NCERT Solutions Class 8 Science Chapter 8 Nature of Matter Elements Compounds and Mixtures

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 8: Nature of Matter :
Elements, Compounds, and
Mixtures

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

116 – 133 19 57 English

Solutions, notes, sample papers & more at 52 pages

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 8: Nature of Matter :
Elements, Compounds, and Mixtures
Almost nothing around you is a single substance. This chapter sorts all matter into mixtures and pure
substances, and then splits pure substances into elements and compounds — using lime water, the
electrolysis of water, heated sugar and the iron–sulfur experiment as the evidence.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 116 – 133

SECTIONS QUESTIONS

19 57

MEDIUM

English

Probe and ponder — Page 116

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Chapter opener

PROBE AND PONDER

Q1

1

10
2

6 3
8
12

7
11 9
5 4

1 Sunlight from the setting sun 5 Football 9 Steps, ramp and metal railing

2 Tree 6 Students (and their uniforms) 10 School building

3 Bushes and flowers 7 White cane 11 Shadows on the ground

4 Grass 8 School bag, water bottle, books 12 Sound of the game

Redrawn sketch of the chapter-opening picture (page 116) — the school ground at the
end of the day.

Which of the entities in the picture above consist of matter, and which of them do
not?

Almost everything you can point to in the school picture is matter — it has mass and takes up
space. What is not matter are the forms of energy in the scene and what is going on inside the
children's heads.

CONSIST OF MATTER DO NOT CONSIST OF MATTER

The trees, bushes, grass and flowers; the students themselves; their The sunlight falling on the ground; the
uniforms and shoes; the football; the books, lunch boxes, water bottle heat of the evening sun; the shadows; the
and school bag; the white cane; the steps, the ramp, the metal railing sound of the children playing; and the
and the tactile yellow tiles; the building, the arches and the doors; and thoughts, ideas and feelings of the students
the air all around them

Page 2 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: Matter is anything that has mass and occupies space, and
everything made of matter is built from elements and compounds. Light, heat,
electricity, sound, thoughts and emotions are real and important parts of our world,
but you cannot weigh them or fill a bottle with them — so they are not matter.

Tip: Air is easy to miss. You cannot see it, but a filled balloon has mass and takes up
space — air is matter, and it is a uniform mixture of gases.

Q2 How can elements be combined to form a compound?

Elements form a compound only when they combine chemically in a fixed ratio — not when they
are merely mixed. Usually some energy, most often heat, has to be supplied to make the
combination happen.
Activity 8.5 in this chapter shows both halves of the idea:

Mixing only: 5.6 g of iron filings stirred with 3.2 g of sulfur powder gives Sample A. The black
iron and the yellow sulfur are still visible, a magnet still pulls the iron out — nothing new has
been made.
Chemical combination: heat the same Sample A and a black mass, Sample B, is formed. It is
iron sulfide, the same colour and texture all through, not attracted by a magnet, and the iron
and sulfur can no longer be taken apart.

Iron + Sulfur (on heating) → Iron sulfide

Element Compound Mixture
one kind of atom two elements joined, fixed ratio particles loose, ratio can vary

In a compound the two kinds of particles are locked together in a fixed ratio; in a mixture they simply
sit side by side.

Page 3 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

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Why it happens: In a compound the particles of the different elements are held to

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each other so tightly that no physical method — no magnet, no sieve, no filter, no
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.c — can pull them apart. That is exactly why the new asubstance
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evaporation behaves

l a se like the elements it came from. Water is made of hydrogen, a fuel, and
g
nothing
aoxygen, which supports burning, in the fixed ratio 2 : 1, and yet water puts out fire.

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a
Q3 How could the discovery of a compound that absorbs carbon dioxide from the air
contribute to solving environmental challenges?

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Such a compound would let us take carbon dioxide out of the air itself, instead of only trying to

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release less of it. Carbon dioxide traps heat in the atmosphere, so removing it directly attacks

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a You have already met one substance that does this in Activity 8.1: calcium hydroxide (lime
the cause of global warming.

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.co agl
water). Left standing in a petri dish, it turns milky because the carbon dioxide of the air reacts
with it.
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a
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

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A good absorbing compound could be used to:
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m .c the exhaust gases of factories, power plants and cementagkilns before they reach the
se open air;
clean
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agl keep the air breathable in closed spaces — submarines, spacecraft and mines;

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lock the carbon dioxide away as a solid carbonate, so it cannot escape again;
slow down global warming, and withcit
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the melting of glaciers and the rise in sea level.

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agl graphene aerogel points to the other half of the
Did you know? The chapter's
problem — it is highly porous, so it has a very high absorbing capacity. A material

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that is both a good absorber and cheap to make in large amounts is what scientists
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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q4 Share your questions

This bullet asks you to add your own questions to the chapter. Write questions that a page of
the chapter could actually answer, and keep them open — starting with why, how or what if
rather than yes/no.
Sample questions:

Air is a mixture of gases, so why does the oxygen not settle at the bottom and the lighter
gases at the top?
If a compound has a fixed ratio, what happens when I mix iron and sulfur in the wrong
proportion and heat it?
Stainless steel is a mixture, yet it looks like one metal — how uniform can a mixture really
get?
Why can dissolved salt be got back from water by evaporating, while hydrogen cannot be got
back from water in the same way?
Only two elements are liquid at room temperature. What decides whether an element is a
solid, a liquid or a gas?

Tip: Keep these questions in your notebook and tick them off as the chapter
answers them. The ones still unticked at the end are the ones worth asking your
teacher.

In-text Questions — Page 117
8.1 What Are Mixtures?

Q1 Can you identify a few more examples of non-uniform mixtures around you?

Yes — any mixture whose components you can still pick out with your eyes or with a magnifying
device is non-uniform.

Sand mixed with water, and muddy water after rain
A dish of rajma-chawal, or vegetable pulao with peas and carrot
Chana and moori (puffed rice) mixed in a chaat
Unfiltered honey with bits of wax in it
Soil — you can see grains of sand, bits of leaf and small stones
Oil floating on water
A handful of mixed dal before it is cooked

Page 5 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: In these mixtures the components stay in separate lumps or layers
large enough for the eye to resolve, so the mixture is not the same everywhere. Take
one spoon from the top of muddy water and one from the bottom and you get two
different things — that is the test of a non-uniform mixture.

Q2 Can you list a few uniform mixtures?

Yes — in these the components are spread evenly and cannot be told apart even under a
microscope.

Sugar dissolved in water, and common salt dissolved in water
Lemonade (nimbu-pani) and soda water
Air
Seawater
Brass, bronze and stainless steel — the alloys
Vinegar (acetic acid in water)
Filtered lime water

Why it happens: Here the components are broken up into their individual particles
and those particles are mixed evenly throughout. A spoonful from anywhere in the
glass tastes and behaves exactly like a spoonful from anywhere else, so the mixture
is uniform.

Check it yourself: A mixture is still a mixture even when it looks uniform. Sugar
solution can be separated by evaporating the water — no chemical reaction has
happened, so the sugar is unchanged.

A step further — Page 118

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.1 What Are Mixtures?

A STEP FURTHER

Q1 Do you know that stainless steel is also a mixture?

