1 If molecules stick together more, they'll be tougher to break apart. 2  Stronger intermolecular forces â higher melting and boiling points. 3  Viscosity is a measure of how well substances flow. 4  Stronger intermolecular forces â higher viscosity. Viscosity is a measure of how well substances flow. 2  Stronger intermolecular forces â higher viscosity. 3  Surface tension is a measure of the toughness of the surface of a liquid. 4  Stronger intermolecular forces â higher surface tension. 5  This is a small amount of gas that is found above all liquids.