Consider a beaker filled with molecules in a liquid state. It may look calm on the outside, but if you could see the tiny electrons moving inside the beaker, then dispersion forces would be obvious. Also called London dispersion forces, after Fritz London, they are electrostatic attractive forces between the electrons. Every molecule exhibits some degree of these forces.
TL;DR (Too Long; Didn't Read)
The attraction between neighboring molecules causes dispersion forces. The electron cloud of one molecule becomes attracted to the nucleus of another molecule, so the distribution of electrons changes and creates a temporary dipole.
What Causes Dispersion Forces
The attraction between molecules falls under the category of Van der Waals forces. The two types of Van der Waals forces are dispersion forces and dipole-dipole forces. The dispersion forces are weak, while the dipole-dipole forces are stronger.
The electrons that orbit molecules can move and have different charge distributions over time. One end of the molecule can be positive while the other end can be negative. A temporary dipole exists when you have two opposite charges that are close to each other. When one molecule comes into contact with another one, it can be attracted to it. The electrons from the first molecule may feel a pull toward the positive charge of the second molecule, so dispersion forces are in action. However, the attraction is weak.
Example of Dispersion Forces
Looking at substances such as bromine (Br2) or dichlorine (Cl2) reveals dispersion forces. Another common example is methane (CH4). The only forces in methane are dispersion forces because there are no permanent dipoles. Dispersion forces help nonpolar molecules turn into liquids or solids because they attract particles.
What Causes a Dipole-Dipole Force
When polar molecules come together, dipole-dipole forces appear. Similar to dispersion forces, opposites attract again. Two molecules become attracted to each other because they have permanent dipoles. Electrostatic interactions happen between these dipoles. The molecules can line up with the positive ends attracted to the negative ones. Dipole-dipole forces are stronger than dispersion forces.
How to Determine Dipole-Dipole Forces
The main way to determine dipole-dipole forces is to look at the molecules and check the polarity. You can examine the electronegativity difference between the atoms to see if they are polar. Electronegativity shows the capability of atoms to attract electrons. In general, if this difference falls between 0.4 and 1.7 on the electronegativity scale, there is polarity and a strong chance of dipole-dipole forces existing.