UPPSC 2022 Prelims GS Paper-I · Question 92 of 150

Correct — B, Methane. Natural gas as it comes out of a well is a mixture of gaseous hydrocarbons in which methane, CH4, is overwhelmingly dominant — commonly around ninety-five per cent of processed pipeline gas. The remainder is a small tail of heavier alkanes (ethane, propane, butane) together with carbon dioxide, nitrogen, sometimes hydrogen sulphide, and traces of helium, all of which are stripped out to varying degrees at the processing plant. Methane's dominance is why the whole downstream vocabulary works the way it does: compressed natural gas is nothing more than this same gas squeezed to two hundred bar or more, so CNG is also essentially methane; liquefied natural gas is the same gas cooled to about minus 162 degrees Celsius; and piped natural gas delivered to homes is again methane. It is also why natural gas is the cleanest of the fossil fuels per unit of energy — methane has the highest hydrogen-to-carbon ratio of any hydrocarbon, so more of its energy comes from making water and less from making carbon dioxide.

• (a)Hexane — Hexane, C6H14, is a liquid at ordinary temperature and pressure, so it cannot be the main constituent of a gas. It belongs to the light-naphtha fraction of crude oil and is used industrially as a solvent, notably for extracting edible oil from oilseeds — not as a component of the gas stream.
• (c)Butane — This is the intended trap, because butane is a real natural-gas component — but only a minor one. Butane, with propane, is the main constituent of LPG, the cooking-gas cylinder, which is a refinery and gas-processing product recovered from the heavier fraction. Keep the two apart: natural gas and CNG are methane; LPG is propane and butane. UPPSC asked exactly this contrast in 2021, where the main constituent of LPG was the answer.
• (d)Benzene — Benzene, C6H6, is an aromatic hydrocarbon, not an alkane, and it is a liquid at room temperature. It is a product of petroleum refining and coal carbonisation and a recognised carcinogen; it is not a constituent of natural gas.

Fossil fuel gases are distinguished by which hydrocarbons dominate them, and each has its own name and use. Natural gas, and therefore CNG, LNG and PNG, is mainly methane. LPG is mainly propane and butane, liquefied under modest pressure. Biogas from anaerobic digestion is roughly two-thirds methane and one-third carbon dioxide. Coal gas is a manufactured mixture of hydrogen, methane and carbon monoxide, and water gas is carbon monoxide with hydrogen. The physical differences follow from the chemistry: methane is lighter than air, so a CNG leak rises and disperses, whereas propane and butane are heavier than air and a leaking LPG cylinder pools at floor level — the single most important safety contrast between the two.

The examiner is testing whether a candidate can hold 'natural gas' and 'LPG' apart, which is why butane sits in the option list. The clean way to reason is by physical state. Natural gas must be made of something that is a gas at ambient conditions and light enough to flow through long pipelines, and among the four options only methane and butane qualify as gases at all; hexane and benzene are liquids and can be struck out immediately. Between methane and butane, methane is the smallest and lightest hydrocarbon and the one that biological and thermal decay of organic matter produces in bulk underground, so it is the natural candidate. India context worth carrying: Delhi's public transport fleet was converted to CNG following Supreme Court directions from 2001-02, city gas distribution is regulated by the Petroleum and Natural Gas Regulatory Board, and methane is also a potent greenhouse gas, which is why leakage from gas infrastructure, paddy fields and landfills is a climate issue in its own right.

• Natural gas is a mixture of gaseous hydrocarbons in which methane, CH4, is the overwhelming constituent, commonly about ninety-five per cent of processed gas.
• Minor components include ethane, propane and butane along with carbon dioxide, nitrogen, hydrogen sulphide and traces of helium.
• CNG is natural gas compressed to roughly 200-250 bar, so it is chemically the same methane-dominated gas; LNG is the same gas liquefied at about minus 162 degrees Celsius.
• LPG, by contrast, is mainly propane and butane; methane is lighter than air while LPG is heavier, which is why LPG leaks collect near the floor.
• Biogas is about two-thirds methane and one-third carbon dioxide; coal gas is hydrogen, methane and carbon monoxide; water gas is carbon monoxide and hydrogen.

• Answering 'butane' because it is a familiar cooking gas — butane is LPG's main constituent, not natural gas's
• Assuming CNG and LPG are the same product in different containers; they are chemically different gases with opposite densities relative to air
• Forgetting that hexane and benzene are liquids at room temperature and so cannot be the main constituent of any gas

UPPSC asks this family as direct one-line recall — the main constituent of natural gas, of LPG, of biogas — and returns to it every few years, so learning the whole set of gas-to-constituent pairs once is more efficient than learning any single one. UPSC prefers to test the contrast, either as a match of fuel gases to their major constituents or as an assertion-reason built on the LPG-versus-methane confusion.

• practice — not a real PYQ

  Which of the following pairs of fuel gas and its principal constituents is correctly matched?

  • (a)LPG — Methane and ethane
  • (b)Water gas — Propane and butane
  • (c)CNG — Methane
  • (d)Coal gas — Carbon dioxide and nitrogen

  Answer(c) CNG — Methane. CNG is natural gas under pressure, so it is methane-dominated; LPG is propane and butane, water gas is carbon monoxide and hydrogen, and coal gas is hydrogen, methane and carbon monoxide.

• practice — not a real PYQ

  A leak from a domestic LPG cylinder tends to collect near the floor, whereas a CNG leak rises and disperses. The reason is that

  • (a)LPG is stored as a liquid while CNG is stored as a gas
  • (b)propane and butane are heavier than air while methane is lighter than air
  • (c)LPG contains an added odorant while CNG does not
  • (d)CNG is stored at a much higher pressure than LPG

  Answer(b) propane and butane are heavier than air while methane is lighter than air — the difference in molecular mass, not the storage method, decides which way a leak moves.