'A' and 'B' are two salts used for washing purposes. Salt 'A' is used for bathing also. Four test tubes I, II, III and IV as mentioned below are taken. I. Rain water + solution of salt 'A' II. Rain water + solution of salt 'B' III. Tubewell water + solution of salt 'A' IV. Tubewell water + solution of salt 'B' The test tubes are shaken one by one almost identically for the same time and the lengths of foam formed in each test tube is noted. (a) In which one of the four test tubes is the foam formed the minimum? (b) Differentiate between salt A and salt B. (c)(i) What are esters? What happens when an ester reacts with an alkali (say sodium hydroxide)? Give chemical equation for the reaction. (c)(ii) What is the cause of hardness of water? Sometimes it is observed that while bathing foam is formed with difficulty and an insoluble substance is formed. Name this substance and write the cause of its formation.

ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 37

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedAcids, Bases and Salts

'A' and 'B' are two salts used for washing purposes. Salt 'A' is used for bathing also. Four test tubes I, II, III and IV as mentioned below are taken.

I. Rain water + solution of salt 'A'
II. Rain water + solution of salt 'B'
III. Tubewell water + solution of salt 'A'
IV. Tubewell water + solution of salt 'B'

The test tubes are shaken one by one almost identically for the same time and the lengths of foam formed in each test tube is noted.

(a)
In which one of the four test tubes is the foam formed the minimum?
1 mark
(b)
Differentiate between salt A and salt B.
1 mark
(c)(i)
What are esters? What happens when an ester reacts with an alkali (say sodium hydroxide)? Give chemical equation for the reaction.
2 marks
OR
(c)(ii)
What is the cause of hardness of water? Sometimes it is observed that while bathing foam is formed with difficulty and an insoluble substance is formed. Name this substance and write the cause of its formation.
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Salt A is used for bathing, so which cleansing agent is it? Which water sample is hard?

Hint 2

Compare the chemical nature of soaps and detergents and how each behaves in hard water.

Hint 3

Think of the reverse of esterification when an ester meets NaOH.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Test tube III (tubewell water + salt A) forms the least foam. Tubewell water is hard, and salt A is soap, which does not lather well in hard water.
1 mark
(b) Salt A is a soap: a sodium or potassium salt of a long-chain carboxylic acid, which does not lather well with hard water. Salt B is a detergent: a sodium salt of a sulphonic acid (or an ammonium salt with chloride or bromide ions), which lathers even with hard water.
1 mark
(c)(i) Esters are sweet-smelling compounds used in perfumes and flavouring agents. They are formed when a carboxylic acid reacts with an alcohol in the presence of an acid catalyst (esterification).
½ mark
When an ester is treated with an alkali such as NaOH, it is converted back into an alcohol and the sodium salt of the carboxylic acid. This reaction is called saponification.
½ mark
$C H_{3} COO C_{2} H_{5} + NaOH \to C_{2} H_{5} OH + C H_{3} COONa$
1 mark
(c)(ii) (Alternative to (c)(i), also worth 2 marks.)
- Cause of hardness: water contains dissolved calcium and magnesium salts (bicarbonates, chlorides, sulphates).
- Insoluble substance: scum.
- Cause of its formation: soap molecules react with the $C a^{2 +}$ and $M g^{2 +}$ ions in the water and form an insoluble precipitate (scum), so little foam is formed.

Answer: (a) Test tube III; (b) A is a soap (Na/K salt of long-chain carboxylic acid, poor lather in hard water), B is a detergent (sulphonate salt, lathers in hard water); (c)(i) esters are sweet-smelling compounds; with NaOH they give alcohol + sodium salt of the acid (saponification): CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa; (c)(ii) Ca/Mg salts cause hardness; the insoluble substance is scum, formed by soap reacting with Ca²⁺/Mg²⁺ ions.

Where the marks are

Identifies test tube III (1 mark)
One valid difference between soap and detergent (1 mark)
(c)(i) Defines esters (½ mark)
(c)(i) States ester + alkali gives alcohol + salt of the acid (½ mark)
(c)(i) Correct equation (1 mark)
OR (c)(ii) Ca and Mg salts cause hardness (½ mark)
OR (c)(ii) Names scum (½ mark)
OR (c)(ii) Soap reacts with Ca/Mg ions to form an insoluble substance (1 mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2025: question 37

Hints

Step-by-step solution

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