blank in your book — but make it a question you can
actually investigate. The chapter is not asking for a clever-sounding question; it is asking for
one that can start an experiment.
A good question for this page passes three tests:

1. It is about something you can see or do — in your kitchen, courtyard, school ground or on
your way home.
2. It is specific. “How does nature work?” cannot be tested. “Does curd set faster in a steel bowl
or an earthen bowl?” can.
3. It can be written as “What happens to ___ if I change ___?” If you can fill both blanks, you
already have an experiment.

Sample answers (adapt one, or better, write your own):

Why does a banana peel turn black in the fridge but not on the table?
Does water in an earthen matka cool faster on a windy day than on a still day?
Do the ants in my courtyard always take the same path to a spilt drop of sugar syrup?
Why does rangoli powder look brighter on a damp floor than on a dry one?
Does a phulka puff faster on a gas flame or on a hot tawa?

Try this: keep a small “question notebook” this year. Write the date, the question,
and what you did to find out. By the end of Grade 8 you will be able to see which of
your own questions actually got answered — and that is exactly how a scientist
works.

In-text Questions — Page 2

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Entering the Investigative World of Science

Q1 Why does dough rise?

Because a gas is being made inside the dough and cannot get out. The gas collects in
thousands of tiny bubbles, each bubble pushes the dough outward, and the whole lump swells.
Where the gas comes from depends on how the dough was made:

Fermented dough (bread, bhatura, idli and dosa batter): tiny living things — yeasts and
certain bacteria — feed on the sugars and starch in the flour and give out carbon dioxide gas
as they respire. This is why such dough is left in a warm place for hours.
Dough raised with baking soda: no living thing is involved. The soda releases carbon
dioxide the moment it meets an acid such as curd or lemon juice. That is why a soda-and-
dahi bhatura batter starts bubbling straight away.

Why it happens: the gas is only half the story — the dough must also be able to
hold it. Kneading wheat flour with water develops gluten into a stretchy, springy
network. Gas cannot slip out through it, so each bubble stretches the dough instead
of escaping. Un-kneaded flour and water would just fizz and go flat. When the dough
is finally baked or fried, the stretched walls set hard around the bubbles — that is
why bread is spongy.

Try this: take two identical balls of fermented dough. Keep one in a warm corner of
the kitchen and one in the fridge. Measure the height of each after 2 hours. You
changed only the temperature, so any difference in rising must be caused by
temperature — living things work faster when it is warm.

Q2 Is the world getting warmer?

Yes. Records from thousands of thermometers on land, from ships and buoys at sea, and now
from satellites all agree: the average temperature of the Earth’s surface has risen by roughly 1.1
°C to 1.3 °C since about 1850.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Why it happens: burning coal, petrol, diesel and natural gas releases carbon dioxide
into the air. Sunlight passes through carbon dioxide easily, but the heat the warm
Earth sends back out is partly absorbed by it and returned downward — the gas acts
like a blanket. Put more of the blanket in the air and less heat leaves, so the average
temperature settles at a higher value.

One degree sounds tiny — why does it matter? Because it is an average over the whole
planet and the whole year. Shifting an average shifts the extremes with it: heatwaves become
longer and more frequent, the monsoon becomes more erratic, and glaciers that were in slow
balance begin to lose more ice than they gain.
How do we know it is not just a hot summer? A single hot day is weather; a rise in the
average that holds for more than a century is climate. And separate lines of evidence agree
with the thermometers — Himalayan glaciers are retreating, sea level is rising as ocean water
warms and expands, and the oceans themselves are measurably warmer.

Tip: the last chapter of this book comes back to this question in full. Notice that the
answer rests on a trend measured over time, not on any one observation — that is
the difference between a scientific claim and an opinion.

In-text Questions — Page 3
A tour of the year — microorganisms and staying healthy

Q1 But what does our body need to stay healthy?

