NCERT Solutions Curiosity Chapter 2 –9Section 2.1.1 Litmus as an indicator — Activity 2.1: Let us explore

Book page 8 Updated on2026-09-05

Q1.

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

Answer

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.

Why it happens: litmus is a dye that has one colour in an acid and another in a base. The four samples that turned it red all contain acids. The rest have no acid to act on the dye, so the blue colour stays.

Tip: use a fresh piece of litmus for every sample and one clean dropper per bottle. A dropper carried from lemon juice to lime water will give you a wrong reading.

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.

Answer

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.

S.No.	Name of the sample	Colour of blue litmus paper after putting a drop of sample	Colour of red litmus paper after putting a 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 water	Turns red	No change (stays red)
5.	Vinegar	Turns red	No change (stays red)
6.	Baking soda solution	No change (stays blue)	Turns blue
7.	Lime water	No change (stays blue)	Turns blue
8.	Tap water	No change	No change
9.	Washing powder solution	No change (stays blue)	Turns blue
10.	Sugar solution	No change	No change
11.	Salt solution	No change	No change
12.	Any other — curd	Turns red	No change (stays red)

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.

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.

Answer

Sorting the twelve entries of Table 2.1 gives Table 2.2.

Group A (turn blue litmus red)	Group B (turn red litmus blue)	Group C (affect neither paper)
Lemon juice	Soap solution	Tap water
Amla juice	Baking soda solution	Sugar solution
Tamarind water	Lime water	Salt solution
Vinegar	Washing powder solution	—
Curd (any other)	—	—

In the words the chapter uses two pages later: Group A is acidic, Group B is basic, Group C is neutral.

Check it yourself: add one sample of your own — orange juice, curd whey, toothpaste solution, floor-cleaning liquid — and see which column it lands in before you read the answer on page 10.

Q4.

How to prepare lime water?

Answer

Mix lime (chuna, i.e. calcium oxide) in water, leave the mixture undisturbed for some time — say an hour — then filter the clear liquid into another container. That filtrate is lime water.

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.

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