Yes. Stainless steel contains iron, nickel, chromium and a small amount of carbon. They are
mixed so uniformly that the whole mixture looks the same throughout and you cannot see the
individual substances in it.
Such uniform mixtures of metals are called alloys. Two more from the chapter:

ALLOY MADE BY MIXING SEEN AS

Stainless steel Iron, nickel, chromium and a little carbon Plates, spoons, cooking vessels

Brass Copper and zinc Lotas, pots, door handles

Bronze Copper and tin Statues, bells, old vessels

Why it happens: The metals are melted together, so their particles get mixed right
down to the level of individual particles — that is why an alloy is a uniform mixture
and not a compound. No chemical reaction has taken place and the proportions are
not fixed by any rule; a maker can change how much zinc goes into brass. That is
also why an alloy can be made stronger or more rust-resistant than the pure metal:
stainless steel is stronger and more durable than pure iron.

Did you know? Our scientific heritage box on the same page calls such a mixture
mishraloha. The Charaka Samhita and Susruta Samhita describe bronze (kamsya) —
copper (tamra, 4 parts) with tin (vanga, 1 part) — being used to improve digestion
and boost immunity.

In-text Questions — Page 118
8.1.1 Is air a mixture?

Q1 Is air a mixture? What kind of mixture is it?

Yes, air is a mixture — and it is a uniform one.

Page 7 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Air is made mainly of nitrogen, oxygen, argon, carbon dioxide and water vapour. These gases
are simply mixed: none of them reacts with the others, and each keeps its own properties inside
the mixture.

Nitrogen makes up about 78% of air and does not take part in combustion.
Oxygen is needed by most living beings to stay alive, and it helps in combustion.
Water vapour is there too — when warm air touches a cool surface, the vapour turns into
liquid water and tiny droplets form on the surface.

Why it is uniform: The gases are mixed particle by particle, so a sample of air taken
from anywhere in this room has the same composition and behaves in the same
way. You cannot see or point out the nitrogen apart from the oxygen even with a
microscope.

Tip: Dust and smoke floating in air are not part of the mixture we call air — they are
pollutants, and they are the reason a sunbeam through a small opening looks full of
shining specks.

Activity 8.1: Let us experiment — Page 118
8.1.1 Is air a mixture?

ACTIVITY

Q1 Take a glass tumbler and fill it half with water. Add a small amount of calcium oxide
(quick lime) slowly to it. What do you observe?

The calcium oxide reacts vigorously with the water. You will see bubbling and hissing, the lump
breaks up into a white powdery paste, and the tumbler becomes distinctly warm to the touch.

Calcium oxide + Water → Calcium hydroxide + heat released

Why it happens: This is a chemical change — a completely new substance, calcium
hydroxide, is formed, and heat is given out during the reaction. That released heat is
what you feel through the glass.

Page 8 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

co m
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Safety first: The book warns you to perform this step carefully. Add the quick lime

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om with bare hands. l
slowly and in a small amount, keep your face away from the tumbler, and do not
touch the.cmixture a g
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Stir continuously to make a solution of calcium hydroxide. This solution is called
.
Q2

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as
lime water. Filter it and observe its colour.

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After filtering, the lime water is a clear, colourless liquid. The milky-white look of the unfiltered
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liquid disappears with the undissolved calcium hydroxide left behind on the filter paper.
em.
m l as
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a s
Whyemit happens: Only a little calcium hydroxide dissolves in water; the rest stays as
a gl fine solid particles that scatter light and make the liquid look white. Filtering
removes those particles and leaves behind only the dissolved part — and a solution
m a s
.co agl
of a colourless substance in water is itself colourless.

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Tip: Starting with a clear, colourless solution is the whole point. Any milkiness you
see later must then have come from something new, not from what you began with.

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q3 Leave this colourless solution in a petri dish for a few hours (Fig. 8.5a). Keep stirring
the solution at regular intervals. What do you observe (Fig. 8.5b)? Does it turn
milky?

Lime water Lime water turns milky

Petri dish Petri dish

(a) (b)

Fig. 8.5, page 119 — redrawn sketch. Lime water in a petri dish: (a) clear and colourless
at the start, so the grain of the table shows through; (b) milky after a few hours in the
air.

Yes. Over a few hours the clear solution slowly turns milky white, and a thin white film may
appear on the surface.

STAGE WHAT YOU SEE

Just after filtering (Fig. 8.5a) Clear, colourless lime water

After a few hours in the open petri dish (Fig. 8.5b) Milky white — turbid throughout

Why it happens: The petri dish is open, so a large surface of the lime water stays in
contact with the air. Stirring keeps bringing fresh solution to that surface, so the
reaction with the air goes on steadily instead of stopping at the top layer.

Page 10 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q4 Can you explain why the solution has turned milky?

Because the carbon dioxide present in the air has reacted with the calcium hydroxide of the lime
water and formed calcium carbonate — insoluble tiny white particles — along with water.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Why it happens: Calcium carbonate does not dissolve in water. It stays suspended
as very fine white particles that scatter light, and that scattering is what your eye
reads as milkiness. Since nothing was added to the dish except air, the carbon
dioxide must have come from the air — so this activity proves that air contains
carbon dioxide.

Try This: Blow gently through a straw into fresh lime water. It turns milky in seconds
instead of hours, because exhaled air has far more carbon dioxide than ordinary air.

In-text Questions — Page 119
8.1.1 Is air a mixture?

Q1 Apart from gases, have you ever observed anything else present in the air? Have
you ever noticed tiny shining particles moving in a beam of sunlight entering a dark
room through a small opening? What are these particles?

Those shining specks are dust particles suspended in the air — not a gas at all, and not part of
what air is made of.

Why they shine: Dust particles are far too small to be noticed in ordinary light.
When a narrow beam of sunlight enters a dark room, each particle scatters a little of
that light towards your eye while the background stays dark — so the particles
suddenly stand out. They dance about because the moving air keeps pushing them
around.

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Dust is not an integral part of the air; it is considered a pollutant. Its nature and number vary
from time to time and from place to place — more on a dry, windy day near a road, less after
rain.

Did you know? The major pollutants in air are particulate matter (dust, soot) and
gases like carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide. The air
quality index (AQI) is the tool used to describe the quality of air.

Activity 8.2: Let us explore — Page 119
8.1.1 Is air a mixture?

ACTIVITY

Q1 Place the black sheet of paper undisturbed near an open window (Fig. 8.6a), or in
the garden, for a few hours. What do you observe?

Open window Dust particles

(a) (b)

Black sheet of paper Magnifying glass

Fig. 8.6, page 119 — redrawn sketch. The black sheet of paper left at an open window: (a)
with no dust particles; (b) with dust particles, examined through a magnifying glass.

Tiny particles have settled on the surface of the sheet. Under a magnifying glass (Fig. 8.6b) you
can see that they are of different sizes, shapes and colours — fine grey dust, specks of soot, bits
of pollen, a stray fibre of cloth.

Page 12 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

SHEET OBSERVATION

Fig. 8.6a — at the start Black, clean, free of any visible dust particle

Fig. 8.6b — after a few hours Small particles settled all over the surface

Why it happens: These particles were floating in the air all along. Once the sheet
lies undisturbed, the particles slowly settle on it under gravity and stay there. This
shows that dust particles are suspended in the air. A black sheet is chosen
deliberately — light-coloured dust shows up clearly against a dark background.

Check it yourself: Repeat the activity with one sheet near a busy road and one
inside a closed room. The difference in the amount of dust tells you why the number
of dust particles varies from place to place.

