Science — CBSE Class 10 board question

(i)

(I) In covalent compounds, electrons are shared between atoms and no charged particles (ions) are formed. Since conduction of electricity requires free ions or electrons, covalent compounds are poor conductors of electricity.

(II) Hard water contains Ca²⁺ and Mg²⁺ ions. Soap (sodium salt of fatty acid) reacts with these ions to form an insoluble precipitate called scum. Since the soap is used up in forming scum, no lather is produced.

(III) C–C bonds are strong and stable, allowing carbon to form long chains (catenation). Si–Si bonds are weaker and Si has a larger atomic size, making long Si–Si chains unstable. Hence silicon does not show catenation.

(ii)

(I) Oxidation of ethanol by acidified K₂Cr₂O₇:

$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Acidified K}_2\text{Cr}_2\text{O}_7} \text{CH}_3\text{COOH}$$

(Ethanol → Ethanoic acid)

(II) Hydrogenation of ethene:

$$\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni, }\Delta} \text{CH}_3\text{CH}_3$$

Source: Chapter 4 – Carbon and its Compounds, Sections 4.1, 4.3, 4.4

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• Part (i)(I): Always link "no ions formed" directly to "no conduction." This is the textbook reason.
• Part (i)(II): Name the precipitate as "scum" and mention Ca²⁺/Mg²⁺ ions — examiners look for these terms.
• Part (i)(III): Compare bond strength of C–C vs Si–Si and mention atomic size; both points earn marks.
• Part (ii): Write balanced/arrow equations with correct reagents. For oxidation of ethanol, "acidified K₂Cr₂O₇" must appear as the reagent. For hydrogenation, Ni catalyst and heat (Δ) must be shown — these are common mark-fetching details.