Molecule (a or b) in each pair has the higher boiling point also indicate force, hydrogen bonding (h), dipolar (p), or dispersion (d) is responsible for the difference so in NH₃ and PH₃ the a molecule is high melting point and forces are dispersion force and in C₂H₅OH and CH₃OHCH₃ the molecule a has high melting point and has a dispersion force and C₄H₈ and C₁₀H₂₀ the b molecule has high melting point and has a dispersion force and in HCl and F₂ the a molecule has high boiling point and has a dipolar dipole interaction forces
The forces are electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons as in covalent bonds and there are four type of forces
So here given molecule in that we have to seen the forces and high melting point so
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