= Solution
The clearest comparison is between the two <amines>, which have the same <molecular formula> and <molar mass>. \b[<isopropylamine> can donate N–H <hydrogen bonds> to neighbouring <nitrogen> <lone pairs>; <trimethylamine> has no N–H donor.] The resulting intermolecular association raises the <primary amine>'s boiling point, allowing it to remain liquid at room <temperature>, whereas <trimethylamine> is gaseous. Independent phase-change data place their normal boiling points on opposite sides of room <temperature>.<NIST Chemistry WebBook, SRD 69>[trimethylamine, CAS 75-50-3, and isopropylamine, CAS 75-31-0, phase-change data]{ref}
<acetone> has a substantial <carbonyl> dipole and London dispersion forces sufficient for a room-<temperature> liquid, even though it cannot build a strong donor–acceptor <hydrogen bond> network by itself. <isobutane> has mainly London dispersion forces and a compact shape and remains gaseous. <acetamide> combines <carbonyl> acceptors with two N–H donors, allowing an extended, directionally bonded crystal lattice; favourable packing as well as intermolecular association stabilizes its solid state.
These compounds have similar molar masses, so mass alone cannot explain the differences. The <hydrogen-bond acceptor parameter> also cannot predict the phase on its own: a pure substance needs complementary donors to exploit strong acceptor sites, and freezing additionally depends on crystal packing and <entropy>. A stronger acceptor does not automatically imply a higher melting point for arbitrary <molecules>.
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