Science (086) — CBSE Class 10 board question

(i)

I. CH₃–CH₂–C(=O)–H

The compound has 3 carbon atoms with an aldehyde (–CHO) functional group.
Name: Propanal

II. CH₃–C≡CH

The compound has 3 carbon atoms with a triple bond (alkyne).
Name: Propyne

(ii)

I. The sweet-smelling compounds produced belong to the class Esters.

II. The reaction is called Esterification. The chemical equation is:

$$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$$

(Ethanoic acid + Ethanol → Ethyl ethanoate + Water)

(iii)

Soap does not lather with hard water because hard water contains dissolved salts of calcium and magnesium. The soap reacts with these salts to form an insoluble precipitate called scum (calcium or magnesium salt of the fatty acid), which reduces the soap available for cleaning and prevents foam formation.

Source: Carbon and its Compounds, 4.2.5 Nomenclature of Carbon Compounds

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• (i): Count the carbon atoms → assign the parent chain name. Aldehyde suffix = -al; triple bond suffix = -yne. Common errors: writing "propanaldehyde" or "methylacetylene" — CBSE expects IUPAC names.
• (ii): Always write the full balanced equation with the acid catalyst shown above the arrow. State the class name clearly as "Esters."
• (iii): The key word examiners look for is scum and the explanation that Ca²⁺/Mg²⁺ ions react with soap to form insoluble salts, consuming the soap. One mark — keep it to 2–3 lines.