NCERT Solutions Class 7 Science Chapter 2 Exploring Substances Acidic Basic and Neutral

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

CLASS 7 · SCIENCE

NCERT Solutions

Chapter 2: Exploring Substances:
Acidic, Basic, and Neutral

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

7 – 22 15 54 English

Solutions, notes, sample papers & more at 54 pages

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

CLASS 7 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 2: Exploring Substances:
Acidic, Basic, and Neutral
Complete NCERT Solutions for Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral
from the NCERT textbook Curiosity. Every question of the chapter is answered — all twelve questions of Let Us
Enhance Our Learning, the Dive Deeper box, every task in Activities 2.1 to 2.7, the Exploratory Projects
and each in-text question from pages 7 to 22 — with what you observe, why it happens and an everyday
Indian example.

TEXTBOOK BOOK PAGES

Curiosity (Class 7) 7 – 22

SECTIONS QUESTIONS

15 54

MEDIUM

English

Activity 2.1: Let us explore — Pages 8–9

Page 1 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Section 2.1.1 Litmus as an indicator

ACTIVITY

Q1 Do you observe any change in the colour of the blue litmus pieces? Record your
observations in Table 2.1.

Dropper

Sample bottles, each with its own dropper

White tile
Small pieces of blue litmus paper — one piece has turned red

Fig. 2.2(a), page 8 — redrawn sketch. One drop of each sample is put, one by one, on
small pieces of blue litmus paper spread on a clean dry white tile.

Page 2 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

S.NO. NAME OF COLOUR OF BLUE COLOUR OF RED LITMUS
THE SAMPLE LITMUS PAPER AFTER PAPER AFTER PUTTING A
PUTTING A DROP OF DROP OF SAMPLE
SAMPLE

1. Lemon juice

2. Soap solution

3. Amla juice

4. Tamarind
water

5. Vinegar

6. Baking soda
solution

7. Lime water

8. Tap water

9. Washing
powder
solution

10. Sugar
solution

11. Salt solution

12. Any other

Table 2.1: Testing the nature of samples with blue and red litmus papers — page 9.
Reproduced as printed, with the two observation columns left blank for you to fill in.

Yes. Only some of the samples change the blue litmus pieces to red; the rest leave them blue.
The blue pieces turn red with lemon juice, amla juice, tamarind water and vinegar. They stay
blue with soap solution, baking soda solution, lime water, washing powder solution, tap water,
sugar solution and salt solution.

Page 3 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

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Why it happens: litmus is a dye that has one colour in an acid and another in a

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base. The four samples that turned it red all contain acids. The rest have no acid to
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act on the.cdye, so the blue colour stays. a g
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aTip: use a fresh piece of litmus for every sample and one clean dropper per bottle. A

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dropper carried from lemon juice to lime water will give you a wrong reading.
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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q2 Repeat the same activity with pieces of red litmus paper as shown in Fig. 2.2b and
record your observations in Table 2.1.

Dropper

Sample bottles, each with its own dropper

White tile
Small pieces of red litmus paper — one piece has turned blue

Fig. 2.2(b), page 8 — redrawn sketch. The same activity repeated with small pieces of red
litmus paper on the white tile.

Page 5 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

S.NO. NAME OF COLOUR OF BLUE COLOUR OF RED LITMUS
THE SAMPLE LITMUS PAPER AFTER PAPER AFTER PUTTING A
PUTTING A DROP OF DROP OF SAMPLE
SAMPLE

1. Lemon juice

2. Soap solution

3. Amla juice

4. Tamarind
water

5. Vinegar

6. Baking soda
solution

7. Lime water

8. Tap water

9. Washing
powder
solution

10. Sugar
solution

11. Salt solution

12. Any other

Table 2.1: Testing the nature of samples with blue and red litmus papers — page 9.
Reproduced as printed, with the two observation columns left blank for you to fill in.

Now exactly the opposite set of samples reacts. The red litmus pieces turn blue with soap
solution, baking soda solution, lime water and washing powder solution, and stay red with
everything else.
Putting both halves of the test together gives the completed Table 2.1.

Page 6 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

S.NO. NAME OF COLOUR OF BLUE LITMUS COLOUR OF RED LITMUS
THE SAMPLE PAPER AFTER PUTTING A PAPER AFTER PUTTING A
DROP OF SAMPLE DROP OF SAMPLE

1. Lemon juice Turns red No change (stays red)

2. Soap solution No change (stays blue) Turns blue

3. Amla juice Turns red No change (stays red)

4. Tamarind Turns red No change (stays red)
water

5. Vinegar Turns red No change (stays red)

6. Baking soda No change (stays blue) Turns blue
solution

7. Lime water No change (stays blue) Turns blue

8. Tap water No change No change

9. Washing No change (stays blue) Turns blue
powder
solution

10. Sugar solution No change No change

11. Salt solution No change No change

12. Any other — Turns red No change (stays red)
curd

Read the table like this: a sample never changes both papers. If blue goes red the
sample is acidic, if red goes blue it is basic, and if nothing moves it is neutral. That
single rule is the whole of Table 2.1.

Page 7 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q3 Now, let us analyse Table 2.1 and sort the samples into three groups as follows —
Group A with samples that turn the blue litmus paper to red. Group B with samples
that turn the red litmus paper to blue. Group C with samples that do not affect
either of the two litmus papers. Record the data in Table 2.2.

S.NO. NAME OF COLOUR OF BLUE COLOUR OF RED LITMUS
THE SAMPLE LITMUS PAPER AFTER PAPER AFTER PUTTING A
PUTTING A DROP OF DROP OF SAMPLE
SAMPLE

1. Lemon juice

2. Soap solution

3. Amla juice

4. Tamarind
water

5. Vinegar

6. Baking soda
solution

7. Lime water

8. Tap water

9. Washing
powder
solution

10. Sugar
solution

11. Salt solution

12. Any other

Table 2.1: Testing the nature of samples with blue and red litmus papers — page 9.
Reproduced as printed, with the two observation columns left blank for you to fill in.

Page 8 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

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GROUP A GROUP B GROUP C

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Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Reproduced as printed,

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blank for you to fill in.

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Sorting the twelve entries of Table 2.1 gives Table 2.2.

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GROUP A
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GROUP B
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GROUP C

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(TURN BLUE LITMUS RED) (TURN RED LITMUS BLUE) (AFFECT NEITHER PAPER)
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Amla juice Baking soda solution Sugar solution

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Tamarind water
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Lime water Salt solution
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Vinegar Washing powder solution —

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Curd (any other) — —

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g B is basic, Group C is
In the.cwords the chapter uses two pages later: Group A is acidic,aGroup
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aneutral.
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om — and see which column it lands in before a
Check it yourself: add one sample of your own — orange juice, curd whey,
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toothpaste solution, floor-cleaning
you read the answer on pagese
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10.

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How to prepare lime water?
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agl Mix lime (chuna, i.e. calcium oxide) in water, leave the mixture undisturbed for some time —
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say an hour — then filter the clear liquid into another container. That filtrate is lime water.
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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Lime (calcium oxide) + Water

→ Calcium hydroxide solution

→ let it settle for about 1 hour
→ filter

→ Lime water

Why you must filter: only a small amount of calcium hydroxide dissolves. The rest
stays as a white powder at the bottom. A milky liquid would hide the colour change
you are trying to see, so you use only the clear liquid above the sediment.

Tip: do not confuse lime water with lime the fruit. Lime water is basic; the juice of the
lime fruit is acidic — the two behave in exactly opposite ways on litmus.

