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 ( $Na$ ) or potassium ( $K$ ) 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:

A hydrophilic (water-loving) ionic head which is soluble in water.
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.

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

Hard water contains dissolved calcium ( $Ca^{2+}$ ) and magnesium ( $Mg^{2+}$ ) 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

Property	Ethanol ( $CH_3CH_2OH$ )	Ethanoic Acid ( $CH_3COOH$ )
Physical Properties
1. Melting Point	$156\text{ K}$ (Very low melting point)	$290\text{ K}$ (Freezes in cold climates to form glacial acetic acid)
2. Boiling Point	$351\text{ K}$	$391\text{ K}$
3. Odour	Pleasant, characteristic alcoholic smell	Pungent, vinegar-like smell
Chemical Properties
1. Reaction with Litmus	Neutral; does not change the color of blue litmus paper.	Acidic; turns blue litmus paper red.
2. Reaction with Carbonates / Hydrogencarbonates	Does not react with sodium carbonate ( $Na_2CO_3$ ) or sodium hydrogencarbonate ( $NaHCO_3$ ).	Reacts to produce sodium acetate, water, and carbon dioxide gas ( $CO_2$ ) with brisk effervescence:<br> $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \uparrow$

sodium or potassium saltsammonium or sulphonate saltshydrophilic headhydrophobic tailmicellescumcalcium and magnesium ionsmelting pointboiling pointlitmussodium hydrogencarbonatebrisk effervescence

Marking Scheme

1Option 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)
2OR 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.

Analysis & 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 ( $Ca^{2+}$ , $Mg^{2+}$ ) 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.

Common Mistakes

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

Interesting Facts

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

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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.