NCERT Solutions for Class 7th Science Chapter 2 Activity 2.1: Let us explore — Litmus as an indicator
Book page 8–9 Updated on2026-09-19
Q1.
Do you observe any change in the colour of the blue litmus pieces? Record your observations in Table 2.1.
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.
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
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.
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.
Fig. 2.2(b), page 8 — redrawn sketch. The same activity repeated with small pieces of red litmus paper on the white tile.
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
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.
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.
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
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.
Group A
Group B
Group C
Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Reproduced as printed, blank for you to fill in.
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.