In-text Questions — Page 120

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.1.2 Types of mixtures
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Mixtures could be of several types depending on the physical state of their
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Q1

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components. Some mixtures with their examples are shown in Table 8.1. Complete

a gl the third column—

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Table 8.1: Different types of mixtures
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S.NO. MIXTURE-
TYPE
a EXAMPLES UNIFORM OR NON-
UNIFORM

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1. Gas and gas Air Uniform

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2. Gas and liquid Aerated water (soda water) ……………………
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a……………………
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Oxygen dissolved in water

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3. Solid and gas Carbon particles in air ……………………

4. Liquid and Acetic acid in water (vinegar) ……………………

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liquid Oil and water ……………………
.
5. Solid and
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and water ……………………

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liquid Seawater ……………………

6. Solid and solid Baking powder (baking soda and ……………………

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tartaric acid) ……………………
Alloys
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The column to be filled is the one headed Uniform or non-uniform. Judge each example by one
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test: can the components be seen separately?

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

S.NO. MIXTURE- EXAMPLES UNIFORM OR NON-
TYPE UNIFORM

1. Gas and gas Air Uniform

2. Gas and liquid Aerated water (soda water) Uniform
Oxygen dissolved in water Uniform

3. Solid and gas Carbon particles in air Non-uniform

4. Liquid and Acetic acid in water (vinegar) Uniform
liquid Oil and water Non-uniform

5. Solid and liquid Sand and water Non-uniform
Seawater Uniform

6. Solid and solid Baking powder (baking soda and Non-uniform
tartaric acid) Uniform
Alloys

Why these answers: Soda water, dissolved oxygen, vinegar and seawater are all
cases where one substance has dissolved fully in the other, so the mixture is the
same everywhere — uniform. Carbon particles in air, oil on water and sand in water
stay as separate specks, layers or grains you can see, so they are non-uniform.
Baking powder is a mechanical mixture of two white powders — under a magnifying
glass the two kinds of grains can be told apart. Alloys are the special case: metals
melted together mix so evenly that even a microscope shows one substance
throughout.

Tip: The English book says "Complete the third column" while the blank column in
Table 8.1 is actually the fourth; the Hindi edition of the same page correctly says चौथा
स्तंभ (fourth column). Fill the Uniform or non-uniform column.

A step further — Page 121

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.2 What Are Pure Substances?

A STEP FURTHER

Q1 According to science, how would you classify milk, packed fruit juice, baking soda,
sugar, and soil — as mixtures or pure substances?

Only baking soda and sugar are pure substances. The other three are mixtures, however "pure"
the label on the pack may claim to be.

SUBSTANCE CLASSIFICATION REASON

Milk Mixture Water, fat, proteins, sugar and minerals together; the fat
can be separated as cream, which is a physical process

Packed fruit Mixture Water, fruit pulp, sugar, acids and often added colour and
juice preservative

Baking soda Pure substance (a Made of only one kind of particle; no physical process can
compound) split it into other kinds of matter

Sugar Pure substance (a Only one kind of particle; on heating it breaks into carbon
compound) and water, but that is a chemical change, not a physical
one

Soil Mixture Sand, clay, humus, water, air and minerals — you can see
the different bits

Why it happens: The everyday word "pure" means unadulterated — nothing cheap
or harmful added. Science asks a stricter question: does the substance consist of
only one type of particle? Milk with nothing added to it is still unadulterated milk, but
it is scientifically impure, because it is made of more than one substance.

Activity 8.3: Let us experiment (Demonstration activity) — Page 121

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.3 What Are the Types of Pure Substances?

ACTIVITY

Q1 Do you observe the formation of any gas bubbles at both the terminals inside the
test tubes?

Yes. Fine bubbles rise steadily from both terminals of the 9 V battery and collect at the closed
top of the two water-filled test tubes, pushing the water down.

Why it happens: Electricity passing through the water breaks it down into two
gases. A few drops of dilute sulfuric acid were added to the beaker for a reason —
pure water hardly lets current pass, and the acid makes the water conduct well
enough for the change to proceed at a visible rate.

Safety first: This must be done under the teacher's supervision, sulfuric acid must
be handled carefully, and a lithium-ion battery must never be used.

Page 17 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q2 Observe the volume of gas collected in each test tube (Fig. 8.8d). Is the volume of
the gas collected the same in both the test tubes?

Hydrogen gas

Oxygen gas

Test tube

Gas bubbles

Water containing a few
drops of sulfuric acid

9 V battery

(d) Gases collected in the test tubes

Fig. 8.8d, page 122 — redrawn sketch of the apparatus for passing electricity through
water. The gas columns are drawn as they stand in the book.

No. The two volumes are clearly unequal — one test tube fills with about twice as much gas as
the other.

TEST TUBE VOLUME OF GAS GAS COLLECTED

Over one terminal Larger — roughly double Hydrogen

Over the other terminal Smaller — roughly half Oxygen

Why it happens: In water the number of hydrogen atoms to oxygen atoms is in the
fixed ratio 2 : 1. When water is broken down, hydrogen is therefore released in about
twice the amount of oxygen — and you are watching that fixed ratio with your own
eyes. Wait for the full 10–15 minutes; over a short time the difference is too small to
judge.

Page 18 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

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Test these gases one-by-one by bringing a burning candle close to the mouth of the
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test tubes. What happens in each case?

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a

The two gases behave in completely different ways.

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Test tube with the larger volume: a sharp pop sound is heard as the gas catches fire at the

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mouth of the tube (Fig. 8.9a).

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Test tube with the smaller volume: no pop; instead the flame of the candle glows brighter
(Fig. 8.9b).

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Why it happens: The pop is the sound of the gas itself burning quickly in air — the
m.
o m g l a
.cburn better — it supports combustion. Two opposite behaviours
gas is a fuel. The brighter flame means the gas does not burn itself but helps the

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candle from two

a glagases got out of one liquid: this is the first clue that water is not a single simple
substance.

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Safety first: Do the gas testing with care and maintain a safe distance from the set-
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up.
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Which gas is present in each test tube?
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The pop sound confirms hydrogen; the brighter flame confirms oxygen.
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TEST OBSERVATION GAS IDENTIFIED

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Burning candle at the mouth of the tube A pop sound is heard Hydrogen (Fig. 8.9a)

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Burning candle at the mouth of the tube The flame glows brighter Oxygen (Fig. 8.9b)

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m .Water (on passing electricity) → Hydrogen + Oxygen a
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What this proves: Water has been broken down into two different constituents. So
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water cannot be an element — hydrogen and oxygen are the pure substances here,
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and both of them are elements.
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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q5 Can these collected gases be water vapour?

No. If they were water vapour they would have condensed back into water as soon as they
cooled inside the test tube — but the gas stays a gas and keeps pushing the water level down.

Why it settles the question: Water vapour is still water; only its state has changed,
and cooling brings it straight back to liquid water, as you saw in Grade 6. These
gases do neither. Moreover water vapour neither pops nor makes a candle burn
brighter. So passing electricity through water is not a change of state at all — it is a
change into two entirely new substances.

Ever heard of ... — Page 123
8.3.1 Elements

EVER HEARD OF ...

Q1 When electric current is passed through water, it breaks down into hydrogen and
oxygen. Is this a chemical change or a physical change? Recall Curiosity, Grade 7,
chapter ‘Changes Around Us: Physical and Chemical’.

It is a chemical change.

TEST OF A CHEMICAL DOES IT APPLY HERE?
CHANGE

New substances are formed Yes — hydrogen and oxygen, neither of which was there before

The new substances have different Yes — one pops when lit, the other makes a flame glow brighter; water
properties does neither

The change is not easily reversed Yes — you cannot get water back simply by cooling the gases

Page 20 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: Compare it with melting ice or boiling water. There, only the state
changes and the particles of water remain the same, so the change is physical and
reversible by heating or cooling. Here the particles themselves are pulled apart into
particles of two different elements — the identity of the substance has changed, and
that is what makes a change chemical.