In-text Questions — Page 10
Section 2.1.1 — What litmus is, and the Holistic Lens box

Q1 I am curious to know — what are these red and blue litmus paper strips made of?
Why do they change colour when drops of some samples are put on them?

Litmus paper is ordinary paper soaked in litmus, a natural dye obtained from lichens. Two
versions are sold — the dye in its blue form and the dye in its red form.
The colour changes because litmus is an acid-base indicator: the dye has one colour when it sits
in an acidic liquid and a different colour when it sits in a basic liquid. Put a drop of acid on the
blue strip and the dye switches to red; put a drop of base on the red strip and it switches to
blue.

Why it happens: the colour of a dye depends on the exact form its molecules are in.
An acid pushes the litmus molecule into its red form; a base pushes it into its blue
form. Nothing is added to the paper — the same dye is simply being flipped
between two forms, which is why the change is instant.

Page 10 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Did you know? Litmus is not the only such dye. The chapter goes on to use red rose
petals and turmeric, and beetroot, purple cabbage, jamun and gudhal flowers work
too.

Q2 The substances in Group C, such as tap water, sugar solution, and salt solution, did
not change the colour of either litmus paper. Can you predict their nature?

They are neutral — neither acidic nor basic.

Why it happens: only an acid can flip blue litmus to red, and only a base can flip red
litmus to blue. A liquid that leaves both papers untouched contains neither, so there
is nothing in it for the dye to respond to. The chapter names this third kind of
substance neutral.

Tip: "no change" is a real result, not a failed test. In science an absence of change is
often the answer itself.

Q3 Lichens are formed by the association of two living organisms, a fungus and an
alga. They grow on rocks and trees in regions that have abundant rainfall and clean
air. Do you find lichens on trees in your neighbourhood?

Look on the shaded side of old tree trunks, on compound walls and on stone surfaces after the
monsoon. Lichens look like flat, crusty or leafy patches — dull green, grey-green, yellow or
orange — that seem painted onto the bark.
Sample answer: "In our locality I found grey-green crusty patches on the trunk of an old mango
tree and on the boundary wall of the school, but none on the trees along the main road."

Why the answer differs from place to place: lichens need clean air. They absorb
whatever is dissolved in rainwater, so smoke and vehicle exhaust kill them. Plenty of
lichens means clean air; almost none, on trees that otherwise look healthy, usually
means polluted air. That is why scientists call lichens bio-indicators of air quality.

Page 11 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Try This: count the lichen patches on ten trees near a busy road and on ten trees
inside a park. Compare the two counts — you have just measured air pollution
without any instrument.

Activity 2.2: Let us relate and explore — Page 11
Section 2.1.1 — Taste, touch and the acids in food

ACTIVITY

Q1 Are all the substances in Group A of Table 2.2 edible? Have you ever tasted these
edible substances? Can you recall their taste?

GROUP A GROUP B GROUP C
(TURN BLUE LITMUS (TURN RED LITMUS (AFFECT NEITHER
RED) BLUE) PAPER)

Lemon juice Soap solution Tap water

Amla juice Baking soda solution Sugar solution

Tamarind water Lime water Salt solution

Vinegar Washing powder solution

Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Column headings as printed;
the entries are the ones recorded in Activity 2.1.

Yes — every substance in Group A is edible: lemon juice, amla juice, tamarind water and
vinegar are all used in Indian kitchens. And every one of them tastes sour.

What this tells you: the four samples that turned blue litmus red are exactly the
four that taste sour. So substances that taste sour tend to contain acids and are
acidic in nature — the tongue and the litmus paper give the same verdict.

Page 12 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Caution: the book's warning matters. Do not taste anything until asked to do so,
and never taste an unknown substance. Group B substances such as washing
powder solution are harmful. Taste is a clue only for foods you already eat.

Q2 Find out and write the names of the most common acids present in the following
substances — Lemon________, Curd________, Tamarind________, Vinegar________.

SUBSTANCE MOST COMMON ACID PRESENT

Lemon Citric acid

Curd Lactic acid

Tamarind Tartaric acid

Vinegar Acetic acid

Fig. 2.4 of the book gives four more: orange — citric acid; amla — ascorbic acid (Vitamin C) and
citric acid; tamarind — tartaric acid; tomato — citric acid and oxalic acid.

Why the names sound like the food: many of these acids were first taken out of
that very food and named after it — citric from citrus fruits, lactic from milk (lac =
milk), tartaric from the tartar left in wine barrels, acetic from acetum, the Latin for
vinegar.

Did you know? Curd is sour only because milk has been fermented. Fresh milk is
almost neutral; the bacteria in the culture make lactic acid from milk sugar, and that
acid both sets the curd and gives it the sour taste.

Q3 Now, let us take one of the substances — baking soda solution — from Group B. Rub
the baking soda solution between your fingers. What do you observe?

It feels soapy or slippery between the fingers.

Page 13 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

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Why it happens: that slipperiness is a standard test for a base. A basic solution

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reacts with the thin layer of oil and protein on your skin and turns some of it into a
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— so you are literally making a trace of soapaon
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Bases also generally taste bitter — but the book adds an important warning: not everything

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bitter contains a base. Bitter gourd (karela) tastes bitter yet is not basic in nature.

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Tip: rinse your fingers well afterwards. Strong bases such as washing powder
solution are slippery for the same reason and can harm the skin, which is why the

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safe test is always litmus, not touch.

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In-text
Section 2.1.1 — Looking for other indicators

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If litmus is not available, are there some other natural substances that can serve as
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acid-base indicators?
a g
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Yes — plenty of them, and most are already in your kitchen or garden. Any

. co whose colour changes in acid or base will work.
plant part
a gl
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agl NATURAL INDICATOR COLOUR IN ACID COLOUR IN BASE

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Red rose extract
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Red Green

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Turmeric (haldi) Red

Purple cabbage extract Reddish pink Green

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Beetroot extract
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Stays red-pink Yellowish
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Red hibiscus (gudhal) Pink to red

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Bright red-pink Dull greenish

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Why so many flowers work: the red, pink and purple colours of petals and fruits
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come from a family of dyes called anthocyanins, and these dyes change form — and

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therefore colour — depending on whether the liquid around them is acidic or basic.
Litmus from lichens is only the most famous member of a very large family.

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Try This: squeeze jamun juice into two glasses. Add lemon juice to one, baking soda
solution to the other. The two glasses will not look like they started from the same
fruit.

Activity 2.3: Let us prepare — Page 12
Section 2.1.2 Red rose as an indicator

ACTIVITY

Q1 Take a fistful of the collected petals of red roses and wash them with water. Crush
the petals using a mortar and pestle. Place them in a glass tumbler. Pour some hot
water into the glass tumbler to ensure that the crushed flower petals are
completely immersed. Cover the glass tumbler with a lid. Wait for 5 – 10 minutes till
the water becomes coloured (Fig. 2.6), and filter it. Prepare the red rose extract.

Glass lid

Glass tumbler

Hot water

Crushed red rose petals

the water becomes deep pink-red

Fig. 2.6, page 12 — redrawn sketch. Crushed red rose petals immersed in hot water in a
covered glass tumbler; after 5–10 minutes the water is deeply coloured.

Follow the six steps and you get a clear pink-red liquid — the red rose extract, which is your
indicator.

1. Collect fallen petals of red roses. Do not pluck flowers; pick petals or flowers that have
dropped on the ground.
2. Take a fistful and wash them with water to remove dust.

Page 15 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

3. Crush them with a mortar and pestle so the coloured juice comes out of the cells.
4. Put the crushed petals in a glass tumbler and pour in enough hot water to cover them
completely. Do this under adult supervision.
5. Cover with a lid and wait 5 – 10 minutes, till the water is well coloured.
6. Filter. The filtrate — the liquid you get after filtration — is the flower extract.

