'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]

(b) Differentiate between salt A and salt B. [1]

(c) (i) What are esters ? What happens when an ester reacts with an alkali (say sodium hydroxide) ? Give chemical equation for the reaction. [2]

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]

Acids, Bases and Salts

Hardness of Water (Washing Soda); Soaps, Detergents and Esters (cross-topic application)

Official Answer

(a) Minimum foam: Foam is formed the minimum in test tube III (Tubewell water + solution of salt A).

Tubewell water is generally hard water (contains dissolved calcium and magnesium salts), and salt 'A' (soap) reacts with the $Ca^{2+}/Mg^{2+}$ ions in it to form an insoluble scum, drastically cutting down lather/foam formation.

(b) Differentiating salt A and salt B:

Feature	Salt A (Soap)	Salt B (Detergent)
Chemical nature	Sodium/potassium salts of long-chain fatty (carboxylic) acids	Ammonium or sulphonate salts of long-chain acids
Use	Used for both bathing and washing	Used mainly for washing clothes, not for bathing
Behaviour with hard water	Reacts with $Ca^{2+}/Mg^{2+}$ ions to form insoluble scum, reducing lather	Does not form scum with hard water; works well (produces good lather) even in hard water
Biodegradability	Biodegradable	Often non-biodegradable, can cause water pollution

(c)(i) Esters and their reaction with alkali:

Esters are sweet-smelling compounds formed by the reaction of an acid with an alcohol. They are used in making perfumes and as flavouring agents in the food industry.

When an ester reacts with an alkali such as sodium hydroxide, the reaction is called saponification, and it produces back the alcohol and the sodium salt of the corresponding fatty acid (i.e., soap). This is essentially the reverse of ester formation and is the industrial reaction used to manufacture soap.

$\text{Ester} + NaOH \rightarrow \text{Sodium salt of fatty acid (Soap)} + \text{Alcohol}$

(c)(ii) Cause of hardness of water and the insoluble substance formed:

Hardness of water is caused by the presence of dissolved calcium and magnesium salts (chlorides, sulphates and bicarbonates of $Ca^{2+}$ and $Mg^{2+}$ ) in it. The NCERT Acids, Bases and Salts chapter (Section 2.4.3, Uses of washing soda) notes that sodium carbonate (washing soda) is itself "used for removing permanent hardness of water."

When soap is used with such hard water, the $Ca^{2+}$ and $Mg^{2+}$ ions react with the soap (sodium salt of fatty acid) to form an insoluble substance called scum (calcium/magnesium salts of the fatty acids). Because part of the soap is consumed in forming this precipitate instead of producing lather, foam is formed with difficulty, and the scum is seen floating/sticking as a curdy insoluble mass.

Marking Scheme

1(a) 1 mark for correctly identifying test tube III with reasoning.
2(b) 1 mark for at least two valid points of difference between soap and detergent.
3(c) 2 marks: (i) 1 mark for definition of ester + 1 mark for correct saponification equation with NaOH; OR (ii) 1 mark for correct cause of hardness (Ca2+/Mg2+ salts) + 1 mark for identifying scum and its formation cause.

Hint

Salt 'A' used for bathing is soap; salt 'B' is a detergent. Recall which type of water (rain/soft vs tubewell/hard) reduces lather when mixed with soap, and that the NCERT Acids, Bases and Salts chapter states washing soda is 'used for removing permanent hardness of water' (Section 2.4.3).

Quick Oral Answer

A is soap, B is detergent; soap reacts with Ca2+/Mg2+ in hard (tubewell) water to form scum, giving least foam in tubewell water + soap. Esters react with NaOH (saponification) to give soap and alcohol; hardness of water is due to dissolved calcium and magnesium salts, which react with soap to form insoluble scum — washing soda (from this chapter) is used to remove this hardness.

Analysis & Explanation

NOTE ON GROUNDING: This case-based question is a cross-topic application question. Its treatment of hardness of water and washing soda (parts a, and c-ii) is directly grounded in Section 2.4.3 of the NCERT Acids, Bases and Salts chapter, which explicitly states washing soda is 'used for removing permanent hardness of water.' However, the detailed chemistry of soap/detergent composition and esters/saponification (parts b and c-i) is standard NCERT Class 10 Science content but belongs to the 'Carbon and its Compounds' chapter, not to this Acids, Bases and Salts chapter — it is included here only because the original case study combines both topics under a common 'salts used for washing' theme. All facts stated are scientifically correct.

(a) Rain water is naturally soft (low dissolved mineral content), while tube-well (ground) water is typically hard due to dissolved calcium/magnesium salts leached from rock strata. Soap, unlike detergent, reacts with these hardness-causing ions to form an insoluble scum instead of producing lather — hence tube-well water + soap (Test tube III) gives the least foam.

(b) The key distinguishing feature is chemical composition and behaviour in hard water: soap is a sodium/potassium salt of a fatty acid that curdles in hard water, while detergents (ammonium or sulphonate salts) remain effective and lather well even in hard water, which is why detergents are preferred for washing clothes with any water quality.

(c)(i) tests recognition of esters as products of acid + alcohol reactions and their alkaline hydrolysis (saponification) back to soap and alcohol — a reaction of direct industrial relevance to soap manufacture.

(c)(ii) tests the underlying chemistry of hardness itself, which IS grounded in this chapter: dissolved $Ca^{2+}/Mg^{2+}$ salts react with the soap's fatty-acid anion to precipitate an insoluble calcium/magnesium soap (scum), which is why lather is difficult to form and a curdy insoluble mass appears while bathing with hard water.

Common Mistakes

1Concluding wrongly that detergents also form scum with hard water — detergents are specifically designed NOT to react with $Ca^{2+}/Mg^{2+}$ , which is their advantage over soap.
2Confusing 'esterification' (acid + alcohol $\rightarrow$ ester) with 'saponification' (ester + alkali $\rightarrow$ soap + alcohol) — they are reverse processes.
3Attributing hardness only to calcium salts and forgetting magnesium salts also contribute.

Interesting Facts

The word 'saponification' comes from the Latin 'sapo', meaning soap — the same reaction that converts fats/esters into soap industrially is the reverse of the esterification reaction that creates fragrant esters used in perfumery.

The Acids, Bases and Salts chapter itself links directly to this topic via washing soda: it states washing soda 'is used for removing permanent hardness of water' (Section 2.4.3, Uses of washing soda).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2025?

This question carries 4 marks in the CBSE Class 10 Science 2025 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Hardness of Water (Washing Soda); Soaps, Detergents and Esters (cross-topic application)" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2025 paper?

This question appears in Section E in the CBSE Class 10 Science 2025 paper.