Final answer:
The strongest attractive force in pure substances depends on whether the substance is molecular or ionic, and on the polarity and size of the molecules involved. For molecular substances, hydrogen bonding (when applicable) is generally the strongest, followed by dipole-dipole interactions, then London dispersion forces. Ionic attractions are the strongest for ionic compounds.
Step-by-step explanation:
To identify the strongest attractive force present in pure substances, one must understand the different types of intermolecular forces. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Among these, ionic attractions also play a role in the aggregation of ions rather than molecules.
London dispersion forces are the weakest and occur in all atoms and molecules as a result of temporary shifts in the electron clouds. Dipole-dipole interactions occur between molecules with permanent dipoles and are stronger than London forces. Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. Ionic attractions, which are not technically van der Waals forces, are present in ionic compounds and are the strongest type of intermolecular force.
The strength of these attractive forces greatly influences physical properties such as boiling point, melting point, viscosity, and surface tension. For instance, water's high boiling point is due to its strong hydrogen bonds, and increased molecular size leads to stronger London forces.