MPSC 2025 Prelims Paper-I · Question 65 of 100

Correct — option (3), Only C. Of the four sources listed, three release energy by oxidising carbon-containing fuel, and only one — geothermal energy — does not. Statement A, wood, is a biomass fuel: it is mostly cellulose and lignin, both carbon-based organic compounds, and burning it releases energy by oxidising that carbon to carbon dioxide, exactly like any other combustion fuel. Statement B, biogas, is produced by anaerobic digestion of organic (carbon-containing) waste and is chiefly a mixture of methane (CH4) and carbon dioxide; burning biogas for energy is, again, the combustion of a carbon compound, methane. Statement D, coal, is the most carbon-dense of all — it is essentially fossilised, carbon-rich plant matter, and coal-fired power generation is textbook carbon combustion. Statement C, geothermal energy, works on an entirely different physical principle: it taps heat that already exists inside the Earth — from the residual heat of planetary formation and from the radioactive decay of elements in the Earth's core and mantle — and brings it to the surface as steam or hot water to drive turbines. No fuel is burned and no carbon compound is oxidised to release this energy; the energy is thermal and geological in origin, not chemical. Because A, B and D all involve carbon combustion while C alone does not, the correct set is 'Only C'.

• (1)Both A and B — This selects wood and biogas as the two sources that do not involve carbon, when in fact both are carbon-based combustion fuels — wood is burned cellulose and lignin, and biogas is burned methane, a single-carbon compound. This option also omits the one source that genuinely does not involve carbon, geothermal energy (C), so it gets both the composition of the included sources and the identity of the correct answer wrong.
• (2)Both B and C — This correctly includes C (geothermal, which indeed does not involve carbon) but incorrectly pairs it with B, biogas — which is chiefly methane, a carbon compound, and is burned to release energy exactly like any other combustible gas. Because biogas does involve carbon, pairing it with geothermal produces a set that is only half right.
• (4)Only D — This names coal alone, but coal is precisely the source that most obviously involves carbon — it is a carbon-rich fossil fuel formed from ancient plant matter, and its combustion is the classic example of carbon-based energy production. Selecting only D also misses that A (wood) and B (biogas) are equally carbon-based, and it misses the actual non-carbon source, C (geothermal), entirely.

Energy sources can be grouped by whether they release energy through the chemical combustion of carbon-containing fuel or through some other physical process. Wood, biogas and coal all belong to the first group: each is, chemically, a store of carbon compounds (cellulose/lignin in wood, methane in biogas, largely elemental and hydrocarbon-bound carbon in coal), and each yields usable energy by being oxidised, releasing carbon dioxide as a combustion product. Geothermal energy belongs to a different category altogether — it is a form of thermal energy harvested directly from the Earth's internal heat, generated by the planet's residual formation heat and by ongoing radioactive decay deep in the crust and mantle. Geothermal plants use naturally occurring or engineered steam and hot water to spin turbines, with no fuel combustion and no carbon compound consumed in the energy-release step itself.

MPSC's environment and energy questions frequently test whether a candidate can classify energy sources correctly by their underlying physical mechanism rather than by loosely grouping them as 'renewable' or 'non-renewable'. This question is a good example: wood and biogas are commonly, and correctly, called renewable or 'green' energy sources, which can tempt a candidate into assuming they must therefore be non-carbon sources like geothermal or solar — but renewability and carbon involvement are independent properties. A fuel can be renewable and still carbon-based (wood, biogas), just as it can be non-renewable and carbon-based (coal); geothermal is unusual in being neither a combustion fuel nor, in the conventional sense, a stored chemical fuel at all.

• Wood is a biomass fuel composed mainly of cellulose and lignin; burning it oxidises carbon to release energy.
• Biogas is chiefly methane (CH4) produced by anaerobic digestion of organic waste; burning it is carbon combustion.
• Coal is a carbon-rich fossil fuel formed from ancient plant matter over geological time.
• Geothermal energy is harvested from the Earth's internal heat (residual formation heat plus radioactive decay) and involves no fuel combustion.
• Renewability and carbon-based combustion are independent properties: wood and biogas are renewable yet carbon-based, while geothermal is neither combustion-based nor a stored chemical fuel.

• Assuming any source labelled 'renewable' or 'green' must also be carbon-free, when biomass and biogas are renewable yet still carbon-based
• Overlooking that biogas is fundamentally a methane-based combustion fuel because the word 'bio' suggests something inherently clean
• Treating coal as the only 'carbon' source in a list and missing that wood and biogas are equally carbon-based
• Not recognising geothermal energy as fundamentally different in mechanism (harvested heat) rather than degree (a 'cleaner' fuel)

MPSC science and environment questions on energy sources often mix genuinely non-carbon sources like geothermal or solar with renewable-but-carbon-based sources like wood and biogas, testing whether a candidate classifies sources by their actual energy-release mechanism rather than by a loose renewable/non-renewable label.

No directly related past PYQ was found.

• practice — not a real PYQ

  Geothermal energy is primarily derived from which of the following ?

  • (a)Combustion of underground carbon deposits
  • (b)The Earth's internal heat from residual formation heat and radioactive decay
  • (c)Solar radiation absorbed by rocks
  • (d)Anaerobic digestion of organic matter underground

  Answer(b) The Earth's internal heat from residual formation heat and radioactive decay — geothermal plants tap this heat directly as steam or hot water to drive turbines, without burning any fuel.

• practice — not a real PYQ

  Biogas produced by anaerobic digestion of organic waste is chiefly composed of which gas, making its combustion a carbon-based energy process ?

  • (a)Hydrogen
  • (b)Methane
  • (c)Nitrogen
  • (d)Oxygen

  Answer(b) Methane — biogas is mainly a mixture of methane and carbon dioxide, and burning the methane component to release energy is a carbon-combustion process, unlike geothermal energy.