'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.
Read the following case and answer the questions that follow.
Generated by claude-sonnet-4-6 · 2026-06-15 06:47 · grounding stimulus
Model Answer
(a) Test tube IV (Tubewell water + salt B) forms the minimum foam.
Tubewell water is hard water, and salt B (washing soda / detergent) forms least lather with hard water compared to soap.
(b)
| Salt A | Salt B |
|--------|--------|
| Soap – sodium salt of a long-chain fatty acid; used for bathing and washing | Detergent (synthetic) – sodium salt of a long-chain sulphonic acid; used only for washing |
Soap does not work in hard water; detergent works in hard and soft water both.
(c)(i)
Esters are sweet-smelling compounds formed by the reaction of an acid with an alcohol.
When an ester reacts with an alkali (NaOH), it undergoes saponification to give sodium salt of the acid (soap) and alcohol:
$$\text{CH}_3\text{COOC}_2\text{H}_5 + \text{NaOH} \xrightarrow{\Delta} \text{CH}_3\text{COONa} + \text{C}_2\text{H}_5\text{OH}$$
(Ethyl ethanoate + Sodium hydroxide → Sodium ethanoate + Ethanol)
Source: Carbon and its Compounds, Soaps and Detergents / Esters section
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Explanation
- Salt A = soap (used for bathing + washing); Salt B = detergent (washing only). This is the key identification.
- Hard water (tubewell) + soap forms scum, not foam → test tube IV gives minimum foam because detergent still lathers but the combination of hard water reducing lather and detergent being less affected actually makes IV the least foamy compared to I (rain + soap gives maximum foam).
- For (c), examiners expect the definition, the term saponification, and a balanced chemical equation — all three earn the 2 marks.
- Keep the equation correctly balanced; the reactants and products must match.