Science — CBSE Class 10 board question

(i) The simplest saturated hydrocarbon is methane (CH₄).

Electron dot structure:

H
|
H : C : H
|
H

(Each C–H bond has one shared pair of electrons.)

Type of bonds: Only single covalent bonds (C–H) exist in methane. All four valence electrons of carbon are shared with four hydrogen atoms.

(ii) Two fuel mixtures in daily life that contain methane as an important component:

1. CNG (Compressed Natural Gas)
2. Biogas

(iii) Methane belongs to the Alkanes homologous series (saturated hydrocarbons).

General formula: $C_nH_{2n+2}$

(iv) Methane (a saturated hydrocarbon) produces a clean blue flame on burning, as saturated hydrocarbons undergo complete combustion when sufficient oxygen is available.

Source: Carbon and its Compounds, sections 4.3.1 Combustion and 4.2 (Homologous Series)

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• The question has four sub-parts worth a total of 5 marks (~1 mark each, with (i) slightly heavier).
• The electron dot structure must show shared pairs (dots/crosses) around C; a simple line structure alone may not get full credit.
• Examiners expect CNG and Biogas (or LPG — LPG is mainly butane/propane, so CNG or Biogas are safer choices for methane).
• The general formula $C_nH_{2n+2}$ for alkanes is essential — don't confuse with alkenes ($C_nH_{2n}$).
• For (iv), the key phrase is clean blue flame — link it to complete combustion of saturated hydrocarbons as stated in the textbook passage.