CAPF AC 2023 Q31: Science & Technology – Chemistry
- (a) methane
- (b) hydrogen
- (c) carbon dioxide
- (d) hydrogen sulphide
Answer: (a) methane
Explanation
Biogas
How to think: biogas forms when cow dung and other organic waste rot without air.
Bacteria break the waste down and release a gas mixture.
Up to three-fourths of it is methane, the part that burns.
| Constituent of biogas | Approximate share |
|---|---|
| Methane | 50 to 75% |
| Carbon dioxide | 25 to 45% |
| Hydrogen, hydrogen sulphide | Traces |
Why it is a good fuel: it burns without smoke, leaves no ash and has a high heating value.
The leftover slurry is a rich manure with nitrogen and phosphorus.
The process: anaerobic digestion by methanogens in a gobar gas plant.
Answer: (a) methane.
Why each option is right or wrong
- (a) CORRECT. Methane makes up as much as 75% of biogas.
- (b) Incorrect. Hydrogen is present only in traces.
- (c) Incorrect. Carbon dioxide is the second gas and does not burn.
- (d) Incorrect. Hydrogen sulphide is a trace gas that gives the smell.
Key point
Biogas (gobar gas): up to 75% methane, rest carbon dioxide with traces of hydrogen and hydrogen sulphide; anaerobic digestion by methanogens; slurry used as manure. CNG and natural gas are also mainly methane. LPG: butane and propane.
Remember
Biogas = mostly methane (up to 75%).
Common mistakes
- Marking carbon dioxide, which is present but is not the main gas.
- Mixing up biogas (methane) with LPG (butane and propane).
- Thinking hydrogen is the burning part of biogas.
Exam tip
Educated guess: marsh gas, natural gas, CNG and biogas all come from rotting organic matter, and all are mainly methane.
Sources
- NCERT Class X Science — Sources of Energy (biogas plant)
- NCERT Class XII Biology — Microbes in Human Welfare (microbes in production of biogas)