A balanced diet, safe water and clean air, regular physical activity, enough sleep, personal
cleanliness, and protection against disease. Health is not merely the absence of illness —
each of these keeps some life process of the body running properly.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

WHAT THE WHY IT NEEDS IT EVERYDAY EXAMPLE
BODY
NEEDS

Balanced diet Carbohydrates and fats give energy; proteins build and Dal–rice with a sabzi, curd
repair the body; vitamins and minerals keep the body’s and a seasonal fruit
processes working; roughage keeps digestion smooth

Safe drinking Carries substances around the body, controls its Boiled or filtered water
water temperature, removes wastes — and unsafe water itself
carries disease

Physical activity Keeps muscles and bones strong and the heart and Kabaddi, cycling, a brisk
lungs efficient walk

Sleep and rest The body repairs itself and the brain settles what you 8–9 hours a night at your
learnt during the day age

Cleanliness Keeps harmful microorganisms from entering the body Washing hands with soap
before eating and after
using the toilet

Vaccines Prepare the body’s defences before a particular germ The vaccines listed in your
ever arrives immunisation card

Why it happens: the body is not a store that simply needs filling. It is constantly
building, repairing and defending itself, and every one of those jobs needs raw
material (food), a medium to work in (water), and time in which to do the repairing
(rest). Leave out any one of them and the others cannot make up for it — which is
why an excellent diet without sleep, or plenty of sleep without a proper diet, still
leaves you unwell.

Q2 How do we fight these infections?

On three levels at once: the body’s own defences, medicines and vaccines, and — most
effective of all — stopping the germ from reaching us in the first place.
1. The body fights first, on its own.

Skin is an unbroken barrier. A cut is a doorway — which is exactly why you wash it and cover
it.
Mucus in the nose and windpipe traps dust and microorganisms; sneezing and coughing
throw them back out.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Acid in the stomach destroys most of what we swallow along with food.
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survivors are the ones that make the medicine useless next time.
3. Vaccines prepare the defence in advance. A vaccine shows the body a harmless preview of

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

CALENDAR WHAT IT ACTUALLY COUNTS HOW LONG IT
UNIT TAKES

Day One rotation of the Earth on its own axis about 24 hours

Month One full cycle of the Moon’s phases, new moon to new about 29.5 days
moon

Year One revolution of the Earth around the Sun about 365.25 days

Why it happens: a calendar is useful only if it lets you predict — when to sow, when
the rains will come, when a festival falls. You can predict only what repeats regularly.
Long before any clock existed, the sky was the one thing that repeated perfectly: the
Sun rose every day, and the Moon went through the same set of phases again and
again. So humans counted the sky, and those counts became the calendar.

The catch: the three motions do not divide neatly into one another, and that is why the world
has more than one calendar.

12 lunar months = 12 × 29.5 ≈ 354 days

1 solar year ≈ 365.25 days

365.25 − 354 ≈ 11 days short every year

A purely lunar calendar (the Hijri calendar) simply lets the months drift, so Ramzan slowly
moves through all the seasons.
A purely solar calendar (the Gregorian one on your wall) keeps the seasons fixed and adds
one leap day every four years to soak up the leftover 0.25 day.
Indian panchangs are luni-solar: the months follow the Moon, but an extra month (adhik
maas) is inserted about every three years so that festivals stay in their proper season.

The calendar printed in this chapter shows Chaitra, the first month of the Indian national
calendar. In 2025 Chaitra ran from 22 March to 20 April — exactly 30 days, and its start is fixed
by where the Earth is in its orbit, not by any decision on Earth.

Did you know? The Moon is about 3,84,000 km away and the Sun about 15 crore km
away. The date on which you celebrate Holi, Eid, Diwali or Pongal is settled by where
those two distant bodies happen to be — which is why the same festival falls on a
different English date each year.

In-text Questions — Page 6

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Thinking like a scientist — investigating the puffing puri

Q1 Have you noticed how a puri or a batura puffs up when placed in hot oil? Or how a
phulka swells when put directly on the flame. Why does it puff up like a balloon?
And why is one side thinner than the other?

It puffs because water inside the dough turns to steam faster than the steam can escape.
A sealed crust forms on the outside within seconds, the steam trapped under it has nowhere to
go, and the soft dough gives way and balloons out.

1 · Disc enters 2 · Outside dries 3 · Trapped steam
hot oil (~180 °C) into a sealed crust inflates it

water inside pressure builds two layers form

The three stages of a puri puffing. Nothing is added to the puri — the balloon is made of the puri’s
own water, turned into steam.