Why each step matters: crushing breaks open the petal cells; hot water pulls the
dye out much faster than cold water; the lid keeps the steam and the smell in and
stops the extract cooling too fast; and filtering removes petal bits, because a colour
change is impossible to judge through a cloudy liquid.

Tip: keep the extract in a clean, covered bottle away from sunlight and use it within a
day or two. A stale extract loses its colour and gives confusing results.

Activity 2.4: Let us find out — Pages 12–13

Page 16 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Section 2.1.2 — Testing samples with the red rose extract

ACTIVITY

Q1 Add 20 – 30 drops of lemon juice in test tube A and 20 – 30 drops of soap solution in
test tube B with the help of droppers. Observe and record any colour changes (Fig.
2.8) to the extract in Table 2.3.

A B

Test tube stand

red rose extract + lemon juice
red rose extract + soap solution

Fig. 2.8, page 13 — redrawn sketch. The colour of the red rose extract after adding
lemon juice (A) and soap solution (B).

S.NO. NAME OF THE COLOUR OF THE RED ROSE NATURE OF THE
THE SAMPLE EXTRACT AFTER ADDING THE SUBSTANCE
SAMPLE

1. Lemon juice

2. Soap solution

3. Amla juice

4. …

Table 2.3: Testing the nature of samples with the red rose extract — page 13.
Reproduced as printed, blank for you to fill in.

Page 17 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

The two test tubes end up looking completely different, exactly as in Fig. 2.8.

Test tube A (lemon juice): the extract turns a clear red.
Test tube B (soap solution): the extract turns green.

Repeating with the other samples of Activity 2.1 fills in Table 2.3.

S.NO. NAME OF THE THE COLOUR OF THE RED ROSE NATURE OF THE
SAMPLE EXTRACT AFTER ADDING THE SAMPLE SUBSTANCE

1. Lemon juice Red Acidic

2. Soap solution Green Basic

3. Amla juice Red Acidic

4. Tamarind water Red Acidic

5. Vinegar Red Acidic

6. Baking soda Green Basic
solution

7. Lime water Green Basic

8. Tap water No change Neutral

9. Washing powder Green Basic
solution

10. Sugar solution No change Neutral

11. Salt solution No change Neutral

Tip: use the same number of drops of extract in every tube. If one tube has more
extract than another, the shades cannot be compared fairly.

Page 18 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

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Are the samples that change the colour of the flower extract to a shade of red the
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same as those that changed the colour of blue litmus paper to red? (Group A, Table
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Lemon juice
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the entries are the ones recorded in Activity 2.1.

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q3 Are the samples that change the colour of the flower extract to a shade of green
the same as those that changed the colour of red litmus paper to blue? (Group B,
Table 2.2)

GROUP A GROUP B GROUP C
(TURN BLUE LITMUS (TURN RED LITMUS (AFFECT NEITHER
RED) BLUE) PAPER)

Lemon juice Soap solution Tap water

Amla juice Baking soda solution Sugar solution

Tamarind water Lime water Salt solution

Vinegar Washing powder solution

Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Column headings as printed;
the entries are the ones recorded in Activity 2.1.

Yes — soap solution, baking soda solution, lime water and washing powder solution, that is, all
of Group B.

Why it happens: these four contain bases. Red litmus answers "base" by turning
blue; the rose extract answers "base" by turning green. The colours differ but the
message is identical.

Page 20 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q4 Are the samples that do not change the colour of the flower extract the same as
those that did not change the colour of red and blue litmus papers? (Group C, Table
2.2)

GROUP A GROUP B GROUP C
(TURN BLUE LITMUS (TURN RED LITMUS (AFFECT NEITHER
RED) BLUE) PAPER)

Lemon juice Soap solution Tap water

Amla juice Baking soda solution Sugar solution

Tamarind water Lime water Salt solution

Vinegar Washing powder solution

Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Column headings as printed;
the entries are the ones recorded in Activity 2.1.

Yes — tap water, sugar solution and salt solution, that is, all of Group C.

Why it happens: these are neutral. Having neither acid nor base, they give an
indicator nothing to respond to, so the rose extract keeps its original pink-red shade
just as the litmus papers kept their original colours.

What the three answers together prove: an indicator is not magic tied to one
particular dye. Any acid-base indicator sorts the same eleven samples into the same
three groups — which is why the red rose extract is a genuine acid-base indicator.

Page 21 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q5 Can you now fill in the nature of the substances in Table 2.3?

S.NO. NAME OF THE COLOUR OF THE RED ROSE NATURE OF THE
THE SAMPLE EXTRACT AFTER ADDING THE SUBSTANCE
SAMPLE

1. Lemon juice Turns red

2. Soap solution Turns green

3. Amla juice Turns red

4. …

Table 2.3: Testing the nature of samples with the red rose extract — page 13. The colours
are the ones observed in Activity 2.4; the last column is the one you have to fill in.

Yes — read the colour and write the nature straight off it.

Red rose extract turns red → the substance is acidic

Red rose extract turns green → the substance is basic

Red rose extract shows no change → the substance is neutral

Applying the rule: lemon juice, amla juice, tamarind water and vinegar are acidic; soap solution,
baking soda solution, lime water and washing powder solution are basic; tap water, sugar
solution and salt solution are neutral. The completed column is shown in the table of Q1 above.

Why the red rose extract is better than turmeric here: it gives three
distinguishable results, so it can separate all three natures. Turmeric paper, as you
will see in Activity 2.5, gives only two and cannot tell an acid from a neutral
substance.

In-text Questions — Page 14

Page 22 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Fascinating Facts — hydrangea, and the idea for turmeric paper

Q1 Hydrangea is a plant that grows in cooler climates in the Himalayan region and the
North-eastern states. It gives flowers of different colours, depending on the nature
of the soil. Acidic soil produces blue-coloured flowers, whereas in basic soil, the
flowers are pink or red. Can gardeners alter the colour of hydrangea flowers by
adjusting the acidic or basic nature of the soil?

Yes, they can — and gardeners actually do this.

To get blue flowers, the gardener makes the soil more acidic — for example by adding
organic matter such as composted leaves or pine needles.
To get pink or red flowers, the gardener makes the soil more basic — for example by
mixing in lime (chuna).

Why it happens: the hydrangea flower contains a dye that behaves like an indicator,
and the nature of the soil decides which form of the dye the plant carries into its
petals. So the bush is, in effect, a living litmus paper rooted in the ground — the
colour you see is a reading of the soil.

Did you know? The change is not instant. The plant has to take up the new soil
condition through its roots, so a gardener starts treating the soil months before the
flowering season.

Q2 We have used blue and red litmus paper strips in Activity 2.1. Can you also make
paper strips with some other natural indicators?

Yes. The recipe is always the same: get a strongly coloured plant extract, soak plain paper in it,
dry it, and cut it into strips.