In-text Questions — Page 124
8.3.2 Compounds

Q1 Why cannot we separate hydrogen and oxygen present in water by physical means?

Because in water the particles of hydrogen and oxygen are so tightly attached to each other
that no physical method can pull them apart. Water is a compound, not a mixture.

Why it happens: A physical method — filtering, sieving, evaporating, using a
magnet, letting things settle — only sorts particles that are already separate and
lying side by side. In a compound the elements have combined chemically in a fixed
ratio and are no longer separate particles of hydrogen and oxygen at all; the
molecule of water is a new particle. To take them apart you have to force a chemical
change, which is exactly what the electric current in Activity 8.3 did.

Tip: This is the sharpest line in the chapter. Salt dissolved in water is a mixture —
evaporate the water and the salt is back. Hydrogen and oxygen in water are a
compound — no amount of evaporating will ever give them back.

Q2 Are common salt and sugar elements or compounds? Let us find out.

Both are compounds.

Common salt is sodium chloride. Sodium is a soft metal and chlorine is a hazardous gas, yet
together they make a harmless, taste-enhancing substance that is essential for our lives —
made of particles of sodium and chlorine in a 1 : 1 ratio.

Page 21 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Sugar is a chemical compound of the elements carbon, hydrogen and oxygen. Activity 8.4
proves it: heated sugar leaves behind charcoal (carbon) and gives off water, and water itself
is hydrogen and oxygen.

Why it happens: The clinching evidence in both cases is the change of properties.
Neither salt nor sugar behaves anything like the elements inside it — a compound is
a genuinely new substance, not the two elements sitting together.

Q3 Is it possible to separate sodium chloride into its elements by physical processes?

No. Sodium chloride is a compound, so no physical process can split it into sodium and chlorine.

Why it happens: Be careful not to confuse two different separations. Dissolved
sodium chloride can be recovered from seawater by evaporation — but that only
separates salt from water, a mixture. It gives you back sodium chloride, not sodium
and chlorine. To break the salt itself into its two elements you would have to break
the chemical combination that holds them in the 1 : 1 ratio, and that needs a
chemical change, not a physical one.

Check it yourself: Ask of every separation — am I separating a mixture into its
components, or a compound into its elements? Only the first can be done physically.

Activity 8.4: Let us experiment — Page 124

Page 22 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.3.2 Compounds

ACTIVITY

Q1 Put a teaspoon of sugar in a boiling tube. Heat it gently as shown in Fig. 8.12a. What
do you observe?

Test tube holder

Boiling tube

Sugar

Spirit lamp

(a) Heating of sugar

Fig. 8.12a, page 125 — redrawn sketch. A teaspoon of sugar being heated gently in a
boiling tube held over a spirit lamp.

The sugar changes in three clear stages.

1. It melts and turns brown (Fig. 8.12b).
2. On further heating it begins to char — it turns blackish (Fig. 8.12c).
3. Small droplets of water appear inside the boiling tube near its open end, and a black solid,
charcoal (carbon), is left behind.

Sugar (on heating) → Carbon + Water

Page 23 of 52

as e
a g l
Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

co m
m.
Why it happens: Sugar decomposes on heating. The water and the black charcoal

m l a se
did not exist in the tube before, so a new set of substances has been formed — this
o g than one
.c change, and it is the proof that sugar is made of more
a
m
is a chemical

l a se
ag
element.

o m
c ag
Safety first: This activity must be performed in the presence of a teacher. Hold the
.
m keep its mouth pointed away from
s e
boiling tube with a test tube holder and
a
agl
everyone.

co m
se m.
o m
In-text Questions — Page 125
g l a
m .c a
se
8.3.2 Compounds

l a
ag Q1 You will find small droplets of water inside the boiling tube near its open end.
m a s
.co agl
Where did this water come from? Was it in the dry sugar, or did it come by the

se m
condensation of water vapour in the air?

g l a
ANSWER a
The water came from the dry sugar itself, not from the air.
com
m .
o m l a se
g air could settle inside a
.ac cool surface, never on a hot one — so nothing from the
Why we can be sure: We are heating the tube. Water vapour from the air condenses
m a
aseboiling tube that is being warmed. The droplets appear near the open end, the
on

agl
m
coolest part of the tube, exactly where a vapour coming from inside would cool and

a se
com l
condense. And the sugar we started with was dry.
. a g
e m
s and one of the products is water. Since water itself
So heating has broken the sugaradown,
g l
a the sugar must have contained hydrogen and oxygen —
consists of hydrogen and oxygen,
sugar cannot be an element.

co m
m .
m as e
. c o a g l
s e mQ2 Can you predict what is left behind?
a
agl c
m .
e

om a s
. c agl
Charcoal — that is, carbon — is left behind in the boiling tube. It is the black, brittle solid you
can scoop out onto a watch glassm
a s e (Fig. 8.12c).

agl

com
m .
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.co


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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: On heating, sugar decomposes into carbon and water. The water
escapes as vapour and condenses near the open end, while the carbon, which does
not vaporise at this temperature, stays behind as a black residue. If you set fire to a
little of it, it burns like coal — the test that it really is carbon.

Putting the whole activity together:

Sugar → Carbon + Water

and Water → Hydrogen + Oxygen

so sugar contains carbon, hydrogen and oxygen — it is a compound.

Activity 8.5: Let us experiment (Demonstration activity) — Page 125
8.3.2 Compounds

ACTIVITY

Q1 Mix them thoroughly in a watch glass. Label this mixture as Sample A (Fig. 8.14). Is
this a uniform or a non-uniform mixture? Can you still observe both iron and sulfur
as separate substances?

Sample A

Iron filings (black specks) Sulfur powder (pale yellow)

Fig. 8.14, page 126 — redrawn sketch. Sample A: 5.6 g of iron filings mixed with 3.2 g of
sulfur powder in a watch glass.

Sample A is a non-uniform mixture, and yes — the iron and the sulfur can still be seen
separately.

Page 25 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: Look closely and you can pick out the greyish-black specks of iron
filings among the pale yellow grains of sulfur powder; a magnifying glass makes it
obvious. Nothing has reacted — the two elements have only been stirred together,
so each keeps its own colour, its own texture and all its other properties. That is the
definition of a mixture, and because the two kinds of grains are big enough to be
told apart, the mixture is non-uniform.

Tip: The masses are not arbitrary. 5.6 g of iron to 3.2 g of sulfur is the ratio 7 : 4 by
mass — the proportion in which the two elements combine completely when they
are later heated, leaving nothing over.

Page 26 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q2 Now, you have two samples — Sample A and Sample B (Fig. 8.16a and 8.16b).
Compare both the Samples A and B step by step and record your observations in
Table 8.2.

(a) Sample A (b) Sample B

Fig. 8.16, page 126 — redrawn sketch. (a) Sample A — the iron–sulfur mixture; (b) Sample
B — the black mass left after heating Sample A, ground to a powder.

Table 8.2: Comparison of Samples A and B

S.NO. EXPERIMENT OBSERVATIONS

SAMPLE A SAMPLE B

1. Appearance
(i) Colour
(ii) Texture

2. Magnet test

3. Gas test
(i) Odour
(ii) Burning

Here is Table 8.2 completed.

