(a) Compare soaps and detergents on the basis of their composition and cleansing action in hard water.

(b) What happens when ethanol is treated with sodium metal? State the behaviour of ethanol in this reaction.

(c) Draw the structure of cyclohexane.

(d) Name the following compound:

$\text{CH}_3-\text{CH}_2-\text{OH}$

Q: (a) Compare soaps and detergents on the basis of their composition and cleansing action in hard water. (b) What happens when ethanol is treated with sodium metal? State the behaviour of ethanol in this reaction. (c) Draw the structure of cyclohexane. (d) Name the following compound: $$\text{CH}_3-\text{CH}_2-\text{OH}$$

(a) **Comparison of Soaps and Detergents**: * **Composition**: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). Detergents are ammonium or sulphonate salts of long-chain carboxylic acids. * **Cleansing Action in Hard Water**: Soaps react with calcium and magnesium ions present in hard water to form an insoluble sticky precipitate called scum, which makes them ineffective. Detergents do not form insoluble precipitates with calcium and magnesium ions, meaning they remain highly effective even in hard water. (b) **Ethanol treated with Sodium Metal**: * **Reaction**: When ethanol reacts with sodium metal, sodium ethoxide is formed and hydrogen gas is evolved with effervescence. * **Chemical Equation**: $$2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{ONa} + \text{H}_2 \uparrow$$ * **Behaviour of Ethanol**: Ethanol behaves as a weak acid in this reaction because it releases a proton ($\text{H}^+$) to bond with the active metal sodium. (c) **Structure of Cyclohexane ($\text{C}_6\text{H}_{12}$)**: Cyclohexane consists of six carbon atoms arranged in a ring, with each carbon atom single-bonded to two other carbon atoms and two hydrogen atoms to satisfy its tetravalency. (d) **Name of the compound**: The compound $\text{CH}_3-\text{CH}_2-\text{OH}$ is **Ethanol**.

Carbon and its Compounds

Versatile Nature of Carbon

Official Answer

(a) Comparison of Soaps and Detergents:

Composition: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). Detergents are ammonium or sulphonate salts of long-chain carboxylic acids.
Cleansing Action in Hard Water: Soaps react with calcium and magnesium ions present in hard water to form an insoluble sticky precipitate called scum, which makes them ineffective. Detergents do not form insoluble precipitates with calcium and magnesium ions, meaning they remain highly effective even in hard water.

(b) Ethanol treated with Sodium Metal:

Reaction: When ethanol reacts with sodium metal, sodium ethoxide is formed and hydrogen gas is evolved with effervescence.
Chemical Equation:

$2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{ONa} + \text{H}_2 \uparrow$

Behaviour of Ethanol: Ethanol behaves as a weak acid in this reaction because it releases a proton ( $\text{H}^+$ ) to bond with the active metal sodium.

Cyclohexane consists of six carbon atoms arranged in a ring, with each carbon atom single-bonded to two other carbon atoms and two hydrogen atoms to satisfy its tetravalency.

(d) Name of the compound:

The compound $\text{CH}_3-\text{CH}_2-\text{OH}$ is Ethanol.

sodium or potassium saltssulphonate saltsscumhard watersodium ethoxidehydrogen gasweak acidcyclohexaneethanol

Marking Scheme

1Comparing composition of soaps and detergents (1 mark)
2Comparing cleansing action in hard water (mentioning scum formation for soaps) (1 mark)
3Stating products of ethanol + sodium reaction with balanced chemical equation (1 mark)
4Explaining the acidic behaviour of ethanol in this reaction (0.5 marks)
5Drawing the correct cyclic structure of cyclohexane (1 mark)
6Naming the compound as Ethanol (0.5 marks)

Hint

For (a), think about scum formation. For (b), recall the reaction of active metals with acids. For (c), remember that cyclohexane is a saturated ring of 6 carbons. For (d), identify the number of carbons and the functional group.

Quick Oral Answer

Soaps are sodium/potassium salts of fatty acids and form scum in hard water, while detergents are sulphonate salts and work well in hard water. Ethanol reacts with sodium to release hydrogen gas, showing its acidic nature. Cyclohexane is a ring of 6 carbons with 12 hydrogens.

Analysis & Explanation

(a) Hard water contains dissolved calcium and magnesium salts. Soaps precipitate out as scum because their calcium/magnesium salts are insoluble. Detergents have charged ends that do not form insoluble precipitates with these ions.

(b) Active metals like sodium displace hydrogen from acidic groups. Ethanol's reaction with sodium is a test for the presence of the hydroxyl group, showing its weakly acidic nature.

(c) Cyclohexane is a saturated cyclic hydrocarbon with the formula $\text{C}_6\text{H}_{12}$ . Each carbon forms 4 single covalent bonds (two with adjacent carbons, two with hydrogens).

(d) The compound has two carbon atoms (prefix 'eth-'), single bonds between carbons ('-an-'), and an alcohol functional group ('-ol'), giving the IUPAC name Ethanol.

Common Mistakes

1Writing the formula of cyclohexane as $\text{C}_6\text{H}_6$ (which is benzene) instead of $\text{C}_6\text{H}_{12}$ .
2Forgetting to balance the chemical equation for the reaction of ethanol with sodium.
3Confusing the composition of soaps (carboxylic acid salts) with detergents (sulphonate/ammonium salts).

Interesting Facts

Friedrich Wöhler disproved the 'vital force theory' in 1828 by synthesizing urea (an organic compound) from ammonium cyanate (an inorganic compound) in a laboratory.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 5 marks in the CBSE Class 10 Science 2020 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 "Versatile Nature of Carbon" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Long Answer question from Section C in the CBSE Class 10 Science 2020 paper.