Which of the hydrocarbons are arranged as per the increasing order of their boiling points ?
- (a)Methane, Butane, Propane, Heptane
- (b)Propane, Butane, Pentane, Octane
- (c)Propane, Butane, Heptane, Methane
- (d)Octane, Ethane, Methane, Propane
Correct — B, Propane, Butane, Pentane, Octane. In a family of straight-chain alkanes the boiling point rises with the number of carbon atoms, because larger molecules have more surface area and stronger van der Waals (London dispersion) forces. Propane (C3), butane (C4), pentane (C5) and octane (C8) are in increasing size order, so their boiling points increase in that same order.
- (a)Methane, Butane, Propane, Heptane — Butane (C4) is placed before propane (C3), but butane boils higher than propane, so this is not a correct increasing order.
- (c)Propane, Butane, Heptane, Methane — Methane (C1) has the lowest boiling point of all, yet it is placed last — the order is not increasing.
- (d)Octane, Ethane, Methane, Propane — Octane (C8) has the highest boiling point but is placed first, so this sequence is jumbled, not increasing.
Alkanes form a homologous series in which each member differs by a CH2 unit. As the carbon chain lengthens, molecular mass and surface area grow, the van der Waals forces between molecules strengthen, and so the boiling point rises steadily up the series.
To pick the increasing-boiling-point order, just line the alkanes up by carbon number: the more carbons, the higher the boiling point. Only option (b) keeps the carbon count rising the whole way (C3 to C4 to C5 to C8).
- Boiling point of straight-chain alkanes increases with carbon number.
- This happens because larger molecules have stronger van der Waals (dispersion) forces.
- Boiling points rise in the order methane < ethane < propane < butane < pentane < ... < octane.
- Branching lowers the boiling point (less surface contact) for the same formula.
More carbons means a higher boiling point, so C3 < C4 < C5 < C8 — option (b).
- Ordering by name rather than by carbon number.
- Forgetting that methane is the lowest-boiling member.
Asked as an 'arrange in increasing/decreasing order of boiling point (or molecular weight)' item.
Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer(a) Methane, ethane, propane and butane
Same reasoning — ordering alkanes by carbon number. UPSC orders them by molecular weight; the NDA item uses the same rising-chain-length logic for boiling points.
The temperature at which "vapour pressure of the liquid in an open vessel becomes equal to the atmospheric pressure" is called
- (a) Melting point
- (b) Boiling point
- (c) Liquid point
- (d) None of the above
Answer(b) Boiling point
Companion concept — this NDA item defines the boiling point that Q74 asks you to compare across alkanes.
- practice — not a real PYQ
Among the following alkanes, which has the highest boiling point?
- (a)Methane
- (b)Propane
- (c)Butane
- (d)Octane
Answer(d) Octane — the longest chain has the strongest van der Waals forces.
- practice — not a real PYQ
The boiling point of straight-chain alkanes increases with chain length because of stronger
- (a)hydrogen bonds
- (b)ionic bonds
- (c)van der Waals forces
- (d)covalent bonds
Answer(c) van der Waals forces — larger molecules attract one another more.