Page 27 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

S.NO. EXPERIMENT SAMPLE A SAMPLE B

1. Appearance — (i) Two colours together — black specks of Uniformly black throughout
Colour iron among yellow sulfur

Appearance — (ii) Grainy and uneven; two kinds of grains Fine, even powder; the same
Texture can be told apart all through

2. Magnet test Iron filings are attracted and lifted away Not attracted at all
by the magnet

3. Gas test — (i) Odour The gas has no smell The gas smells of rotten eggs

Gas test — (ii) Burns with a "pop" sound (hydrogen) Does not burn with a pop; it is
Burning hydrogen sulfide

What the table shows: Every single row is different. Sample A behaves like iron and
sulfur kept side by side; Sample B behaves like neither of them. That is the whole
difference between a mixture and a compound, written out as observations.

Q3 Step 1 — Appearance. Compare the appearance of Sample A and Sample B like
colour and texture.

They do not look the same at all.

Sample A: a speckled mixture — black iron filings clearly visible among yellow sulfur powder;
the texture is coarse and uneven.
Sample B: a uniformly black mass, ground to a fine powder, with the same colour and
texture at every point.

Why it happens: In Sample A the iron and the sulfur are still themselves, so you see
both their colours. In Sample B the two elements have combined chemically into a
single new substance, iron sulfide. There is no iron left to look black-grey and no
sulfur left to look yellow — there is only one substance, so there is only one
appearance.

Page 28 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

co m
m.
Step 2 — Magnet test. Take a magnet and move it over the Samples A (Fig. 8.17a)
e
Q4

m l as
.co
and B (Fig. 8.17b), one by one. What do you observe?

a g
se m
g l a
a

com
Magnet Magnet

e m . ag
a s
agl

co m
e m.
m l as
m .co a g
l a se
a g (a) Sample A (b) Sample B

m a s
.co B. agl
Fig. 8.17, page 126 — redrawn sketch. A magnet moved over (a) Sample A and (b) Sample

se m
g l a
a
co m
m .
The magnet pulls iron filings out of Sample A, leaving the yellow sulfur behind. It has no effect
m as e
.co
at all on Sample B.
a g l
a s em
agl SAMPLE RESPONSE TO THE MAGNET WHAT IT TELLS YOU

se m
com a
A Iron filings jump to the magnet; sulfur is Iron is still present as iron — A is a mixture, and it
left behind
.
can be separated
a g l
Nothing is attracted as
em
agl
B There is no free iron left — a new substance has
been formed

co m
m .
e
Why it happens: Being attracted by a magnet is a property of iron. In a mixture each
m l as
.co g
component keeps its own properties, so the iron in Sample A is still magnetic and

em a
s
the magnet becomes a tool for separating the mixture. In Sample B the iron has
l a
ag combined chemically with sulfur; the compound iron sulfide is a different substance
c
with its own properties, and being attracted by a magnet is not one of them.
m .
m a s e
e m . co agl
g l as
a

co m
m .
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.co


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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q5 Step 3 — Gas test. Take a small amount of Sample A in a test tube and add a few
drops of dilute hydrochloric acid (Fig. 8.18a). What do you observe? Gently smell the
evolved gas by wafting it towards your nose (Fig. 8.19).

Test tube

Test tube stand

Waft the fumes towards the nose
(a) Sample A

Fig. 8.18a and Fig. 8.19, page 127 — redrawn sketches. Left: Sample A with a few drops of
dilute hydrochloric acid in a test tube standing in a test tube stand. Right: the correct
way to waft the fumes of an evolved gas towards the nose.

Bubbles of a gas come off briskly, and a yellow solid stays at the bottom of the test tube. Wafted
towards the nose, the gas has no smell at all.
What is happening is that only the iron in the mixture reacts:

Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

The sulfur is left as a yellow solid at the bottom — it does not react with hydrochloric acid.

Why it happens: Sample A is a mixture, so its two components meet the acid
separately and behave exactly as they would on their own. The gas released is
hydrogen, which is colourless and odourless.

Safety first: Never smell anything directly. Waft the fumes towards your nose with
your hand, as shown in Fig. 8.19, and be careful while handling hydrochloric acid.

Page 30 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q6 Test the evolved gas by bringing a burning splinter or a lighted candle near the
mouth of the test tube (Fig. 8.20a). What do you observe? Repeat the above steps
with Sample B as well (Fig. 8.18b and 8.20b).

POP!

(a) Sample A (b) Sample B

Fig. 8.20, page 127 — redrawn sketch. A burning splinter brought to the mouth of the
test tube: (a) Sample A — the gas burns with a 'pop'; (b) Sample B, the same steps
repeated (Fig. 8.18b and 8.20b) — the splinter goes on burning, with no pop.

The gas from Sample A burns with a 'pop' sound — it is hydrogen. The gas from Sample B is
different in every way: it is colourless but has a strong rotten egg-like odour, and it is hydrogen
sulfide.

Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide

SAMPLE A SAMPLE B

Gas evolved Hydrogen Hydrogen sulfide

Odour Odourless Smells of rotten eggs

Burning test Burns with a pop sound Does not give a pop sound

Page 31 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: The gas tells you what the acid actually attacked. In Sample A the
acid met free iron, so hydrogen came off. In Sample B there is no free iron and no
free sulfur — the acid met the compound iron sulfide, and a gas containing sulfur
came off instead. A completely different product is the clearest possible proof that
Sample B is a new substance and not the old mixture.

Safety first: Hydrogen sulfide is poisonous. This is why the book says to do the
activity in a fume hood or a well-ventilated area, and not to inhale the gases.

Some discussion points — Page 128
8.3.2 Compounds

LET US DISCUSS

Q1 Do the Samples, A and B look the same?

No. Sample A is speckled — black iron among yellow sulfur; Sample B is a uniformly black mass
with the same colour and texture throughout.

Why it happens: In Sample A both elements are still present as themselves, so you
see both of their appearances at once. In Sample B they have combined into one
new substance, iron sulfide, and a single substance can have only one appearance.

Q2 Which sample exhibits magnetic properties?

Sample A — the iron filings in it are attracted to the magnet. Sample B is not attracted at all.

Why it happens: Attraction to a magnet is a property of iron. Sample A is a mixture,
and each component of a mixture keeps its own properties, so the iron in it is still
magnetic. In Sample B there is no free iron left; the iron has become part of the
compound iron sulfide, which has its own — different — properties.

Page 32 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q3 Can we separate the components of Samples A and B?

Sample A: yes. Sample B: no.

SAMPLE CAN IT BE HOW, AND WHY
SEPARATED?

A (mixture) Yes A magnet pulls out the iron and leaves the sulfur behind — a
physical method is enough

B No The iron and sulfur are chemically combined in a fixed ratio; no
(compound) physical method can undo that

Why it happens: This single question is the practical test that separates a mixture
from a compound. The components of a mixture are only mixed and can always be
got back physically; the constituent elements of a compound cannot be separated by
any physical method.

Q4 On adding dilute hydrochloric acid, do gases evolve in both Samples A and B?

Yes — both give off a gas. But they are two different gases.

Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide

Why it happens: In Sample A the acid reacts with the free iron of the mixture, while
the sulfur simply sits at the bottom unreacted. In Sample B there is no free iron; the
acid reacts with the compound iron sulfide as a whole, and the sulfur that is locked
inside it leaves with the gas.

