Question 235 marksLong AnswerSection A

What is the difference between the chemical composition of soaps and detergents? State in brief the action of soaps in removing an oily spot from a shirt. Why are soaps not considered suitable for washing where water is hard ?


Or


List in tabular form three physical and two chemical properties on the basis of which ethanol and ethanoic acid can be differentiated.

Carbon and its Compounds
Soaps and Detergents / Properties of Ethanol and Ethanoic Acid

Official answer

Option 1: Soaps and Detergents


#### 1. Difference in Chemical Composition

  • Soaps: Soaps are sodium (NaNa) or potassium (KK) salts of long-chain carboxylic acids (fatty acids).
  • Detergents: Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids.

#### 2. Cleansing Action of Soap on an Oily Spot

  • A soap molecule consists of two parts:
  1. A hydrophilic (water-loving) ionic head which is soluble in water.
  2. A hydrophobic (water-fearing) hydrocarbon tail which is soluble in oil/grease.
  • When soap is applied to an oily spot on a shirt, the hydrophobic tails align themselves towards the oil droplet and dissolve in it, while the hydrophilic ionic heads project outwards into the water.
  • This arrangement forms a spherical cluster of molecules called a micelle.
  • During agitation, the oily dirt is trapped in the center of the micelle and is pulled away from the fabric into the water, forming a stable emulsion that can be easily rinsed away.

``diagram {"xrange": [199.0, 201.0], "yrange": [-247.8, -12.2], "points": [{"x": 200.0, "y": -125.0, "label": "Oily Dirt"}, {"x": 200.0, "y": -35.0, "label": ""}, {"x": 200.0, "y": -225.0, "label": ""}]} ``


#### 3. Why Soaps are Unsuitable for Hard Water

  • Hard water contains dissolved calcium (Ca2+Ca^{2+}) and magnesium (Mg2+Mg^{2+}) ions.
  • When soap is added to hard water, it reacts with these calcium and magnesium ions to form an insoluble, sticky precipitate called scum.
  • This precipitation prevents the soap from lathering easily, wasting a large amount of soap before any cleansing action can take place.


OR Option: Differentiation between Ethanol and Ethanoic Acid


PropertyEthanol (CH3CH2OHCH_3CH_2OH)Ethanoic Acid (CH3COOHCH_3COOH)
Physical Properties
1. Melting Point156 K156\text{ K} (Very low melting point)290 K290\text{ K} (Freezes in cold climates to form glacial acetic acid)
2. Boiling Point351 K351\text{ K}391 K391\text{ K}
3. OdourPleasant, characteristic alcoholic smellPungent, vinegar-like smell
Chemical Properties
1. Reaction with LitmusNeutral; does not change the color of blue litmus paper.Acidic; turns blue litmus paper red.
2. Reaction with Carbonates / HydrogencarbonatesDoes not react with sodium carbonate (Na2CO3Na_2CO_3) or sodium hydrogencarbonate (NaHCO3NaHCO_3).Reacts to produce sodium acetate, water, and carbon dioxide gas (CO2CO_2) with brisk effervescence:<br>2CH3COOH+Na2CO3→2CH3COONa+H2O+CO2↑2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \uparrow
Answer keywords:sodium or potassium saltsammonium or sulphonate saltshydrophilic headhydrophobic tailmicellescumcalcium and magnesium ionsmelting pointboiling pointlitmussodium hydrogencarbonatebrisk effervescence

Marking scheme

  1. 1.Option 1: Difference in chemical composition of soaps and detergents (1 mark); Cleansing action of soap with micelle explanation (2 marks); Hard water reaction and scum formation (2 marks) (5 marks)
  2. 2.OR Option: Three physical properties correctly tabulated (3 marks); Two chemical properties correctly tabulated with equations where applicable (2 marks) (5 marks)

Hint

For the soap question, focus on the structure of a micelle. For the differentiation question, use the physical constants from Table 4.1 and the reaction with sodium bicarbonate to distinguish an acid from an alcohol.

Quick oral answer

Soaps are sodium or potassium salts of long-chain fatty acids, whereas detergents are ammonium or sulphonate salts. Soaps form an insoluble precipitate called scum with calcium and magnesium ions present in hard water, making them ineffective.

Common mistakes

  1. 1.Confusing the hydrophilic head with the hydrophobic tail.
  2. 2.Stating that detergents form scum in hard water; detergents actually work perfectly in hard water.
  3. 3.Forgetting to write the chemical equation for the reaction of ethanoic acid with sodium carbonate/bicarbonate.

Analysis and explanation

This question tests the core chemical and physical differences between key carbon compounds.

  • For the first option, the cleansing action relies on the dual-affinity nature of soap molecules (amphiphilic nature), which self-assemble into micelles. Hard water contains divalent cations (Ca2+Ca^{2+}, Mg2+Mg^{2+}) that displace the monovalent sodium/potassium ions in soap, forming insoluble calcium/magnesium carboxylates (scum).
  • For the second option, the physical constants (melting and boiling points) are directly sourced from Table 4.1 of the NCERT textbook. The chemical tests rely on the acidic nature of ethanoic acid compared to the neutral nature of ethanol.

Interesting facts

Pure ethanoic acid is called 'glacial acetic acid' because its melting point is 290 K290\text{ K} (17∘C17^\circ\text{C}), which causes it to freeze during winter in cold climates, resembling ice.

Question details

Q23 of 41 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2012
Section
Section A
Marks
5
Type
Long Answer
Chapter
Carbon and its Compounds
Topic
Soaps and Detergents / Properties of Ethanol and Ethanoic Acid
Bloom's level
Analyzing
Syllabus topic
Soaps and detergents, cleansing action of soap, hard water; Properties of ethanol and ethanoic acid

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 5 marks in the CBSE Class 10 Science 2012 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Soaps and Detergents / Properties of Ethanol and Ethanoic Acid" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Long Answer question from Section A in the CBSE Class 10 Science 2012 paper.