UPPSC 2021 Prelims Q88: Which one of the following is the main constituent of

Q: What is the answer to UPPSC 2021 Prelims Q88?

C. Correct — C, Butane. LPG (Liquefied Petroleum Gas) is a mixture of light hydrocarbons obtained from petroleum refining and natural-gas processing, and its principal components are butane (C4H10) and propane (C3H8). In Indian domestic cooking-gas cylinders butane is the major constituent (blended with propane), which is why 'Butane' is the correct choice among the options given. These gases liquefy easily under moderate pressure, so a large amount of fuel can be stored as liquid in a portable cylinder.

UPPSC 2021 Prelims GS Paper-I · Question 88 of 150

Verified against the official question paper. Answer as per the official UPPSC answer key.

Which one of the following is the main constituent of LPG?

(a)Hexane
(b)Pentane
(c)Butane
(d)Methane

Answer

Why

Correct — C, Butane. LPG (Liquefied Petroleum Gas) is a mixture of light hydrocarbons obtained from petroleum refining and natural-gas processing, and its principal components are butane (C4H10) and propane (C3H8). In Indian domestic cooking-gas cylinders butane is the major constituent (blended with propane), which is why 'Butane' is the correct choice among the options given. These gases liquefy easily under moderate pressure, so a large amount of fuel can be stored as liquid in a portable cylinder.

Why the others are wrong

(a)Hexane — Wrong — hexane (C6H14) is a liquid at ordinary temperature and pressure and is a component of petrol/solvents, not of the gaseous LPG blend.
(b)Pentane — Wrong — pentane (C5H12) is essentially a volatile liquid at room temperature; it is not a main constituent of LPG, which is dominated by the lighter butane and propane.
(d)Methane — Wrong — methane (CH4) is the main constituent of natural gas / CNG (compressed natural gas), NOT of LPG. This is the classic trap: LPG (petroleum gas, butane+propane) is confused with piped/natural gas (methane).

Concept

LPG is a portable liquefied fuel made mainly of butane and propane — hydrocarbons light enough to be gases at room temperature but which liquefy under modest pressure, so they can be stored in cylinders. It is distinct from CNG/PNG (natural gas), whose main constituent is methane. An odourless gas, LPG is deliberately given a smell by adding ethyl mercaptan so leaks can be detected.

The examiner's trap is 'methane', because methane is the most familiar cooking-fuel gas (piped natural gas). But the question names LPG — Liquefied PETROLEUM Gas — which comes from refining crude oil and is chiefly butane (with propane). Eliminate hexane and pentane (too heavy — liquids), reject methane (that is natural gas/CNG), and butane remains.

Key facts

LPG's main constituents are butane (C4H10) and propane (C3H8); in Indian domestic cylinders butane predominates.
Methane (CH4) is the main constituent of natural gas / CNG — not LPG.
LPG is odourless; ethyl mercaptan is added as an odourant to detect leaks.
Butane and propane liquefy under moderate pressure, allowing compact storage as a liquid.

A worn orange domestic cooking-gas cylinder with a blue regulator and a green rubber hose, standing against a wall. — the everyday LPG cylinder stores liquefied butane and propane under pressure — butane being the main constituent that makes option (c) correctImage: Ks.mini · Wikimedia Commons (CC BY-SA 3.0)

Study next

Hydrocarbons: alkanes methane, ethane, propane, butane and their usesLPG vs CNG vs PNG vs LNG — constituents and sourcesFossil fuels and petroleum refining/fractional distillationCalorific (fuel) value and complete vs incomplete combustionOdourants and safety additives in domestic fuels

Common traps

Answering 'methane' for LPG — methane is natural gas/CNG, while LPG is butane + propane
Assuming heavier alkanes (pentane, hexane) — which are liquids at room temperature — are the main gaseous constituents

UPSC/UPPSC repeatedly test fuel-gas constituents — remember LPG = butane + propane, CNG/natural gas = methane, coal gas = hydrogen + methane + CO, water gas = CO + hydrogen, biogas = mainly methane.

Related PYQs

UPSC_2005_GS1_Q882005Open & attempt →

Assertion (A): The main constituent of the Liquefied Petroleum Gas is methane. Reason (R): Methane can be used directly for burning in homes and factories where it can be supplied through pipelines.

(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R are individually true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Answer(d) A is false but R is true

Directly the same concept — the Assertion 'LPG's main constituent is methane' is FALSE (LPG is butane + propane), the very fact this UPPSC item tests when it marks butane correct.

UPSC_2004_GS1_Q1242004Open & attempt →

Match List I (Fuel Gases) with List II (Major Constituents) and select the correct answer using the codes given below: List I: (A) CNG (B) Coal gas (C) LPG (D) Water gas List II: 1. Carbon monoxide, Hydrogen 2. Butane, Propane 3. Methane, Ethane 4. Hydrogen, Methane, Carbon monoxide

(a) A 2 B 1 C 3 D 4
(b) A 3 B 4 C 2 D 1
(c) A 2 B 4 C 3 D 1
(d) A 3 B 1 C 2 D 4

Answer(b) A-3, B-4, C-2, D-1

Same concept — matching each fuel gas to its major constituents. It fixes LPG → 'Butane, Propane' (C-2), confirming butane as LPG's principal component.

Practice

practice — not a real PYQ
The main constituent of CNG (Compressed Natural Gas) used as a vehicle fuel is
- (a)Butane
- (b)Propane
- (c)Methane
- (d)Ethane
Answer(c) Methane — CNG/natural gas is chiefly methane, whereas LPG is chiefly butane and propane.
practice — not a real PYQ
Which substance is added to odourless LPG so that gas leaks can be detected by smell?
- (a)Carbon dioxide
- (b)Ethyl mercaptan
- (c)Chlorine
- (d)Nitrogen
Answer(b) Ethyl mercaptan — this strong-smelling sulphur compound is added to LPG (and natural gas) as a warning odourant for leaks.