Step by step:

1. Oil at about 180 °C is far above the boiling point of water, so the surface of the disc loses its
water and stiffens almost at once into a thin, sealed crust.
2. Under the crust the water flashes into steam. As steam the same water needs more than a
thousand times the room, so the pressure inside rises very fast.
3. The dough underneath is still soft and stretchy. Instead of tearing, it splits along the middle
into two layers and balloons out — exactly like blowing into a balloon, except the blowing is
done from inside.
4. Fried a little longer, both layers set hard and the shape holds.

And the thin side? The face that touched the oil first sets first, so it can barely stretch. The
other face is still soft when the pressure arrives, so it does all the stretching — and the same
dough spread over a bigger area is thinner. (See Probe and ponder Q1 for the full picture.)

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Why it happens: a phulka on an open flame works the same way — the flame dries
and seals the surface, the moisture inside becomes steam and blows it up. And
notice what happens on your plate: the puri sags after a few minutes. The steam has
cooled, condensed back into liquid water, and the pressure holding the balloon open
is gone.

Tip: this also explains why cold or dry dough puffs badly. Less water inside means
less steam, and less steam means less pressure to lift the layers apart.

Q2 What are the different things that may change the way a puri puffs up when fried?

Sort them into two lists — what you can change or control, and what you can observe or
measure. That single step turns a vague curiosity into an experiment, and it is exactly what the
chapter asks you to do.

WHAT WE CAN CHANGE OR CONTROL WHAT WE CAN OBSERVE OR MEASURE

Thickness of the rolled dough Did it puff up at all? (yes / no)

Size (diameter) of the rolled disc Time taken to puff (seconds)

Type of flour — atta, maida, and so on Does a very thick disc still give one thin side?

Temperature of the oil — boiling hot, hot, not very hot How far it puffs — fully round, half, not at all

How the disc is put in — dropped vertically, slid at an angle, slid Did the oil splatter, smell or smoke?
slowly

How much water the dough was kneaded with; how long it rested Colour, and whether the puri stays puffed on the
plate

The rule that makes the whole thing work: change only one of these at a time and keep
every other condition the same. If you want to compare boiling hot, hot and not-very-hot oil,
then use discs of the same thickness, of the same flour, put in the same way. Otherwise, when a
puri fails to puff, you will have no idea which of your changes was responsible.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Ask a scientific List what you can Change ONE Observe, measure New questions
question change and what thing at a time and write arise
you can observe everything down

…and the cycle starts again

A systematic investigation. The last box is the important one — a good experiment ends with better
questions than it began with.

Why it happens: when two conditions change together you cannot tell which one
caused the result — the two effects are mixed and cannot be separated afterwards.
Holding everything else steady is not fussiness; it is the only way an experiment can
give an answer you are entitled to trust.

Tip: not every observation is a number. “Did it puff — yes or no?” is a perfectly good
scientific observation, and so is “the oil smelt burnt”. Record both kinds.

In-text Questions — Page 7
Keeping notes, and the new questions an experiment raises

Q1 Did the oil splatter, smell, or smoke?

Write down all three — each one is telling you something about the experiment you would
otherwise miss. They are not stray details; they are data.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

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OBSERVATION WHAT IT IS TELLING YOU WHAT TO DO ABOUT

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The oil splattered Free water reached the hot oil — from a damp disc, Dry the ladle; roll the disc

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The oil smoked The oil has crossed its smoke point — it is too hot. Lower the flame at once and
Beyond this the puri browns outside before the inside
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Safety first: hot oil at 180 °C causes serious burns, and smoking oil can catch fire.
Do this only with an elder present — the Hindi edition of the book says so in as many
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Freshly kneaded dough that has been allowed to rest for about 15–20 minutes usually
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matters more than the answer.

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It still holds all the water it was kneaded with, and that water is what becomes the steam
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even disc seals evenly.
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Dough kept for hours, especially in warm weather, starts to ferment and turn sour. The

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Dough taken straight from the fridge is cold, and it cools the oil around it when it goes in.
Cooler oil means a slower crust and a poorer puff.

How to actually test it — a fair test:

1. Knead one batch of dough. Use half of it now; keep the other half covered for 8 hours.
2. Keep everything else identical: same flour, same thickness, same diameter, same oil,
same oil temperature, same way of sliding it in. Only the age of the dough differs.
3. Fry at least 5 puris from each batch — one puri proves nothing, since a single one may fail by
chance.
4. Record two things each time: did it puff (yes/no), and how many seconds it took.
5. Compare: out of 5, how many puffed in each batch, and what was the average time?