Coloured plant extract or paste

→ dip filter paper in it

→ dry it in the shade

→ cut into thin strips

→ your own indicator paper

Page 23 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Turmeric is the easiest — that is exactly what Activity 2.5 does. Purple cabbage extract, beetroot
co m
extract and red hibiscus extract all give usable strips too.
e m.
m l as
.co a g
s em
Why paper strips are handy: a strip needs only one drop of the sample, cannot
a
gl dries quickly and can be carried in a pocket to test soil or pond water on the
aspill,
spot. A bottle of extract cannot do any of that.

co m
m . ag
l a se
agPages 14–15
Activity 2.5: Let us prepare —

m
Section 2.1.3 Turmeric as an indicator

co
em.
as
ACTIVITY

m l
.co a g
a s em
gl to make a paste (Fig. 2.9a). Carefully dip a piece of filter paper in the turmeric paste
Q1 Take a spoonful of turmeric (haldi) in a petri dish or container and add a little water

a
until it gets yellow colour. Take it out and allow it to dry. Cut this yellow paper into

m a s
.co agl
thin strips, which are used as ‘turmeric paper’ (Fig. 2.9b). Prepare the turmeric
paper.
se m
g l a
a
Petri dish
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
(a) Turmeric paste (b) Turmeric paper strips

c o m
.
Fig. 2.9, page 14 — redrawn sketch. (a) Turmeric paste made in a petri dish; (b) the dried

s e m paper’.
yellow filter paper cut into thin strips — the ‘turmeric
m a
m . co agl
l a se
ag c
.

s e m
m a
Four steps and you have your own indicator paper.

e m . co
1. Put a spoonful of turmeric in a petri dish and add a little water — just enough for a thick agl
g l as
paste. Fresh turmeric, ground, works equally well.
a
2. Dip a piece of filter paper in the paste until it is evenly yellow.

co m
m .
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.co


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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

3. Take it out and let it dry completely.
4. Cut the dry yellow paper into thin strips — this is turmeric paper. Do this under adult
supervision.

Why the paste must be thick and the paper fully dry: a watery paste leaves a
pale, patchy yellow that makes a colour change hard to spot, and a damp strip
spreads the sample drop all over instead of holding it in one place. An evenly yellow,
bone-dry strip gives a sharp red mark the moment a base touches it.

Tip: if filter paper is not available, plain white blotting paper or a piece of an
unglazed white sheet will do.

Q2 What did you observe? Do all samples change the colour of the turmeric paper?

No. Only some samples change it — and every one of them turns the yellow paper red.

S.NO. NAME OF THE THE COLOUR OF THE TURMERIC PAPER AFTER PUTTING A
SAMPLE DROP OF THE SAMPLE

1. Lemon juice No change (stays yellow)

2. Soap solution Turns red

3. Amla juice No change (stays yellow)

4. Tamarind water No change (stays yellow)

5. Vinegar No change (stays yellow)

6. Baking soda Turns red
solution

7. Lime water Turns red

8. Tap water No change

9. Washing powder Turns red
solution

10. Sugar solution No change

11. Salt solution No change

Page 25 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why it happens: the yellow colour of turmeric changes only in a basic medium.
Acids and neutral liquids leave it exactly as it was, so only the four Group B samples
show a change.

Q3 Group the samples which do not change the colour of the turmeric paper. Compare
them with the samples in Group A, Group B, and Group C in Table 2.2.

GROUP A GROUP B GROUP C
(TURN BLUE LITMUS (TURN RED LITMUS (AFFECT NEITHER
RED) BLUE) PAPER)

Lemon juice Soap solution Tap water

Amla juice Baking soda solution Sugar solution

Tamarind water Lime water Salt solution

Vinegar Washing powder solution

Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Column headings as printed;
the entries are the ones recorded in Activity 2.1.

The samples that leave the turmeric paper yellow are: lemon juice, amla juice, tamarind
water, vinegar, tap water, sugar solution and salt solution.
Compared with Table 2.2, that is all of Group A plus all of Group C — acidic and neutral
substances lumped together. Only Group B (the bases) is separated out.

TABLE 2.2 GROUP NATURE TURMERIC PAPER

Group A Acidic No change — stays yellow

Group B Basic Turns red

Group C Neutral No change — stays yellow

Page 26 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why this matters: turmeric gives only two possible readings — red or unchanged
— but there are three natures to identify. Two answers cannot label three things, so
Groups A and C have to share the same reading.

Q4 Can turmeric paper be used as an indicator for acidic substances? Discuss your
observations with your classmates.

No. Turmeric paper can be used to test basic substances only. It cannot differentiate between
acidic and neutral substances, because both leave it yellow.

Turmeric paper turns red → the substance is basic

Turmeric paper stays yellow → the substance is acidic or neutral — you cannot say which

What to do instead: if a sample leaves turmeric paper yellow, follow up with blue
litmus. If the blue litmus turns red the sample is acidic; if it too shows no change,
the sample is neutral. Two indicators used one after the other settle all three cases.

Did you know? This is why a curry stain on a white shirt turns red when soap is
applied — the soap is basic and the turmeric in the curry is acting as an indicator on
your shirt. Rinse with plenty of water and the red fades back to yellow.

In-text Questions — Pages 15–16
Fascinating Facts, the Guru Purnima greeting card and olfactory indicators

Q1 Why is turmeric known as a ‘Golden’ spice?

Because of its bright golden-yellow colour and the value it has long been given in Indian
homes and in Ayurveda.

Turmeric is a member of the ginger family, grown in India and other countries.
Beyond the taste and colour it gives to food, it is being researched for other benefits.
In the Ayurvedic system of medicine it is considered to have numerous health benefits and
is used in many traditional home remedies.

Page 27 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

And for this chapter: the very pigment that makes turmeric golden is what makes it
an indicator. Golden-yellow in acid and neutral liquids, red in a base — the colour is
both its identity and its scientific use.

Page 28 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

co m
m.
Ashwin created a greeting card to pay gratitude to his teacher on the day of Guru
se
Q2

o m l a
Purnima. He applied turmeric paste on white paper and dried it. He wrote his

m .cin the teacher's mother tongue (Odia language) on theagdried paper using
wishes

l a se of the solutions tested in Table 2.4. Which solution can be used to write the
one
a g message?

co m
m .
e COLOUR OF THE TURMERIC PAPER AFTER ag
S.NO. NAME OF THE
l a sTHE
SAMPLE
ag PUTTING A DROP OF THE SAMPLE
1. Lemon juice
co m
e m.
com l as
2. Soap solution

. 3. a g
e m
as
Amla juice

agl 4. Tamarind water

m a s
.co agl
5. Vinegar

se m
g l a
a
6. Baking soda
solution

co m
.
7. Lime water

e m
m l as
.co g
8. Tap water

m a
l a se 9. Washing powder

ag solution

se m
10. Sugar solution
com g l a
m . a
gl ase
11. Salt solution

a
Table 2.4: Testing the nature of samples with turmeric paper — page 15. Reproduced as
printed, with the observation column left blank; the eleven samples are the ones used
co m
in Activity 2.1.
m .
m ase
.co a g l
a s em
agl ANSWER
.c
Any basic solution from Table 2.4 — soap solution is the easiest, and baking soda solution, lime
s e m
m a
co agl
water or washing powder solution would work just as well.

m .
as e
a g l

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.co


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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Yellow turmeric paper + basic solution

→ the wet letters turn red

→ a red message on a golden background

Why only a base works: turmeric changes colour in a basic medium alone. Writing
with lemon juice or vinegar would leave no mark at all, since acids do not affect
turmeric — and writing with plain water or sugar solution would leave nothing
either.

Try This: make one yourself for a birthday. Brush turmeric paste on a sheet, dry it,
then write with a cotton bud dipped in soap solution. The message appears in red as
you write, like invisible ink in reverse.

Q3 Are there any substances whose odours change on adding acidic or basic
substances?

Yes. Substances whose odour changes in an acidic or basic medium are called olfactory
indicators.

Onion — the sharp onion smell on a cloth strip is strongly reduced by a basic solution such
as baking soda solution, while an acidic solution such as tamarind water leaves it much as it
was.
Vanilla essence and clove oil behave the same way — their characteristic smell disappears
in a base.