Page 33 of 52

as e
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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

co m
m.
In both the cases, do the gases smell the same or different?
e
Q5

m l as
.co a g
a
s em
a gl The gas from Sample A is colourless and has no smell; the gas from Sample B is
Different.
colourless but has a strong rotten egg-like odour.

o m
e m . c ag
s
Why it happens: The gas from A is hydrogen, which is odourless and burns with a

a ga
pop. The gas from B is hydrogenlsulfide, and the sulfur it carries is what gives it that
unmistakable smell. The smell is therefore direct evidence that the sulfur in Sample
B is no longer lying free but has been carried into a new compound.
co m
se m.
o m
Safety.cfirst: Waft the gas towards your nose with your hand — l a
g never put your nose
m a
l a
tosethe mouth of the test tube.
a g
a s
. comin this activity into mixtures, compounds, and agl
em
Also, categorise the substances used
s
Q6
a
agl
elements.

com

m .
m as e
.co l
ELEMENTS COMPOUNDS MIXTURES

a g
s m (iron filings), Sulfur
eIron
a
gl (sulfur powder), Hydrogen
Iron sulfide (Sample B), Sample A (iron filings + sulfur powder),

a Hydrochloric acid, Iron chloride, Dilute hydrochloric acid (hydrochloric

m
Hydrogen sulfide acid in water)

a se
. com a g l
m
ase
How each was decided: Iron and sulfur cannot be broken down into anything

a gl
simpler — elements. Sample A is the two of them merely stirred together, each
keeping its properties — a mixture. Iron sulfide, iron chloride and hydrogen sulfide

c o m
are each made of two different elements combined chemically in a fixed ratio, with
.
m acid is the
m s e
new properties of their own — compounds. And dilute hydrochloric
a the dropper is a mixture
hydrochloric acid mixed with water, so what islin
.co ag
compound

se m even though the acid itself is a compound.
g l a
a c
m .
m a s e
.co
In-text Questions — Page 128
m agl
l a se
a g

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.co


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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

8.3.2 Compounds

Q1 Can you explain now why the magnet has no effect on Sample B?

Because there is no iron left in Sample B as iron. On heating, the iron and the sulfur combined
chemically to form a new substance — iron sulfide — and a compound has properties
completely different from those of the elements that formed it.

Iron + Sulfur (on heating) → Iron sulfide

Why it happens: Being attracted by a magnet belongs to iron, not to iron sulfide. In
Sample A the iron particles were only mixed with sulfur, so they kept that property
and the magnet could pull them out. In Sample B the iron particles are chemically
bound to sulfur particles in a fixed ratio and are no longer free; the compound they
form is a different substance with its own set of properties, and magnetic attraction
is not one of them. This is also why the iron and sulfur can no longer be separated
from Sample B by any physical means.

Keep the curiosity alive — Page 131
End-of-chapter questions

KEEP THE CURIOSITY ALIVE

Q1 Consider the following reaction where two substances, A and B, combine to form a
product C: A + B → C Assume that A and B cannot be broken down into simpler
substances by chemical reactions. Based on this information, which of the following
statements is correct? (i) A, B, and C are all compounds and only C has a fixed
composition. (ii) C is a compound, and A and B have a fixed composition. (iii) A and B
are compounds, and C has a fixed composition. (iv) A and B are elements, C is a
compound, and has a fixed composition.

Correct answer: (iv) A and B are elements, C is a compound, and has a fixed composition.

Page 35 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why: The one fact you are given is that A and B cannot be broken down into simpler
substances. That is precisely the definition of an element, so A and B are elements. C
is formed by two different elements combining chemically, which makes C a
compound — and every compound has its elements combined in a fixed ratio, so C
has a fixed composition.

OPTION VERDICT REASON

(i) Wrong A and B cannot be compounds — a compound can be broken down into
simpler substances

(ii) Wrong C being a compound is right, but "fixed composition" belongs to C; A and B
are single elements, not combinations

(iii) Wrong Again treats A and B as compounds, which contradicts the information given

(iv) Correct Matches both the definition of an element and the definition of a compound

Q2 Assertion: Air is a mixture. Reason: A mixture is formed when two or more
substances are mixed, without undergoing any chemical change. (i) Both Assertion
and Reason are true and Reason is the correct explanation for Assertion. (ii) Both
Assertion and Reason are true, but Reason is not the correct explanation for
Assertion. (iii) Assertion is true, but Reason is false. (iv) Assertion is false, but
Reason is true.

Correct answer: (i) Both Assertion and Reason are true and Reason is the correct
explanation for Assertion.

Why: The Assertion is true — air is a uniform mixture of nitrogen, oxygen, argon,
carbon dioxide and water vapour. The Reason is a correct statement of what a
mixture is. And the Reason explains the Assertion: air counts as a mixture precisely
because those gases are simply mixed together without any chemical change, each
keeping its own properties. Nitrogen in air still refuses to support burning and
oxygen in air still supports it, exactly as they would on their own. That is the test the
Reason describes, applied to air.

Page 36 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Tip: In an assertion–reason question, first check each statement on its own, then ask
the second question separately — does the reason actually explain the assertion, or is it
merely another true fact?

Q3 Water, a compound, has different properties compared to those of the elements
oxygen and hydrogen from which it is formed. Justify this statement.

The properties of water are not a blend of the properties of hydrogen and oxygen — they are
entirely new. That is what happens when elements combine chemically to form a compound.

PROPERTY HYDROGEN OXYGEN WATER

State at room Gas Gas Liquid
temperature

Behaviour with fire A fuel — burns with Supports combustion — Extinguishes fire
a pop sound makes a flame glow
brighter

Can we drink it? No No Essential for life

Can it be separated Both are elements — cannot be broken down at Cannot be separated into
physically? all hydrogen and oxygen by any
physical method

Why it happens: In water the hydrogen and oxygen are not lying side by side as two
gases; they are chemically combined in the fixed ratio of 2 : 1, and their particles are
held together so tightly that a new kind of particle exists. A new particle means a
new substance, and a new substance has its own set of properties. If water were
only a mixture of the two gases, it would still burn and still support burning — but
water does the opposite of both, which is exactly why we use it to put out fires.

Page 37 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q4 In which of the following cases are all the examples correctly matched? Give
reasons in support of your answers. (i) Elements — water, nitrogen, iron, air. (ii)
Uniform mixtures— minerals, seawater, bronze, air. (iii) Pure substances— carbon
dioxide, iron, oxygen, sugar. (iv) Non-uniform mixtures — air, sand, brass, muddy
water.

Correct answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar.

Why (iii) is right: A pure substance consists of only one type of particle, and every
element and every compound qualifies. Carbon dioxide and sugar are compounds;
iron and oxygen are elements. All four are therefore pure substances.

OPTION THE ITEM REASON
THAT SPOILS
IT

(i) Elements water and air Water is a compound of hydrogen and oxygen; air is a mixture
of gases. Only nitrogen and iron are elements.

(ii) Uniform minerals Minerals are not mixtures at all — most are compounds and a
mixtures few are pure elements. Seawater, bronze and air are indeed
uniform mixtures.

(iii) Pure — none — All four are elements or compounds. This is the correct set.
substances

(iv) Non-uniform air and brass Air is a uniform mixture and brass is an alloy, which is also
mixtures uniform. Only sand and muddy water are non-uniform.

Q5 Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to
form magnesium oxide. Classify all the substances involved in the above reactions
as elements, compounds or mixtures, with justification.

