NCERT Solutions Curiosity Chapter 2 –21End-of-chapter questions — Let Us Enhance Our Learning

Book page 20 Updated on2026-09-05

Q1.

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) Vinegar (iv) Common salt solution

Answer

(iii) Vinegar

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

Answer

(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.

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.

Answer

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

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.

Figure	Colour of the dipped end	Nature of the solution in the 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.

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.

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?

Answer

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.

Answer

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.

Written on the paper with	Sprayed with	Colour of the writing obtained
Soap solution (base)	Turmeric solution	Red on a yellow sheet
Baking soda solution (base)	Turmeric solution	Red on a yellow sheet
Turmeric solution	Lime water (base)	Red on a pale yellow sheet
Lemon juice (acid)	Red rose extract	Red writing
Soap solution (base)	Red rose extract	Green writing
Red rose extract	Lemon juice (acid)	Red writing
Vinegar (acid)	Purple cabbage extract	Reddish-pink writing
Washing soda solution (base)	Purple cabbage extract	Green writing

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 message appear all at once.

Tip: a combination such as "salt solution + turmeric" will not work. Salt solution is neutral, so nothing appears. Always keep one acid or base and one indicator in the pair.

Q7.

Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.

Answer

The mixture will change from red to green.

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?

Answer

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.

Let the orange-juice writing dry fully — it becomes almost invisible on white paper.
Spray or gently brush red rose extract over the whole sheet.
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.

Q9.

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

Answer

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)

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

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

Make a paste of a spoonful of turmeric with a little water.
Dip filter paper in the paste until it is evenly yellow.
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.

Answer

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

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?

Answer

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.

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 ____________.

Answer

Here is the completed flowchart.

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

Blank in the flowchart	Answer
The soil can be ______ in nature.	Acidic
The soil can be ______ in nature.	Basic
Which indicator can be used to test the nature of the soil?	Litmus paper (red and blue); turmeric paper or red rose extract may also be used
The acidic soil can be treated with ______.	Lime (chuna), which is a base
The basic soil can be treated with ______.	Organic matter such as manure and composted leaves, which release acids

How to test soil at home: stir a spoonful of soil in a glass of water, let it settle, and touch the clear liquid above with red and blue litmus papers. Blue turning red means acidic soil; red turning blue means basic soil.

Did you know? The chapter warns that the soil may also be perfectly neutral and the plants still unhealthy — that would point to a deficiency of nutrients, which no amount of lime or manure-for-acidity will fix.

Was this helpful? Report an error