Why they are useful: a colour indicator is no help to a person who cannot see the
colour, or in a dark room. An olfactory indicator gives the answer through the nose
instead of the eye — the very idea used in Question 5 of the exercise, where Manya
is blindfolded.

Activity 2.6: Let us investigate — Page 16

Page 30 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Section 2.1.3 — Onion as an olfactory indicator

ACTIVITY

Q1 Take some finely chopped onions in a container, along with some strips of clean
cotton cloth or filter paper. Tightly close the container and leave it overnight. Take
two of the cotton cloth or filter paper strips from the container and check their
odour.

After a night in the closed container the strips smell strongly of onion, even though no onion is
touching them any more.

Why it happens: the substance responsible for the onion smell escapes into the air
of the closed container and settles into the cloth or paper. Keeping the container
tightly closed is what makes this work — in an open dish the smell would simply
escape into the room and the strips would pick up almost nothing.

Tip: check the odour by wafting the strip towards your nose with your hand rather
than pushing it right against your nostrils — the standard laboratory way to smell
anything.

Q2 Keep them on a clean surface and put a few drops of tamarind water on one strip
and a few drops of baking soda solution on the other. Allow the drops to spread on
the strips. Check the odour again. Do you notice any change in the odour of the
onion strips before and after putting tamarind water and baking soda solution on
them? Note your observations.

Yes, there is a clear difference between the two strips.

STRIP SUBSTANCE ADDED NATURE ODOUR AFTER ADDING

Strip 1 Tamarind water Acidic Onion smell still clearly there

Strip 2 Baking soda solution Basic Onion smell almost gone

Page 31 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why it happens: the smelly substance in onion reacts with a base and is changed
into something that has no smell. An acid does not do this, so the acidic strip keeps
its onion odour. The nose is therefore giving you the same acid/base information
that litmus gives your eyes — which is why onion is an olfactory indicator.

Q3 Similarly, test the change in the odour with other acidic and basic substances and
record your observations.

Repeat with more samples from Activity 2.1 and the pattern holds. Here is a record you can
compare yours with.

SUBSTANCE PUT ON THE ONION STRIP NATURE CHANGE IN ODOUR

Lemon juice Acidic Onion smell remains

Vinegar Acidic Onion smell remains

Soap solution Basic Onion smell greatly reduced

Lime water Basic Onion smell greatly reduced

Tap water Neutral No noticeable change

The conclusion: the onion strip separates basic substances from the rest by smell
alone. Like turmeric paper, it cannot tell an acid from a neutral substance — but
unlike turmeric paper, it needs no light and no eyesight.

Try This: repeat the whole activity with a few drops of vanilla essence on the strips
instead of onion. The result is the same and the laboratory smells much better.

Activity 2.7: Let us experiment — Page 17

Page 32 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Section 2.2 What Happens When Acidic Substances Mix with Basic Substances?

ACTIVITY

Q1 Take one drop of lemon juice in a test tube and add around twenty drops of water
to it. Observe the colour. Add a drop of blue litmus solution to it. Do you observe
any colour change (Fig. 2.10a)?

one drop of lemon juice
+ about twenty drops of water
+ one drop of blue litmus solution

Test tube stand

Fig. 2.10(a), page 17 — redrawn sketch. The colour of the solution after adding blue
litmus solution to the diluted lemon juice.

The diluted lemon juice by itself is colourless. The moment a drop of blue litmus solution goes
in, the whole solution turns red (Fig. 2.10a).

Why it happens: lemon juice contains citric acid. Even diluted about twenty times,
the solution is still acidic, and blue litmus in an acid becomes red.

Tip: why dilute at all? Undiluted lemon juice is cloudy and pale yellow, and the colour
of the litmus would be hard to judge against it. Twenty drops of water make the tube
almost clear so that the red is unmistakable.

Page 33 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

co m
m.
Slowly add drops of lime water to this test tube with the help of a dropper and swirl
e
Q2

m l as
.co
it well. What do you observe? Is there any change in the colour of the solution?

a g
se m
g l a
a

Nothing seems to happen for the first several drops — the solution stays red. Then, quite

m
suddenly, a stage comes when the colour changes from red to blue (Fig. 2.10b).
. co ag
e m
g l as
a
Acidic solution (red)

+ lime water, drop by drop

co m
m.
→ acid being used up …

m as e
.co
→ at one particular drop the solution stops being acidic
a g l
s m flips from red to blue
ecolour
gl a
→
a
om a s
c agl
Why it happens: lime water is a base. Each drop cancels a little of the citric acid.
.
m Once the last of the acid is used up and a
s e
While any acid is left, litmus stays red.
a the litmus switches to blue. Lime water has
agl
slight excess of base is present,
neutralised the effect of the acid.

. com
m a s em and you will see
Tip: swirl after every drop. Without swirling, the base sits at the top
. co patch that vanishes when it mixes, which is confusing.
a gl
sem
a blue

a
agl
se m
com
Again, add one drop of lemon juice to the above solution. Can you predict why there
g l a
.
Q3

m a
ase
is a change in colour?

ANSWER agl
The solution goes back to red.
co m
m .
m as e
.co a g l
m Blue (basic) → add acid → Red (acidic)
Red (acidic) → add base → Blue (basic)

l a se
ag
.c
s e m
m a
co agl
The reason for every colour change here is the same: the litmus is simply reporting whether

m .
e
acid or base is in excess at that moment.

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a

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.co


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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why it happens: when an acid solution and a base solution are mixed in sufficient
quantity, the result is neither acidic nor basic. Such reactions are called
neutralisation reactions, and in them salt and water are formed with the evolution
(release) of heat.

Acid + Base → Salt + Water + Heat

Check it yourself: hold the test tube in your palm while you add the lime water. You
can often feel it turn slightly warm — that is the heat the equation promises.

In-text Questions — Pages 18–19
Section 2.3 Neutralisation in Daily Life, and Let us wrap up!

Q1 Situation 1: Keerthi was observing a butterfly in the garden with her hand resting
on a tree trunk. Suddenly, a red ant bit her, leaving her skin red with stinging pain
(Fig. 2.11). Her brother helped her by applying moist baking soda to the affected
area, which relieved the pain. What do you think might be the reason for this?

Tree trunk with red ants on it

Keerthi's hand resting on the trunk

Butterfly

Red ant (magnified)

Flowering garden

Fig. 2.11, page 18 — redrawn sketch. Keerthi stands in a flowering garden watching a
butterfly, her hand resting on a tree trunk on which red ants are crawling; the inset
shows the red ant magnified.

The ant bite injects an acid, and the baking soda is a base that neutralises it.

Page 35 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Ant bite → injects formic acid into the skin → burning, stinging pain

Moist baking soda (a base) rubbed on

→ neutralisation → acid destroyed → pain relieved

Why it must be moist baking soda: dry powder just sits on the skin. Water lets the
baking soda spread into the bitten spot and actually reach the acid. This is the same
neutralisation reaction as Activity 2.7 — only here the acid is under your skin instead
of in a test tube.

Tip: wash the spot with plenty of water first. Do not scratch it — scratching pushes
the acid deeper and can cause an infection.

Q2 What remedies do people use to treat ant bites in your region?

Most traditional remedies turn out to be either a base that neutralises the acid or something
that soothes the swelling.

COMMON REMEDY WHAT IT IS HOW IT HELPS

Moist baking soda Basic Neutralises the formic acid
paste

Lime (chuna) paste Basic Neutralises the acid; used in many villages

Toothpaste Mildly basic Neutralises and cools

Calamine lotion Contains zinc carbonate, a Neutralises and soothes
base

Ice or cold water Neutral Does not neutralise, but numbs the pain and reduces
swelling

Sample answer: "In my area people rub moist baking soda or a little chuna on an ant bite, and
elders sometimes press a crushed tulsi leaf on it."