Iron + Moist air → Iron oxide

Magnesium + Oxygen → Magnesium oxide

Page 38 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

co m
m.
SUBSTANCE CLASSIFICATION JUSTIFICATION

mElement as e
Iron
.co a g l
Cannot be broken down into simpler substances; made of

a s em identical atoms

l
g air
aMoist Mixture Nitrogen, oxygen, argon, carbon dioxide and water vapour
mixed together, each keeping its own properties

co m
m . ag
se — reddish-brown, brittle, not shiny — are quite
Iron oxide (rust) Compound Iron and oxygen combined chemically in a fixed ratio; its

l a
ag unlike those of iron
properties

Magnesium Element A metal that cannot be broken down further
co m
em.
com l as
Oxygen Element A non-metal that cannot be broken down further

. a g
m
ase
agl
Magnesium Compound Magnesium and oxygen combined chemically in a fixed ratio; a
oxide white powder, unlike the shiny metal or the colourless gas

m a s
.co agl
Why it happens: In both reactions an element combines chemically with oxygen

a s em
and a compound is formed. Note the interesting point in the first one — iron reacts

agl that happen to be in the mixture we call air. That is
with the oxygen and water vapour
why iron rusts in moist air but hardly at all in dry air.

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Q6 Classify the following as elements, compounds, or mixtures in Table 8.3. Carbon
dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen,
rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur,
hydrogen, baking soda. Identify pure substances amongst these and list them
below.

Table 8.3

ELEMENTS COMPOUNDS MIXTURES

PURE SUBSTANCES

Table 8.3

ELEMENTS COMPOUNDS MIXTURES

Aluminium, Gold, Oxygen, Carbon dioxide, Magnesium oxide, Rust, Iron Sand, Seawater, Muddy
Nitrogen, Sulfur, Hydrogen sulfide, Glucose, Water, Sodium chloride, Baking water, Air, Fruit juice
soda

Pure substances — every element and every compound in the list:

Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen,

Carbon dioxide, Magnesium oxide, Rust, Iron sulfide, Glucose, Water, Sodium chloride,

Baking soda

— that is all 14 substances from the first two columns.

Page 40 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

How the doubtful ones were decided:

Rust is iron oxide — iron and oxygen chemically combined, so a compound.
Sand is largely quartz but comes with other minerals and grains mixed in, so it is
treated as a mixture; its grains can be told apart, making it non-uniform.
Seawater is a uniform mixture — water with salts dissolved in it — because the
salt can be recovered by evaporation.
Fruit juice contains water, pulp, sugars and acids together, so it is a mixture.
Baking soda is a single compound. Do not confuse it with baking powder, which
is a solid–solid mixture of baking soda and tartaric acid (Table 8.1).

Matter

Pure substances Mixtures

Elements Compounds Uniform Non-uniform

gold, oxygen water, glucose air, seawater sand, muddy water

Ask first: is it one substance or more than one? Then: can it be broken down further?

The two questions that classify any sample of matter.

Q7 What new substance is formed when a mixture of iron filings and sulfur powder is
heated, and how is it different from the original mixture? Also, write the word
equation for the reaction.

The new substance is iron sulfide, a black mass — and it is a compound, while the original was
a mixture.

Iron + Sulfur (on heating) → Iron sulfide

Page 41 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

POINT OF MIXTURE OF IRON AND SULFUR IRON SULFIDE (SAMPLE
COMPARISON (SAMPLE A) B)

Appearance Black specks among yellow powder; grainy Uniformly black, same texture
throughout

Magnet Iron is attracted and can be pulled out Not attracted

With dilute hydrochloric Gives hydrogen — odourless, burns with a Gives hydrogen sulfide —
acid pop; sulfur left as a yellow solid smells of rotten eggs

Can the components be Yes, by physical means No physical method can
separated? separate them

Proportion of iron to sulfur Can be varied at will Fixed

Why it happens: Heating supplies the energy the two elements need to combine
chemically. Once combined, they are no longer iron and sulfur but a single new
substance with its own properties — which is why every test in the table gives a
different result.

Q8 Is it possible for a substance to be classified as both an element and a compound?
Explain why or why not.

No — never. The two definitions rule each other out.

ELEMENT COMPOUND

Cannot be broken down into simpler substances Can be broken down into the elements it is made of

Made of one kind of atom Made of two or more different elements combined chemically

No question of a ratio — there is only one element Its elements are combined in a fixed ratio

Why it happens: A substance is an element exactly when it cannot be split into
anything simpler. To be a compound it must be splittable into at least two different
elements. A single substance cannot both be splittable and not splittable, so no
substance can be in both categories. Both, however, are pure substances — that is
the one label they share.

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Tip: Water is the classic trap. It looks like one simple thing, but Activity 8.3 splits it
into hydrogen and oxygen — so water is a compound, never an element.

Q9 How would our daily lives be changed if water were not a compound but a mixture
of hydrogen and oxygen?

There would be no liquid water at all — and therefore no life as we know it.

Why: Hydrogen and oxygen are both gases at room temperature. A mere mixture of
them would also be a gas, because in a mixture each component keeps its own
properties. Rivers, wells, seas and the water in your bottle exist only because the two
elements are chemically combined into a completely new substance that happens to
be a liquid.

Following that through:

Nothing to drink, cook with or wash in. You cannot carry a gas in a pot or a bottle.
Everything would be dangerously flammable. A mixture of a fuel (hydrogen) and a gas
that supports combustion (oxygen) would explode at the smallest spark, instead of putting
fires out.
The proportion would keep changing. A mixture has no fixed ratio, so "water" from one
place would not be the same as "water" from another.
No rain, no rivers, no crops. There would be no water cycle, no irrigation and no
agriculture.
No life. The bodies of all living things are mostly water; blood, sap and cell fluids all depend
on it being a liquid.

Did you know? This is why the girl in the chapter finds it fascinating — hydrogen is a
fuel and oxygen supports combustion, yet the compound they form extinguishes
fire. Combining chemically really does make a brand new substance.

Page 43 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Check it yourself: If the tube had contained iron sulfide instead of plain iron filings,
the gas would have been hydrogen sulfide, recognisable at once by its rotten egg
smell. The smell test alone tells the two reactions apart.

Q11 Write the names of any two compounds made only from non-metals, and also
mention two uses of each of them.

Two such compounds are water (hydrogen and oxygen) and carbon dioxide (carbon and
oxygen). Hydrogen, oxygen and carbon are all non-metals, as the chapter states.

COMPOUND NON-METALS IN TWO USES
IT

Water Hydrogen, Oxygen 1. Drinking, cooking and washing in every home.
2. Irrigating crops; also used as a coolant and as a solvent in
industry.

Carbon dioxide Carbon, Oxygen 1. Filling fire extinguishers, because it does not support combustion.
2. Making aerated drinks (soda water) fizzy; plants also use it to
make their food.

Another good pair: Glucose (carbon, hydrogen and oxygen) — a quick source of
energy for the body, and used in medicines and drips for patients.

Q12 How can gold be classified as both a mineral and a metal?

Gold fits both labels because the two words answer two different questions — where does it
come from? and what kind of element is it?

CALLED BECAUSE
A

Mineral It is a natural solid substance found in the Earth with a fixed chemical composition. Gold is one of
the native minerals — the few minerals that are pure elements rather than compounds.

Metal As an element it belongs to the class of metals, along with silver, magnesium, iron and aluminium —
it is shiny, dense and can be beaten into shape.

Page 45 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Why it happens: Most minerals are compounds made of more than one element —
quartz, calcite, mica, pyroxene and olivine. A handful, however, occur in the Earth
already in their pure elemental form; these are the native minerals, and they may be
metals such as gold, silver and copper, or non-metals such as sulfur and carbon.
Gold is simply the best-known example of an element that we dig straight out of the
ground.

Tip: "Mineral" describes the source and the composition; "metal" describes the
properties of the element. Silver and copper can be classified both ways too.