Why it happens: repeating the trial five times is what turns an anecdote into a
result. Kitchens are full of small uncontrolled differences — a slightly hotter patch of
oil, a slightly thicker disc — and repeating lets those average out so the effect you
are actually looking for can show through.

Q3 What happens if I prick a hole in the puri before frying?

It will not puff. It will fry flat and go crisp instead — you will have made something closer to a
papdi or a mathri than a puri.

Why it happens: the whole balloon depends on the crust being sealed. Steam forms
inside either way, but a pricked hole is an open escape route. The steam simply
streams out through it, so the pressure never builds, and without pressure nothing
pushes the two layers apart. The disc stays as one flat sheet, keeps losing water to
the oil, and ends up dry and crisp — and because it stays flat it also spends longer
touching the oil, so it soaks up more of it.

This is a prediction, so test it properly: roll two discs from the same dough, of the same
thickness and diameter. Prick one with a fork; leave the other whole. Fry them in the same oil,
one after the other, put in the same way. If the pricked one fails to puff and the whole one puffs,
your explanation has passed its test.

Tip: the same idea explains a puri that fails for no obvious reason. A tiny tear or
crack made while rolling acts as a hole you did not intend — the seal is broken, and
no seal means no balloon.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Chapter at a glance
Grade 6 showed us that science begins with wonder; Grade 7 showed us that science keeps
evolving. Grade 8 puts the two together and takes us into the investigative world of
science — we no longer only learn new facts, we learn how to find new facts.
Investigation means more than looking and asking simple questions. You ask a focused
question, design a simple experiment to answer it, and then use your own observations to
improve your understanding.
The chapter is a guided tour of the whole year — microorganisms in a drop of water, staying
healthy with food, exercise, medicines and vaccines; the heating and magnetic effects of
electric current; force, pressure, winds and cyclones; the particles that make up matter;
elements, compounds, mixtures and solutions; reflection, mirrors and lenses; the phases of
the Moon and the calendars built from them; ecosystems; and finally what makes Earth ‘just
right’ for life.
You do not need a fancy laboratory. Your kitchen will do. All you need is curiosity, careful
observation, and the habit of asking “what happens if…?”
The method the chapter teaches with the puri: ask a scientific question → list what you can
change or control and what you can observe or measure → change only one thing at a
time → write down everything you see, smell and hear → let the results raise new
questions.
The root at the bottom of every left-hand page and the kite at the top of every right-hand
page are the chapter’s message in a picture: stay grounded in real observation, and let your
ideas take flight. And remember — even the puffing of a puri is not completely
understood by scientists today.

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Class 8 Science Chapter 1 Exploring the Investigative World of Science AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS WHERE IT EVERYDAY EXAMPLE
COMES IN THE
CHAPTER

Investigation A planned way of answering a question Pages 2, 7 Finding out what makes a
— observing, experimenting, explaining puri puff
— not just looking at something

Scientific A focused question that can actually be Page 6 “What changes the way a
question tested by an experiment puri puffs up when fried?”

What we can The condition you deliberately alter in Page 6 Dough thickness, size, type
change or control an experiment of flour, oil temperature,
way of dropping

What we can What you watch, count or time to see Pages 6–7 Did it puff (yes/no); time
observe or whether your change made a difference taken to puff (seconds)
measure

Fair test Changing only one thing at a time and Pages 6–7 Same thickness, same way
keeping every other condition the same of dropping — only the oil
temperature changed

Keeping notes Recording everything you see, smell Page 7 “Did the oil splatter, smell,
and hear, not only the numbers or smoke?”

Systematic The full cycle — question → Page 7 One round of frying leads to
investigation experiment → observation → fresh the next question
questions — used in all science, simple
or complicated

Root symbol Bottom of the left-hand pages — the Page 2 Staying grounded in what
solid foundation of careful observation you actually see
and of our heritage

Kite symbol Top of the right-hand pages — curiosity Page 2 Letting your ideas soar to
must take flight to explore the unknown new horizons

Chaitra First month of the Indian national Page 4 Chaitra 2025 ran from 22
(Shaka) calendar, shown in the chapter’s March to 20 April 2025
calendar picture

An open question A question science has not yet fully Page 7 Exactly why a puri puffs is
answered still not fully understood

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