Page 36 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why acids are not used: lemon juice or vinegar on an ant bite would add more acid
to the acid already there and make the sting worse. The remedy must be the
opposite of the problem.

Q3 Situation 2: On the Farmer’s Portal (an online platform from the Department of
Agriculture, Cooperation and Farmers Welfare), a query from a farmer states, “My
plants are not growing well lately”. After a detailed discussion, it was found that
the excessive use of chemical fertilisers (substances added to soil to help plants
grow better) made the soil acidic. What remedy might be provided to him?

Treat the field with lime, which is a base (Fig. 2.12). The lime neutralises the excess acid and
brings the soil back towards neutral, after which the plants grow well again.

Excess chemical fertilisers → soil becomes acidic → poor plant growth

Add lime (a base) → neutralisation → soil fit for plants again

The chapter also gives the opposite case: if the soil is basic, organic matter such as manure
and composted leaves is added — organic matter releases acids that neutralise the basic
nature of the soil.

Why acidic soil hurts plants: roots can only take up nutrients from the soil water
within a certain range of conditions. Too much acid locks the nutrients up, so the
plant starves even though the food is right there in the soil.

Did you know? The chapter adds a third possibility — the soil may be perfectly
neutral and the plants still unhealthy. That points to a deficiency of nutrients, not
to acid or base at all. Always test before you treat.

Q4 Situation 3: Ashwin’s friend Gurbir stays near an industrial area. He shared with him
that the fish population in his neighbourhood lake was declining day by day! What
do you think might be the causes for this?

The most likely cause is factory waste being released into the lake.

Page 37 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

If the waste is acidic or basic, it changes the nature of the lake water. Fish can survive only in
water that is close to neutral.
The waste may also make the water dirty and cut down the oxygen dissolved in it.
Other everyday causes worth mentioning: sewage from houses, and fertiliser washed off
nearby fields into the lake.

Why fish are the first to suffer: a fish lives with the water constantly passing over
its gills. Anything harmful in the lake reaches its blood within minutes — it cannot
walk away from a polluted patch as a land animal can.

Q5 If the factory waste is acidic in nature, what could be done to save the fish in the
lake?

Neutralise the waste before it is released. Basic substances are added to the acidic factory
waste in a treatment tank, and only after it has become neutral is it allowed into the lake.

Acidic factory waste

+ basic substance (for example lime)

→ neutralisation → waste becomes neutral

→ only then released into the lake

Why treat it at the factory and not in the lake: the waste is concentrated and
small in volume at the outlet pipe, so a manageable amount of base can fix it. Once
it has spread through a whole lake, no practical quantity of base could reach every
part of it — and the fish would already be dead. Treating pollution at its source is
always cheaper and kinder than cleaning it up afterwards.

Q6 Let us wrap up! Now, can you explain why the words ‘Welcome to the Wonderful
World of Science’ appeared on Ashwin and Keerthi’s paper sheets when the liquid
was sprayed on them? Do you think that one possibility could be using a turmeric
solution for the spraying liquid and a soap solution for writing on the paper?

Yes — that is exactly how the trick works, and it is nothing but the turmeric indicator used
cleverly.

Page 38 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

1. The message was written on the white sheet beforehand with a soap solution (basic) and
co m
allowed to dry. Dry soap solution on white paper is practically invisible.
se m.
o m g l a
.c
2. At the gate the volunteer sprayed a turmeric solution on the sheet. The whole sheet turned
m a
se the turmeric met the dried base, it turned red. So the letters appeared in red on a
pale yellow.
l a
agyellow background.
3. Wherever

co m
em
. ag
s
Invisible writing = soap solution (base), dried
a
agl
Spray = turmeric solution (indicator)

m
Turmeric + base → red
co
em.
as
→ the message appears
m l
.co a g
a s emthe writing is invisible before spraying: a colourless liquid dried on white
a gl Why
paper reflects light almost exactly like the paper itself. Nothing coloured is on the

m a s
.co agl
sheet until the indicator arrives and creates the colour on the spot.

a s em
agl — write with turmeric solution and spray with soap
Try This: swap the two around
solution. The message still appears in red, because it is the meeting of base and
turmeric that makes the colour, not the order in which they arrive.
co m
m .
m as e
.co a g l
em
las Us Enhance Our Learning — Pages 20–21
agEnd-of-chapter questions
Let

se m
com g l a
. a
LET US ENHANCE OUR LEARNING

m
ase
Q1 agl
A solution turns the red litmus paper to blue. Excess addition of which of the
following solution would reverse the change? (i) Lime water (ii) Baking soda (iii)

co m
.
Vinegar (iv) Common salt solution

em
m l as
.co a g
em(iii) Vinegar

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

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m .
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.co


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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Red litmus → blue means the solution is basic

To reverse it you must destroy the base → add an acid

Among the four choices, only vinegar (acetic acid) is acidic
Answer: (iii) Vinegar

Why the others fail: lime water and baking soda are themselves basic — adding
them makes the solution even more basic, so the litmus stays blue. Common salt
solution is neutral; it only dilutes the base and never flips the colour back.

Tip: the word "excess" matters. A little vinegar only neutralises part of the base; you
need enough for an acid excess before the litmus turns red again.

Q2 You are provided with three unknown solutions labelled A, B, and C, but you do not
know which of these are acidic, basic, or neutral. Upon adding a few drops of red
litmus solution to solution A, it turns blue. When a few drops of turmeric solution
are added to solution B, it turns red. Finally, after adding a few drops of red rose
extract to solution C, it turns green. Based on the observations, which of the
following is the correct sequence for the nature of solutions A, B, and C? (i) Acidic,
acidic, and acidic (ii) Neutral, basic, and basic (iii) Basic, basic, and acidic (iv) Basic,
basic, and basic

(iv) Basic, basic, and basic

SOLUTION INDICATOR USED OBSERVATION WHAT IT MEANS

A Red litmus solution Turns blue Basic

B Turmeric solution Turns red Basic

C Red rose extract Turns green Basic

Why all three are basic: each indicator has its own way of announcing a base — red
litmus goes blue, turmeric goes red, red rose extract goes green. Three different
colour changes, one and the same conclusion. The trick of the question is that the
three colours look unrelated until you remember what each indicator does.

Page 40 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q3 Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips
are used. Label the nature of solutions present in each of the containers.

Paper strip soaked in red rose extract Container with the solution

Fig. 2.13 Fig. 2.14 Fig. 2.15

Figs. 2.13, 2.14 and 2.15, page 20 — redrawn sketch. A paper strip soaked in red rose
extract is dipped into each container; only the dipped end of the strip is shown after the
dip.

Read each strip against the two rules for red rose extract — red in acid, green in base,
unchanged in neutral.

Fig. 2.13 — turns green Fig. 2.14 — no change Fig. 2.15 — turns deep red
BASIC NEUTRAL ACIDIC

The dipped end of each red rose extract strip. Only the part inside the liquid changes colour; the rest
keeps the original pink of the extract.

Page 41 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

FIGURE COLOUR OF THE DIPPED NATURE OF THE SOLUTION IN THE
END CONTAINER

Fig. 2.13 Green Basic

Fig. 2.14 No change — still pink-red Neutral

Fig. 2.15 Deep red Acidic

How to be sure about Fig. 2.14: compare the dipped end with the part of the strip
still held between the fingers. In Fig. 2.14 both are the same pink — nothing has
happened, so the solution is neutral. In Fig. 2.15 the dipped end is clearly darker
and redder than the held end, which is the acid at work.

