Liquids

Every property this chapter asks about comes back to one thing, the attraction between whole molecules. These forces are physical rather than chemical, electrostatic in nature, and are collectively called intermolecular forces, secondary forces, or more famously van der Waals forces. They are far weaker than the covalent bonds inside a molecule, and they fall into a small number of types the paper expects you to name on sight: the London dispersion force, the dipole-dipole interaction, the dipole-induced dipole or Debye force, the ion-dipole force, and hydrogen bonding, which is a specially strong case of dipole-dipole attraction treated on its own. Two strength statements are safe and worth keeping: the weakest intermolecular force in a liquid is the instantaneous dipole-induced dipole force, and the strongest is hydrogen bonding, though even that is only about five to ten per cent of the strength of an ordinary covalent bond.

London dispersion forces arise from temporary dipoles. In a nonpolar molecule such as $I_2$ the electron cloud is never perfectly steady, so at any instant it can be lopsided enough to give the molecule a fleeting dipole; that dipole distorts the cloud of a neighbouring molecule and induces an opposite dipole in it, and the two attract. The force is named the instantaneous dipole-induced dipole force, and it is the reason nonpolar substances such as $H_2$, $Cl_2$, $CH_4$, $He$ and $Ne$ can be liquefied at all. Its strength climbs with the number of electrons and the size of the molecule, so it gathers down a group: iodine molecules are held far more firmly than the tiny helium atom, which has the fewest electrons of any common example and the weakest dispersion forces in the set.

Dipole-dipole forces act between molecules that already carry a permanent dipole, such as $HCl$ or iodine monochloride, $I-Cl$, where the partly negative chlorine end of one molecule lines up with the partly positive iodine end of the next. Their strength rises with the electronegativity difference across the bond and is roughly one per cent of a normal covalent bond. The Debye force is the same idea one step removed, where a permanent dipole polarises an otherwise nonpolar neighbour, and the ion-dipole force draws an ion towards the charged end of a polar molecule. None of these is strong on its own; a puddle holds together only because there are very many of them.