Intermolecular forces, or IMFs, are forces of attraction between different molecules or particles. There are 4 different IMFs in liquid which affect the properties of substances.

The first IMF is in all nonpolar and is known as the London/Dispersion Force, or LDF, which comes from the idea of polarizability. Polarizability is the ease of distortion of an electron cloud, which means that the random movement of particles in a substance sometimes lets all the electrons onto one side of the molecule, causing it to be polarized for a moment. In other words, since the electrons are moving, sometimes they all gather on one side causing the molecule to become slightly polar and therefore attracted to other polar molecules. Size is the only thing that affects polarizability, as larger molecules are more polarizable than smaller molecules. This means the C₈H₁₈ (gasoline) will be more polarizable than H₂O.

Even though LDFs can create a large attraction occasionally between some of the larger molecules, they are often the weakest of the IMFs, especially in small-medium sized molecules. Because of their small strength of attraction, they have a relatively low effect on the boiling/melting points of the substance, only raising it by a small amount. However, the opposite is true about the freezing point, as LDFs lower it by more than any other IMF.

Dipole-dipole attractions occur between polar molecules only. For these attractions to work, the dipoles need to be permanent, meaning a polarizable molecule does not experience a dipole-dipole attraction. The strength of a dipole-dipole attraction is usually larger than LDF’s, but it is still only a medium attraction. The medium attraction of a dipole-dipole attraction correlates to a medium elevation of the boiling point, and a medium-high depression of the freezing point.

Hydrogen bonding also can only occur between polar molecules, but only when hydrogen is attracted to fluorine, oxygen, or nitrogen. Once again, it needs a permanent dipole to attract. Hydrogen bonds, such as the ones in H₂O, are much stronger than dipole-dipole LDFs, but still aren’t the strongest of the IMFs. They have a medium-high effect on the boiling point, but only a medium effect on the freezing point.

Finally, the ion-dipole attraction is the strongest of all of the IMFS. Occurring only in polar molecules, an ion is extremely attracted to an oppositely charged dipole. The high strength of attraction causes a very high boiling point elevation, yet a very small effect on the boiling point.

IMFs have many effects on the properties of the liquids they are involved in. When two things are miscible, they are mutually soluble with one another. This happens when they share the same IMFs. So if two liquids have LDFs, dipole-dipole, and hydrogen bonding, they will be miscible

Vapor pressure is the tendency to evaporate. At higher temperatures, liquids have a higher temperature to evaporate. Also, a higher vapor pressure means weaker IMFs, or a more volatile substance. A liquid will boil when its vapor pressure is the same as the atmospheric pressure, as the molecules are no longer held together by IMFs. This is why the stronger the IMF, the higher the boiling point.

Surface tension comes from the surface molecules feeling a net force towards the rest of the liquid that comes from IMFs. Water can have a lot of surface tension due to its strong hydrogen bonding. If you put a drop of water on a penny, it will not flow off. If you keep putting water droplets on that penny, the water line will go way above the ridge on the side of a penny, yet it will still not overflow. In fact, depending on the size of the droplets, you can fit 20-25 drops of water on a penny before it overcomes surface tension and overflows.