Q4 A liquid sample from the laboratory was tested using various indicators: Red litmus
— No change; Blue litmus — Turned red; Turmeric — No change in colour. Based on
the tests, identify the acidic or basic nature of the liquid and justify your answer.

The liquid is acidic.

INDICATOR CHANGE OBSERVED WHAT IT RULES IN OR OUT

Red litmus No change Not basic — a base would have turned it blue

Blue litmus Turned red Definitely acidic

Turmeric No change in colour Confirms it is not basic

Justification in one line: only an acid turns blue litmus red, and the two "no
change" results independently confirm that no base is present. Together the three
tests point to a single answer — the liquid is acidic. It could be something like lemon
juice, vinegar or tamarind water.

Tip: notice how the blue litmus does the identifying while the other two do the
confirming. A good scientist looks for a test that could have proved her wrong —
here, the turmeric test.

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q5 Manya is blindfolded. She is given two unknown solutions to test and determine
whether they are acidic or basic. Which indicator should Manya use to test the
solutions and why?

Manya should use an olfactory indicator — for example onion strips, vanilla essence or clove
oil.

Why: being blindfolded, Manya cannot see any colour, so litmus paper, turmeric
paper and red rose extract are of no use to her at all. An olfactory indicator gives its
answer as a change in smell, which she can detect with her nose. If the sharp onion
smell almost disappears the solution is basic; if the smell stays, it is not basic.

Did you know? This is not merely a puzzle. Olfactory indicators are genuinely used
by students who are visually impaired, so that every child can do the same
experiment and reach the same conclusion.

Q6 Could you suggest various materials which can be used for writing the message on
the white sheet of paper (given at the beginning of the chapter) and what could be
in the spray bottle? Make a table of various possible combinations and the colour of
the writing obtained.

The rule is simple: one of the two must be the indicator and the other must be the acid or
the base. Here are combinations that work.

Page 43 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

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WRITTEN ON THE PAPER SPRAYED WITH COLOUR OF THE WRITING

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WITH OBTAINED

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Soap solution (base) Turmeric solution Red on a yellow sheet

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Turmeric solution Lime water (base) Red on a pale yellow sheet

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Lemon juice (acid) Red rose extract Red writing

Soap solution (base)
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Red rose extract Lemon juice (acid) Red writing

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Vinegar (acid) Purple cabbage
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extract

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Why the writing liquid must dry colourless: soap solution, lemon juice, vinegar
and baking soda solution all leave a nearly invisible mark on white paper. The colour
is created only when the sprayed indicator meets them — that is what makes the
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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Grape juice + red rose extract → red → grape juice is acidic

Add baking soda (a base), drop by drop

→ the acid is neutralised

→ with a little excess base, the extract turns green

Justification: red rose extract gives red in acid and green in base. The red tint
proves that grape juice contains acid (mainly tartaric acid). Baking soda is a base, so
it first neutralises that acid; once enough has been added, the mixture becomes
basic and the same extract now reports green.

Check it yourself: add the baking soda a pinch at a time. You may briefly see a dull,
in-between shade at the moment the mixture is almost neutral, before the clear
green appears.

Q8 Keerthi wrote a secret message to her grandmother on her birthday using orange
juice. Can you assist her grandmother in revealing the message? Which indicator
would you use to make it visible?

Yes. Orange juice is acidic (citric acid), so the grandmother needs an indicator that shows a clear
colour in acid. Red rose extract is the best choice; blue litmus solution or purple cabbage
extract works too.

1. Let the orange-juice writing dry fully — it becomes almost invisible on white paper.
2. Spray or gently brush red rose extract over the whole sheet.
3. The letters appear in red, because the acid of the orange juice turns the extract red wherever
it was written.

Why not turmeric here: turmeric changes colour only in a base. Orange juice is an
acid, so a turmeric spray would leave the sheet uniformly yellow and the message
would stay hidden. Match the indicator to the nature of the writing liquid.

Tip: with blue litmus solution the message appears in red on a blue sheet — even
easier to read from a distance.

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q9 How can natural indicators be prepared? Explain by giving an example.

A natural indicator is prepared by taking the coloured juice out of a plant part and using it,
or paper soaked in it, as the indicator. There are two standard methods.
Method 1 — an extract (example: red rose extract)

1. Collect fallen petals of red roses and wash them with water.
2. Crush a fistful of petals with a mortar and pestle.
3. Put them in a glass tumbler and pour in hot water until they are completely immersed. Do
this under adult supervision.
4. Cover with a lid and wait 5 – 10 minutes till the water becomes coloured.
5. Filter. The filtrate is the red rose extract — red in acid, green in base.

Method 2 — an indicator paper (example: turmeric paper)

1. Make a paste of a spoonful of turmeric with a little water.
2. Dip filter paper in the paste until it is evenly yellow.
3. Dry it, then cut it into thin strips — turns red in a base.

Why the same recipe works for many plants: the coloured substance sits inside
the cells of the petal, root or rhizome. Crushing breaks the cells open and hot water
dissolves the colour out. Beetroot, purple cabbage, jamun and red hibiscus can all
be turned into indicators exactly this way.

Q10 Three liquids are given to you. One is vinegar, another is a baking soda solution,
and the third is a sugar solution. Can you identify them only using turmeric paper?
Explain.

Only partly — turmeric paper can pick out the baking soda solution, but it cannot
separate the vinegar from the sugar solution.

LIQUID NATURE TURMERIC PAPER IDENTIFIED?

Baking soda solution Basic Turns red Yes — identified

Vinegar Acidic No change No — cannot be told apart

Sugar solution Neutral No change No — cannot be told apart

Page 46 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Why: turmeric paper gives only two possible readings — red or unchanged — and it
changes only in a base. Since vinegar and sugar solution both leave it yellow, one
single test cannot distinguish three liquids.

How to finish the job: take the two that left the turmeric yellow and touch each with blue
litmus paper.

Blue litmus → red = vinegar (acidic)

Blue litmus → no change = sugar solution (neutral)

Tip: you can also tell vinegar apart by its sharp smell — but litmus is the reliable
scientific test.

Q11 The extract of red rose turns the liquid X to green. What will the nature of liquid X
be? What will happen when excess of amla juice is added to liquid X?

Liquid X is basic. When excess amla juice is added, the green will change to red.

Red rose extract in X → green → X is basic

Amla juice is acidic (ascorbic acid and citric acid)

Acid added to base → neutralisation

With an excess of amla juice the mixture becomes acidic

→ the extract turns red

Why the word "excess" is the key: if you added only a little amla juice, the mixture
would still be basic and would stay green. Only when enough acid has been added
to use up all the base — and then some more — does the extract flip to red. Exactly
the reversal you saw in Activity 2.7 with lemon juice and lime water.

Page 47 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Q12 Observe and analyse the information given in the following flowchart. Complete
the missing information. — Imagine a garden with plants showing signs of poor
health. → The soil can be ______ in nature. / The soil can be ______ in nature. → Which
indicator can be used to test the nature of the soil? _________ → The acidic soil can be
treated with ____________. / The basic soil can be treated with ____________.

Imagine a garden with plants
showing signs of poor health.

The soil can be ______ in nature. The soil can be ______ in nature.

Which indicator can be used
to test the nature of the soil?
______________

The acidic soil can be treated The basic soil can be treated
with ______________. with ______________.

The flowchart printed with Question 12 on page 21, with its blanks.

Here is the completed flowchart.

Imagine a garden with plants
showing signs of poor health.

The soil can be acidic in nature. The soil can be basic in nature.

Which indicator can be used
to test the nature of the soil?
Litmus paper (red and blue)

The acidic soil can be treated The basic soil can be treated
with lime (chuna). with manure / compost.