Discover, design, and debate — Page 133
Science • Society • Interdisciplinary Projects

DISCOVER, DESIGN, AND DEBATE PROJECTS

Q1 Design and create comic strips from real-life examples to differentiate between
elements, compounds, and mixtures with diagrams and illustrate their properties
and uses.

Build the strip around one story, not three separate ones — that is what makes the difference
stand out.
A good comic strip must contain:

three characters, one for each category, with their particles drawn: an element as identical
circles, a compound as two kinds of circles joined together, a mixture as two kinds lying
loose;
at least one real example of each from your own home;
one property that gives each away — a magnet, a filter, a change of colour on heating;
one use of each in daily life;
a closing panel with the rule the reader should carry away.

Sample strip (six panels):

1. The kitchen. Meera picks up a steel spoon, a packet of salt and a bowl of sprout salad.
"Three things — are they the same kind of stuff?"
2. Salad speaks: "Look, you can pick out my moong, my chana, my onion, my tomato. I am a
mixture — everyone here keeps their own taste, and you can pull us apart."
3. Salt speaks: "I am made of sodium, a soft metal, and chlorine, a dangerous gas, joined in a 1
: 1 ratio. I am a compound — and I am safe to eat, which neither of them is. Try separating
me with a sieve!"

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

4. The spoon speaks: "Careful — I am stainless steel, which is iron, nickel, chromium and a
little carbon. I look like one substance, but I am a uniform mixture, an alloy."
5. Enter Gold, from Meera's mother's ring: "I am an element. Break me down as much as
you like — you will still get gold."
6. The rule panel: "Can I separate it physically? Then it is a mixture. If not — can it be broken
into simpler substances at all? If yes, a compound; if no, an element."

Tip: Draw the particles in every panel, in the same style. Readers remember the
picture of the particles long after they forget the words.

Q2 Search for discoveries of some elements (such as phosphorus, sodium), compounds
(such as penicillin) and mixtures (such as brass, bronze, stainless steel). Present
your findings in the class.

Organise the search by asking the same four questions of each substance, so your presentation
has a shape: who found it, when, how they found it, and why it mattered.

SUBSTANCE TYPE POINTS WORTH REPORTING

Phosphorus Element Found in the 17th century by an alchemist who was boiling down urine
while searching for gold; it glowed in the dark, which is how it got its
name. A reminder that many discoveries were accidents.

Sodium Element Obtained early in the 19th century by passing electricity through a
molten compound — the same idea you used in Activity 8.3 on water.
Electricity became the tool that prised open compounds.

Penicillin Compound Noticed when a mould accidentally contaminated a dish of bacteria and
killed them. It became the first widely used antibiotic and has saved
millions of lives.

Bronze Mixture Copper and tin. So important that an entire age is named after it. In
(alloy) India it is kamsya, described in the Charaka Samhita as copper (4 parts)
with tin (1 part).

Brass Mixture Copper and zinc — golden in colour, easy to shape, and still the metal
(alloy) of choice for lotas, bells and door fittings.

Stainless steel Mixture Iron, nickel, chromium and a little carbon. Invented while looking for a
(alloy) steel that would not rust in gun barrels; stronger and more durable
than pure iron.

Page 47 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Tip: End the presentation with a pattern rather than a list: elements were mostly
discovered by taking things apart, compounds by observing an effect and then chasing
its cause, and alloys by craftspeople trying to make a better tool.

Q3 Let us search: Read labels on items like detergents or snacks, and try to list the
mixtures and compounds they contain.

Take the packet to the light and read the "Ingredients" line. Then sort each name by asking: is
this one substance, or several mixed?
Sample answer — a packet of biscuits and a packet of detergent:

ITEM NAMED ON THE LABEL COMPOUND OR MIXTURE

Biscuits Sugar Compound

Salt (sodium chloride) Compound

Baking soda Compound

Raising agent (baking powder) Mixture — baking soda and tartaric acid

Wheat flour, edible vegetable oil, milk solids Mixtures

The biscuit itself Mixture

Detergent Sodium carbonate (washing soda) Compound

Sodium chloride Compound

Perfume, colour, optical brightener Mixtures

The detergent powder itself Mixture

What the exercise teaches: Every packaged product you buy is a mixture, however
"pure" the front of the pack claims to be. The individual ingredients listed on the
back are the pure substances — and they are almost always compounds, because
free elements are rare in everyday products.

Tip: Look for a percentage or a "not more than" figure. A compound has a fixed
composition, so a printed proportion is a sign that you are reading about a mixture.

Page 48 of 52

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

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Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

A pure substance contains only one kind of particle and cannot be separated into other
kinds of matter by any physical process. Every pure substance is either an element or a
compound.
Elements cannot be broken down into simpler substances. Each is made of identical atoms;
two or more atoms usually join into a molecule. 118 elements are known — mostly solids,
eleven gases, and only mercury and bromine liquid at room temperature.
Compounds are formed when different elements combine chemically in a fixed ratio. The
compound's properties are completely different from those of its elements, and the
elements cannot be separated by any physical method.
Minerals are natural solids of the Earth with a fixed chemical composition — mostly
compounds (quartz, calcite, mica), but a few, the native minerals such as gold and sulfur,
are pure elements.

Page 50 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS HOW YOU CAN TELL EXAMPLE FROM THE
CHAPTER

Mixture Two or more substances Components can be Poha, sprout salad, air,
mixed; each keeps its own separated by a physical seawater
properties, no chemical method; composition can
reaction vary

Component An individual substance that It is still itself inside the Green gram, chickpeas,
makes up a mixture mixture onion, tomato in a sprout
salad

Uniform mixture Components evenly spread; Looks the same throughout Sugar in water, air, brass,
cannot be distinguished even bronze, stainless steel
with a microscope

Non-uniform Components visible to the You can point to the Sprout salad, sand and
mixture naked eye or with a separate bits water, muddy water
magnifying device

Alloy A uniform mixture of metals Looks like one metal, but is a Brass (copper + zinc),
(and sometimes a little mixture bronze (copper + tin),
carbon) stainless steel

Pure substance Matter made of only one kind Cannot be separated into Oxygen, iron, water,
of particle other kinds of matter by any sodium chloride, sugar
physical process

Element A substance that cannot be Made of identical atoms; it is Hydrogen, oxygen, gold,
broken down into simpler a building block of matter silver, sulfur, carbon
substances

Atom The identical particle an Different from the particle of An atom of hydrogen; an
element is made of every other element atom of oxygen

Molecule A stable particle formed when Two hydrogen atoms give Molecule of hydrogen;
two or more atoms combine one hydrogen molecule (Fig. molecule of oxygen
8.10)

Compound Different elements combined New properties; elements Water (hydrogen : oxygen
chemically in a fixed ratio cannot be separated = 2 : 1), sodium chloride (1
physically : 1), iron sulfide, sugar

Metals / non- The two main classes of Gold, iron are metals; Silicon and boron have
metals / elements, and the in-between carbon, sulfur are non- properties between the
metalloids class metals; silicon, boron are two
metalloids

Page 51 of 52

Class 8 Science Chapter 8 Nature of Matter : Elements, Compounds, and Mixtures AglaSem · NCERT Solutions

TERM WHAT IT MEANS HOW YOU CAN TELL EXAMPLE FROM THE
CHAPTER

Mineral A natural solid of the Earth Mostly a compound; a native Quartz, calcite, mica,
with a fixed chemical mineral is a pure element pyroxene, olivine, talc; gold
composition and sulfur are native
minerals

Page 52 of 52