Compressed Natural Gas (CNG) is mainly -

Correct — option (3), Methane.

NCERT's Class 10 Science chapter Carbon and its Compounds says: "Methane is widely used as a fuel and is a major component of bio-gas and Compressed Natural Gas (CNG)."

CNG is natural gas squeezed into strong cylinders. Wikipedia's article on CNG (August 2021 revision) describes it as a fuel gas made of natural gas, mainly composed of methane (CH₄), compressed to less than 1% of the volume it occupies at standard atmospheric pressure.

So CNG is mainly methane, the hydrocarbon with a single carbon atom.

The idea to remember: CNG is compressed natural gas, and natural gas is mostly methane (CH₄).

Why the others are wrong

• (1)
  Ethane — Ethane, C₂H₆, is the two-carbon member of the same family of saturated hydrocarbons in NCERT's table.

  Wikipedia's Natural gas article (October 2021 revision) says raw natural gas sometimes contains a significant amount of ethane, but that such heavier hydrocarbons are removed for commercial use before the methane is sold as a consumer fuel.

• (2)
  Propane — Propane, C₃H₈, is a main gas of LPG, not CNG. Wikipedia's article on liquefied petroleum gas (October 2021 revision) describes LPG as a flammable mixture of hydrocarbon gases such as propane and butane.

  In natural gas processing, propane is taken out as a by-product before the gas is sold as fuel.

• (4)
  Butane — Butane, C₄H₁₀, is the four-carbon saturated hydrocarbon and, with propane, a gas of LPG. Wikipedia notes that LPG is sold as mixes that are mostly propane, mostly butane, or both.

  Wikipedia says a full LPG cylinder is mostly liquid under pressure; CNG, by contrast, is natural gas compressed at ambient temperature and is mainly methane.

Methane, ethane, propane and butane are saturated hydrocarbons with 1, 2, 3 and 4 carbon atoms. NCERT's Class 10 table gives their formulae as CH₄, C₂H₆, C₃H₈ and C₄H₁₀.

Wikipedia's Natural gas article (October 2021 revision) describes natural gas as a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes. Before it is sold as fuel, heavier hydrocarbons such as ethane, propane and butane are removed.

CNG keeps this gas at ambient temperature and high pressure; LNG, liquefied natural gas, is stored at low temperature and nearly ambient pressure.

Key facts

• NCERT: methane is a major component of biogas and Compressed Natural Gas (CNG).
• Methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀ have 1, 2, 3 and 4 carbon atoms (NCERT Class 10).
• CNG is compressed to less than 1% of its volume at standard atmospheric pressure and stored at 20–25 MPa (Wikipedia, August 2021).
• LPG is a flammable mixture of hydrocarbon gases such as propane and butane (Wikipedia).
• NCERT Class 8 lists calorific values of 50000 kJ/kg for methane and CNG and 55000 kJ/kg for LPG.

Common traps

• CNG versus LPG: CNG is mainly methane; LPG is propane and butane.
• Present is not main: raw natural gas can contain ethane, propane and butane, but these are removed and methane is what is sold as fuel.
• Compressed is not liquefied: CNG is stored at ambient temperature and high pressure, while LNG is stored as a liquid at low temperature.

UnlockIAS will link similar questions from RAS Pre 2016 and 2013 here once those papers are published on this site.

• practice — not a real PYQ

  Liquefied Petroleum Gas (LPG) is mainly a mixture of –

  • (a)Methane and ethane
  • (b)Propane and butane
  • (c)Methane and hydrogen
  • (d)Carbon monoxide and hydrogen
  Answer(2) — Wikipedia describes LPG as a flammable mixture of hydrocarbon gases such as propane and butane. Option (1) points to natural gas, which is mainly methane, and neither option (3) nor option (4) is the LPG mixture.
• practice — not a real PYQ

  Which hydrocarbon has the formula C₃H₈?

  • (a)Methane
  • (b)Ethane
  • (c)Butane
  • (d)Propane
  Answer(4) — NCERT's table gives propane as C₃H₈, with three carbon atoms. Option (1) is CH₄, option (2) is C₂H₆ and option (3) is C₄H₁₀.