Page 48 of 54

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

The completed flowchart of Question 12 — poor plant health, the two possible soil natures, the
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indicator used to find out, and the treatment for each.

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Dive Deeper — Page 22
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Thinking beyond the chapter
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DIVE DEEPER

Q1 Aman accidentally spilt vinegar on some pieces of an eggshell or marble and
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marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with
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Because eggshell and marble are made of the same substance — calcium carbonate — and

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Eggshell / marble = calcium carbonate

Vinegar = an acid (acetic acid)

Acid + carbonate → salt + water + carbon dioxide gas
→ the gas escapes as the bubbles Aman saw

Soap solution is basic. A base does not react with calcium carbonate in this way, so no gas is
produced and no bubbles appear.

Why it happens: the bubbling is not the acid "dissolving" the shell in a general way
— it is a specific reaction that releases carbon dioxide. The bubbles are therefore a
quick field test: drop vinegar or lemon juice on a stone, and if it fizzes, the stone
contains carbonate.

Did you know? This is exactly why marble monuments are damaged by acid rain,
and why the marble floors of a temple should never be cleaned with lemon or
vinegar. It is also why chalk (also calcium carbonate) fizzes in dilute acid in the
laboratory.

Exploratory Projects — Page 22
Projects and further exploration

EXPLORATORY PROJECTS

Q1 Create rangoli using acidic or basic substances and natural indicators.

The idea is to let the colour appear by itself as you spray, instead of using coloured powders.

1. Make the base. Spread a thin layer of turmeric powder, or brush turmeric paste in a design
on a large sheet of white paper, and let it dry. You now have a golden-yellow rangoli.
2. Draw with a base. Using a cotton bud or a squeeze bottle, draw lines, petals and dots with
soap solution, baking soda solution or lime water. Every stroke turns red the moment it
touches the turmeric.
3. Add a second colour. On another part of the sheet spread a layer soaked in red rose
extract or purple cabbage extract, then draw with lemon juice for red and with soap
solution for green.

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Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

4. Show the science. Write a small card beside the rangoli listing which substance made which
colour, and why.

BASE LAYER DRAWN WITH COLOUR THAT APPEARS

Turmeric Soap or baking soda solution Red

Purple cabbage extract Lemon juice Reddish pink

Purple cabbage extract Washing soda solution Green

Red rose extract Lime water Green

Tip: work on newspaper, keep the liquids in labelled droppers, and wash your hands
afterwards. Do this rangoli on paper, not on a floor you will have to scrub.

Q2 You may discuss in your class the acidic, basic, or neutral nature of water obtained
from various sources. You may like to test the water samples available from sources
such as rain, taps, rivers, etc.

What a good project must contain: clean labelled bottles, the same test done the same way
on every sample, a table of results, and one honest conclusion.

1. Collect samples in clean bottles and label each with the source, date and place — rainwater,
tap water, river or pond water, hand-pump water, boiled water, bottled water.
2. Test each with blue litmus and then with red litmus, using a fresh strip every time.
3. Record the results in a table and discuss them with your class.

SOURCE OF WATER BLUE LITMUS RED LITMUS NATURE

Tap water No change No change Neutral

Rainwater (collected in the open) Slightly red No change Slightly acidic

Pond water No change No change Neutral

River water near a drain No change Slightly blue Slightly basic

Why rainwater is often slightly acidic: as it falls, rain dissolves carbon dioxide and
other gases from the air. Near factories and heavy traffic there are more such gases,
so the rain is more acidic — the phenomenon called acid rain.

Page 51 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Check it yourself: collect the first rain of a shower and the rain from half an hour
later in two separate bottles. The first sample is usually the more acidic of the two,
because it has washed the dust and gases out of the air.

Q3 Collect a soil sample of your area and find out whether it is acidic, basic, or neutral
in nature.

Method

1. Dig out a little soil from about 10 cm below the surface — not the loose dusty top layer. Take
samples from two or three spots in the field or garden and mix them.
2. Put two spoonfuls in a glass, add clean water to about three times the height of the soil, and
stir well.
3. Let it stand until the soil settles and the water above is fairly clear.
4. Touch the clear water with a blue litmus strip and then with a red litmus strip.

Blue litmus → red = acidic soil → treat with lime (chuna)

Red litmus → blue = basic soil → treat with manure or composted leaves

Neither changes = neutral soil → poor growth would mean a nutrient deficiency

Sample answer: "Soil from our school garden turned neither litmus paper, so it is neutral. Soil
collected from the field behind the school turned blue litmus faintly red, so that field is slightly
acidic and would benefit from a little lime."

Why use settled water and not the soil itself: litmus reads a liquid. Wet mud
smeared on the paper hides the colour change under a brown stain, so you must let
the soil release its dissolved substances into water and test that water.

Tip: use rainwater or distilled water if you can. Tap water in some areas is itself
slightly basic and would shift your reading.

Page 52 of 54

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic, and Neutral AglaSem · NCERT Solutions

Chapter at a glance
The chapter opens at a school science fair on National Science Day, where a sprayed liquid
makes hidden writing appear on a white sheet of paper. The whole chapter is the
explanation of that trick.
Substances around us are of three kinds — acidic, basic and neutral. A substance that
shows different colours in acidic and basic solutions is an acid-base indicator.
Litmus, obtained from lichens, is the standard natural indicator. Acids turn blue litmus red;
bases turn red litmus blue; neutral substances change neither.
Red rose extract is another indicator — red in acid, green in base. Turmeric paper turns
red only in a base, so it can spot bases but cannot tell an acid from a neutral substance.
Acids taste sour (lemon, amla, tamarind, vinegar); bases feel soapy or slippery and
generally taste bitter — but not everything bitter is basic (karela is not).
When an acid and a base are mixed in the right amounts they cancel each other. This is a
neutralisation reaction: Acid + Base → Salt + Water + Heat.
Neutralisation solves real problems — moist baking soda on a red-ant bite (formic acid),
lime on acidic soil, organic manure on basic soil, and basic substances added to acidic
factory waste before it reaches a lake.
Some substances change their smell in acidic or basic media; these are called olfactory
indicators — chopped onion strips are the chapter's example.

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Quick revision
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TERM CHAPTER WHERE IT

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Acidic Turns blue litmus red; tastes sour Lemon juice, amla juice, tamarind Page 10

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substance water, vinegar

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Basic substance Turns red litmus blue; feels soapy Soap solution, baking soda solution, Page 10
slippery lime water, washing powder
solution

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Neutral Changes neither red nor blue litmus Tap water, sugar solution, salt

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substance solution

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Acid-base A substance that shows different Litmus, red rose extract, turmeric Page 10

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indicator colours in acidic and basic solutions

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Litmus A natural dye obtained from lichens; Blue litmus paper, red litmus paper Page 10

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sold as solution or as paper strips

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Lichen An association of two organisms Grows on rocks and trees where Page 10
rain is plenty and air is clean

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Red rose Filtrate of crushed red rose petals in Red in acid, green in base Pages 12–13

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extract hot water

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Turmeric cpaper Filter paper dipped in turmeric paste
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Turns red in a base; Pages 14–15

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and dried acid or neutral

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A substance whose odour changes in Onion strips, vanilla, clove oil Page 16
indicator an acidic or basic medium
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Neutralisation An acid and a base cancelling each Acid + Base → Salt + Water + Heat Page 18
other to give salt, water and heat

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Lime water Solution of calcium hydroxide, made Used to neutralise lemon juice in Page 8
by mixing chuna (calcium oxide) in Activity 2.7
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Formic Neutralised by moist baking soda Page 18
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